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Section 6: Freeways

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Overview

Freeways are arterial highways with full control of access. They are intended to provide high levels of safety and efficiency in the movement of large volumes of traffic at high speeds. This section discusses the features and design criteria for freeways and includes the following subsections:

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Basic Design Criteria

Specific references to Freeway Geometric Design criteria are shown in Table 3-15:

Anchor: #i1065690Table 3-15: Freeway Geometric Design Criteria

Design Criteria

Reference

General

Design Speed Mainlanes (urban and rural)

Table 3-17

Design Speed Frontage Roads (urban and rural)

Chapter 3, Frontage Roads

Minimum Horizontal Radius

Table 2-4 and 2-5

Vertical Curvature

Figures 2-6 and 2-7

Maximum Grades (%)

Table 2-9

Stopping Sight Distance

Table 2-1

Vertical Clearance, New Structures

Table 2-11

Superelevation

Chapter 2, Superelevation Rate, Superelevation Transition Length, Supereleveation Transition Placement

Pavement Cross Slope

Chapter 2, Pavement Cross Slope

Lane and Shoulder Widths

Table 3-18

Clear Zone

Table 2-12

Capacity and LOS Analysis

Highway Capacity Manual

Turning Radii

See Chapter 7, Minimum Designs for Truck and Bus Turns



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Access Control

This subsection discusses access control and includes the following topics:

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General

The entire Interstate Highway System and portions (see TxDOT Statewide Planning Map https://www.txdot.gov/apps/statewide_mapping/statewideplanningmap.html) of the State Highway System have been designated by the Texas Transportation Commission as Controlled Access Highways. It can be necessary to limit or deny an abutting owner's access rights along certain sections of highways, which includes the right of ingress and egress and the right of direct access to and from the owner's abutting property to the highway facility. Such access may be controlled under the State's Police Power, which is an inherent right of sovereignty. However, the existing right of access to an existing public way is an increment of ownership and one of the rights vested in the owner of abutting property. This legal right may be limited or completely denied under the State's police power, which may entitle the owner to potential damages suffered by the loss of such access.

The abutting owners are denied access to any controlled access highway on new location, unless there is a specific grant of access, and no damages may be claimed for the denial of access to the new facility. The theory behind this law is the understanding that the owner cannot be damaged by the loss of something which the owner never had. The denied access or Access Denial Line (ADL) (a.k.a., Control of Access line) is established as part of the property rights acquired by the State for the new location access controlled highway. For ADL established in this manner, if an adjacent property owner requests a change in ADL then the change should be valued under the appropriate Department procedures. The ADL change must be supported by an engineering review to prevent access to that part of the roadway designated as an access controlled highway, as outlined in TxDOT's Access Management Manual.

An owner's rights are taken if an existing road is converted into a controlled access facility, the design of which does not contemplate the initial construction of frontage road(s), and the abutting owner is to be denied access to such facility pending frontage road construction. If an existing road is converted into a controlled access facility, the design of which does contemplate frontage road(s) in the initial construction, and the abutting owner is not to be denied access to such frontage road(s), there is not taking or denial of access rights. Access to the frontage road(s) constitute access to the facility. Further control of movements, once upon the frontage road, such as one-way traffic, no U-turns, no left or right turns, denial of direct access to the through lanes, and circuitous routes are all controlled under police power and inflict no more control over the abutting owner than is inflicted upon the general public.

If an existing road is converted into a controlled access facility and no part of the abutting owner's property is taken for right of way, but access is to be denied to the controlled access facility, and by reason of such denial of access it is found that such owner will suffer damages measured by the diminution of the market value of said abutting land, said owner should be requested to release and relinquish said access rights for consideration equal to the State's approved value for such damages. If the owner is not willing to negotiate on these terms, then the access right may be acquired through eminent domain proceedings. In some instances, the State's appraisal and approved value may indicate that there is no diminution in value by reason of the access denial. In those cases the Department will not proceed with acquisition of the access control and will cancel the A.C. Parcel. There will be no offer of compensation to the owner and the owner will not be required or asked to sign a Release and Relinquishment of Access Rights to a Highway Facility. The ADL will remain on the State Right of Way Map memorializing the denial of access under the Department's police powers and its status will be annotated on the State Right of Way map.

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Mainlane Access

Freeway mainlane access, either to or from abutting property or cross streets, is only allowed to occur through a ramp. This control of mainlane access may be achieved through one of the following methods:

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  • Through access restrictions whereby the access to the highway from abutting property owners is denied with ingress and egress to the mainlanes only at selected freeway or interchange ramps; or
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  • Through construction of frontage roads permitting access to the mainlanes only at selected ramps.

    In either case, direct access from private property to the mainlanes is prohibited without exception.

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Frontage Road Access

Information on the driveway clearance from the cross-street intersection is contained in TxDOT's Access Management Manual (Chapter 2 Section 3) and should be considered when locating driveways on projects involving the construction or reconstruction of ramps and/or frontage roads. The remainder of this section addresses driveway and side street access in relation to ramps.

Where frontage roads are provided, access at ramp junctions should be controlled through access restrictions or the use of the State's police powers to maintain operations. The “Control of Access Methods” Section below discusses this further. The placement of streets and driveways near ramp junctions with frontage roads should be carefully considered and permitted only after a traffic operations and safety evaluation.

On reconstruction projects, it may be necessary to close or relocate driveways to meet these guidelines. If the closure/relocation is not feasible, and adjustment of the location of the ramp gore along the frontage road is not feasible, then deviation from these recommended guidelines should be supported by a traffic operations and safety evaluation.

Access Beyond Exit Ramps. Figure 3-17 shows the recommended access control strategy for planned exit ramps and should be used where practical. This figure along with the accompanying values in Table 3-16 show the desirable spacing to be used between exit ramps and driveways, side streets, or cross streets if practical. The number of weaving lanes is defined as the total number of lanes on the frontage road downstream from the ramp. Increased weaving, resulting in operational degradation, occurs when driveway or side street access on the frontage road is in close downstream proximity to exit ramp terminals. For this reason it is important to maintain appropriate separation between the Intersection of Travel Lanes (see Figures 3-17 and 3-18) and downstream driveways or side streets.

It is recognized that there are occasions when meeting the exit ramp separation distance values in Table 3-16 may not be possible due to the nature of the existing development. In these cases, at least 250-ft of separation should be provided from the intersection of the exit ramp and frontage road travel lanes to the downstream driveway or side street. Careful consideration should be given in situations like this, since the minimal separation distance may negatively impact the operation of the frontage road, exit ramp, driveway and/or side street traffic. When a minimum 250-ft separation distance cannot be obtained, consideration should be given to channelization methods that would restrict access to driveways within this 250-ft distance. Refer to the Texas MUTCD for specific types of channelization.

Recommended Access Control at Exit Ramp
Junction with Frontage Road. (click in image to see full-size image) Anchor: #i1032526grtop

Figure 3-17. Recommended Access Control at Exit Ramp Junction with Frontage Road.

Anchor: #i1065730Table 3-16: Desirable Spacing between Exit Ramps and Driveways, Side Streets, or Cross Streets

Total Frontage Road + Ramp Volume

(vph)

Driveway or Side Street Volume (vph)

Desirable Spacing (ft)

Number of Weaving Lanes on Frontage Road

2

3

4

< 2500

< 250

460

460

560

--

> 250

520

460

560

--

> 750

790

460

560

--

> 1000

1000

460

560

> 2500

< 250

920

460

560

--

> 250

950

460

560

--

> 750

1000

600

690

--

> 1000

1000

1000

1000



Access Prior to Entrance Ramps. The presence of driveways or side streets in close upstream proximity to entrance ramp terminals, similar to exit ramps, increases weaving and results in operational degradation of the frontage roads. Therefore, maintaining proper separation is important. Figure 3-18 shows the recommended access control strategy for entrance ramps and should be used where practical. There will be occasions when meeting the entrance ramp separation distance values shown in Figure 3-18 may not be possible due to existing development conditions. In these cases, at least 100-ft of separation distance should be provided between the intersection of the entrance ramp and frontage road travel lanes and the upstream driveway or side street.

This limited separation negatively impacts the operation of the frontage road, entrance ramp, driveway, and/or side street traffic, therefore, careful consideration should be given to its use. When the 100-ft separation distance for entrance ramps cannot be obtained, consideration should be given to channelization methods that would restrict access to driveways within this 100-ft distance. Refer to the Texas MUTCD for specific types of channelization.

Recommended Access Control at Entrance
Ramp Junction with Frontage Road. (click in image to see full-size image) Anchor: #i1032528grtop

Figure 3-18. Recommended Access Control at Entrance Ramp Junction with Frontage Road.

Ramp Location. During schematic development, care should be exercised to develop design in sufficient detail to accurately tie down the locations of ramp junctions with frontage roads and the location of access control limits; reference Chapter 1, Section 3 for specific guidance on schematic development. These drawings are often displayed at meetings and hearings and become the basis for right-of-way instruments or the Department's regulation of driveway location for that project. Figures 3-17 and 3-18 provide recommended access control at exit and entrance ramp junctions with frontage roads.

In some instances, ramps must be shifted to satisfy level of service considerations or geometric design controls. When this is necessary, the access control limits should also be shifted if right-of-way has not been previously purchased.

Direct Access to a Ramp. The following requirements apply to Direct Access to a ramp from an adjacent property or street:

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  • All ramps with a frontage road: Direct access to a ramp is always prohibited for the full length of the ramp.
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  • Interstate freeway ramp without a frontage road or an Interstate interchange connector: Direct access is always prohibited under 23 CFR 625.3 and 625.4 for the full length of the ramp or Interstate interchange connector.
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  • Non-Interstate facility without a frontage road, with or without access controlled by designation: Direct access is strongly discouraged. If allowed, the access location must be determined through the procedures and spacing criteria contained in this chapter, and in the TxDOT Access Management Manual.
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Control of Access Methods

A controlled access highway may be developed in either of two ways:

Control of Access by Designation. When the Texas Transportation Commission designates a freeway to be developed as a controlled access facility under Transportation Code §203.031, the State is empowered to control access through access restrictions. All Interstate Highways are designated as controlled access and certain other routes have been or may be designated. These designated freeways may or may not have frontage roads, whichever arrangement is determined to be appropriate as discussed in Planning Development of freeways by designation, rather than solely by design, is the preferred design approach especially for all new location freeways.

Not Along an Existing Public Road. Per Transportation Code §203.031, wherever designated controlled access freeways include frontage roads and the planned location is not along an existing public road, access should be controlled through access restrictions at ramp junctions with frontage roads as shown on Figure 3-17 and Figure 3-18.

Where no frontage roads are provided, access is controlled to the mainlanes by access restriction.

Along an Existing Public Road. Per Transportation Code §203.031, whenever a designated controlled access freeway is to be provided along an existing public road, frontage roads are generally provided to retain or restore existing access. This is subject to the guidelines presented in the Planning section of this document.

Frontage road access should be controlled by imposing recommended access restrictions in accordance with Figure 3-17 and Figure 3-18 whenever all of the following conditions prevail:

Access may be controlled by use of the State's police power to control driveway location and design where any of the following conditions prevail:

Whenever the State's police powers are used, the denial of access zone should be free of driveways insofar as practical.

(Some designated controlled access freeways may be a combination of new location and along an existing road. Both Figure 3-17 and Figure 3-18 and police powers may be used at appropriate locations)

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Control of Access by Design

If an existing highway is to be developed as a controlled access facility solely by design (not designated by the Transportation Commission), TxDOT is not empowered to purchase access rights. Instead, TxDOT must achieve access control by construction of continuous frontage roads and by the utilization of the State's police power to control driveways, particularly at ramp junctions with frontage roads.

In the interest of providing for highway safety and utility, the State may regulate driveway location and design through its police powers. Landlocking through complete denial of access is beyond the State's regulatory power (without Commission designation under the Transportation Code). The State, however, may effectively regulate driveway location in accordance with statewide policy as long as the following two conditions are met:

The TxDOT Access Management Manual governs design and location of driveways.

The design philosophy from the section on Frontage Roads applies whenever new or relocated ramps are to be provided along existing freeways. Access should be controlled at frontage road junctions through access restriction as illustrated in Figure 3-17 and Figure 3-18.

Whenever access is to be controlled solely by provision of frontage roads, State police power to regulate driveway location and design should be used to control access near ramp junctions. However, where designation by the Transportation Commission is practical, use of access restrictions as illustrated in Figure 3-17 and Figure 3-18 is preferred over controlling access solely by design (police power).

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Mainlanes

This subsection discusses mainlanes and includes information on the following topics:

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Design Speed

The design speed of urban freeways should reflect the anticipated operating conditions during non- peak hours. However, the design speed should not exceed the limits of prudent construction, right-of-way, and socioeconomic costs. Minimum design speeds for rural and urban freeways are shown in Table 3-17.

Anchor: #i1065800Table 3-17: Design Speed for Controlled Access Facilities

Facility

Minimum (mph)

Mainlanes - Urban

50

Mainlanes - Suburban

60

Mainlanes - Rural

70



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Level of Service

For acceptable degrees of congestion, urban freeways and their auxiliary facilities should generally be designed for Level of Service (LOS) C, as defined in the Highway Capacity Manual, in the design year. In heavily developed urban areas, LOS D may be acceptable. In heavily congested areas, other Measure of Effectiveness (MOEs) may include travel time, speed and queue lengths.

In rural areas, LOS B is desirable for freeway facilities; however, LOS C may be acceptable for auxiliary facilities (i.e., ramps, direct connections and frontage roads) carrying unusually high volumes.

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Lane Width and Number

The minimum and usual mainlane width is 12-ft. The number of lanes required to accommodate the anticipated traffic in the design year is determined by the level of service evaluation as discussed in the Highway Capacity Manual. See Table 3-18 and Figure 3-20 for further information.

After the number of lanes has been determined, the balance in the number of lanes should be confirmed based on the following principles:

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  1. At entrances, the number of lanes beyond the merging of two traffic streams should not be less than the sum of all traffic lanes on the merging roadways minus one but may be equal to the sum of all traffic lanes on the merging roadways.
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  3. At exits, the number of approach lanes on the highway should be equal to the number of lanes on the highway beyond the exit, plus the number of lanes on the exit, minus one. Exceptions to this principle occur at clover leaf loop-ramp exits that follow a loop-ramp entrance and at exits between closely spaced interchanges. (Closely spaced interchanges are those where the distance between the end of the taper of the entrance terminal and the beginning of the taper of the exit terminal is less than 1,500-ft, and a continuous auxiliary lane between the terminals is being used). In these cases, the auxiliary lane may be dropped in a single-lane exit such that the number of lanes on the approach roadway is equal to the number of through lanes beyond the exit plus the lane on the exit.
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  5. The traveled way of the highway should be reduced by not more than one traffic lane at a time.
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  7. To satisfy lane-balance at two-lane exits, an auxiliary lane upstream of the exit should be provided.
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  9. To satisfy lane-balance at two-lane entrances, at least one additional lane should be provided downstream. This lane may be another through lane if needed for capacity or an auxiliary lane that may be reduced with the appropriate taper or at the next interchange.

Typical examples of lane balance are shown in Figure 3-19.

Typical Examples of Lane Balance. (click in image to see full-size image) Anchor: #JRTQJFPJgrtop

Figure 3-19. Typical Examples of Lane Balance.

See AASHTO's A Policy on Geometric Design of Highways and Streets for design details.

See the latest version of the Texas MUTCD, Freeway Pavement Marking Standard and the TxDOT Freeway Signing Handbook for additional guidance on signing and pavement marking design.

Typical Freeway Sections. (click in image to see full-size image) Anchor: #i1034630grtop

Figure 3-20. Typical Freeway Sections.

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Shoulders

Continuous surfaced shoulders are provided on each side of the mainlane roadways, both rural and urban, as shown in Figure 3-20. For four-lane freeways, the minimum shoulder widths must be 10-ft on the outside and 4-ft on the inside (median). On freeways of six lanes or more, 10-ft inside shoulders must be provided for emergency pull-offs. A 10-ft outside shoulder must be maintained along all speed change lanes. However, in weaving areas with LOS A or LOS B, a 6-ft shoulder can be considered. See Table 3-18 and Figure 3-20 for further information. For shoulders that are designated as emergency evacuation lanes, a 12-ft outside shoulder is recommended.

Anchor: #i1781394Table 3-18: Roadway Widths for Controlled Access Facilities

Type of Roadway

InsideShoulder Width(ft.)

OutsideShoulder Width(ft.)

Traffic Lanes(ft.)

Mainlanes:

4-Lane Divided

4

10

24

6-Lane or more Divided

10

10

361

1-Lane Direct Conn.2

2 or 44 Rdwy.; 44 Str.

8

14

2-Lane Direct Conn.2

2 or 44 Rdwy.; 44 Str.

8

24

Ramps:

 

Minimum

Desirable

 

Ramps2 (uncurbed)

2 or 44 Rdwy.; 44 Str.

6

8

145

Ramps3 (curbed)

-

-

-

225

Speed Change Lanes

-

6

10

12

Notes:

  1. For more than six lanes, add 12-ft width per lane.
  2. If sight distance restrictions are present due to horizontal curvature, swapping the widths of the inside and outside shoulders is allowed (i.e. the shoulder width on the inside of the curve may be increased to 8-ft and the shoulder width on the outside of the curve decreased to 2-ft (Rdwy) or 4-ft (Str)) but can be no less than the values stated in this footnote (see Note 4).
  3. The curb for a ramp lane will be mountable and limited to 4-inches or less in height. The width of the curbed ramp lane is measured face to face of curb. Existing curb ramp lane widths of 19-ft may be retained.
  4. Roadways with longitudinal traffic barriers must provide a minimum 4-ft shoulder from the travel lane edge. The shoulder across a bridge will be measured from the travel lane edge to the nominal face of rail. This provides additional offset for high-speed operation and door-opening space for vehicles stopped on the shoulder of the roadway.
  5. 12-ft lanes are permissible on ramps with multiple lanes.


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Medians

The width of the median is the distance between the inside edges of the travel lanes. For depressed freeway sections, 76-ft medians are generally used. Where topography, right-of-way, or other special considerations dictate, depressed freeway median width may be reduced from 76-ft to a minimum of 48-ft. A median width of 24-ft to 30-ft is generally used on freeway sections with flush medians. On freeways including six or more travel lanes and a flush 24-ft median, the resulting section provides for 10-ft inside shoulders and a usual 2-ft offset to barrier centerline. See Figure 3-20 for further information.

Design the ultimate freeway section initially where feasible to reduce the adverse impact to high-speed and high-volume traffic resulting from construction in multiple phases. Several benefits could be realized from initially constructing the ultimate section such as lower levels of traffic handling and traffic shifts, minimal temporary pavement and interim transitions, construction and inflation cost savings, and plans to accommodate future bridge columns and other appurtenances. Under those circumstances where future additional lanes will be provided in the median area, the usual median width of 24-ft should be increased by the appropriate multiple of 12-ft in anticipation of need for additional lanes. Provisions should be made, or retained, for any future high occupancy vehicle lanes in the median.

On freeways where median barrier is used, the horizontal alignment must be checked for adherence to the stopping sight distance criteria accounting for any restriction caused by the barrier.

For information on freeway median crossings, refer to Chapter 7, Section 6 Emergency Median Crossovers.

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Outer Separation

The portion of the freeway between the mainlanes and frontage road or right-of-way, where frontage roads are not provided, should be wide enough to accommodate shoulders, speed change lanes, side slopes and drainage, retaining walls and ramps, and signs and other appurtenances necessary for traffic control. Because of right-of-way limitations in urban areas, the outer separation may oftentimes be narrower than desired; however, in rural areas, where opposing headlights along a two-way frontage road tend to reduce a driver's comfort and perception on the freeway, the outer separation should be as wide as possible. Clear zones, as defined in Chapter 2, are desirable for errant vehicle recovery and safety.

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Crossing Facilities

The following references show the appropriate widths for facilities crossing the freeway:

The Bridge Project Development Manual should also be referenced for appropriate structure widths.

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Vertical and Clear Zones at Structures

Vertical. All controlled access highway grade separation structures, including railroad underpasses, should provide the minimum vertical clearance over the usable roadway shown in Table 2-11. Specific railroad company guidelines may also require additional vertical clearance requirements as a function of structure type. This information can be found in the applicable railroad company guidelines. Additionally, all freeways are part of the Texas Highway Freight Network (THFN), and therefore may require additional THFN vertical clearance requirements as stated in Chapter 2, Section 6 and Chapter 3, Section 8.

Structures over the mainlanes of Interstate or controlled access highways must meet the minimum vertical clearance requirement in Table 2-11 except within cities where the vertical clearance is provided on an Interstate loop around the particular city. See Chapter 1, Section 2 for specific guidance and coordination for design exceptions for Interstates.

Roadways under the mainlanes of interstate or controlled access highways must meet the minimum vertical clearance requirements for the appropriate undercrossing roadway classification.

Minimum vertical clearances for pedestrian crossover structures are specified in Table 2-11. This is due to the increased risk of personal injury upon impact by over-height loads and the relative weakness of such structures to resist lateral loads from vehicular impact.

The clearances specified above apply to the entire width of roadway mainlanes including usable shoulders and include an allowance of 6-inches for future pavement overlays. It is recognized that it is impractical to arrive at the exact clearance dimensions on the structure plans.

Vertical clearance for railroad overpasses is discussed in the Bridge Project Development Manual.

Horizontal. For the minimum clear zone to bridge parapets and piers see Table 2-12.

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Frontage Roads

This subsection discusses frontage roads and includes information on the following topics:

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Function and Uses

Frontage roads serve a multitude of purposes in addition to controlling or providing access. In urban settings, frontage roads reduce the “barrier” effect of urban freeways by providing for some of the circulation of the local street. They provide invaluable operational flexibility, serving as detour routes during mainlane crashes, mainlane maintenance activity, for over-height loads, as bus routes, and during inclement weather. Continuous frontage roads provide the operational flexibility required to manage saturation for freeways that include freeway surveillance and control.

In addition to these functions, frontage roads can prove advantageous when used as the first stage of construction for a future freeway facility. By constructing frontage roads prior to the mainlanes, interim traffic demands can often be satisfied and a usable section of highway can be opened to the traveling public at a greatly reduced cost.

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Planning

Frontage roads may be incorporated into a project at various points during the project development, including the planning stage, subsequent to the planning stage, or after freeway construction. However, later incorporation of frontage roads will be more difficult.

Frontage road construction may be funded by TxDOT, a local government, or shared by both. The Texas Transportation Commission has adopted rules governing the construction and funding of frontage roads. All frontage road development must be in accordance with the rules contained in 43 Texas Administrative Code (TAC) §15.54. TxDOT’s Project Development Process Manual can also be referenced for additional information.

Changes in control of access must be in accordance with 43 TAC §15.54(d)(4).

Subsequent changes in the control of access will be as shown on approved construction plans or as provided in instruments conveying right-of-way on authorized projects, or as authorized by Commission Minute Order, and specified in TxDOT's ROW Preliminary Procedures for the Authority to Proceed. Where access is permitted to adjacent properties and the adjacent property owners approach the State for a change in access, ingress and egress will be governed by the issuance of permits to construct access driveway facilities. This process is shown in Department established policy, guidelines and procedures, designed to provide reasonable access, in accordance with the Texas laws.

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Design Speed on Frontage Roads

Design speeds for frontage roads are a factor in the design of the roadway. For consistency, design speeds that match values for collector streets or highways should be used. The desirable design speed for urban frontage roads is 50 mph, and the minimum design speed is 30 mph. See Table 3-5 for design speeds for suburban frontage roads (suburban collector criteria), and Table 3-19 for rural frontage roads (rural collector criteria).

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Capacity and Level of Service

Although techniques to estimate capacity and level of service on freeways and urban arterials are detailed in the Highway Capacity Manual, these procedures should not be applied directly to frontage roads, as frontage roads have characteristic of both freeways (i.e., exit and entrance ramps) and urban arterials (i.e., driveways, cross streets and signalized intersections). The following report was developed to suggest techniques for estimating capacity and level of service on frontage roads.

Kay Fitzpatrick, R. Lewis Nowlin, and Angelia H. Parham. Procedures to Determine Frontage Road Level of Service and Ramp Spacing. Research Report 1393-4F, Texas Department of Transportation, Texas Transportation Institute, 1996.

Additional information can be found in TTI Research Report 1393-4F, which contains procedures for the following:

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Frontage Road Design Criteria

A frontage road should be designed to provide one-way operation. There may be exceptions in certain isolated instances, where a one-way pattern would impose severe restrictions on circulation within an area. These exceptions must be approved by the Design Division at the schematic stage.

Frontage road design criteria can be found in the following tables:

Anchor: #CHDIAGBITable 3-19: Design Criteria for Rural Frontage Roads

Design Speed2 (mph)

Min. Width1 for Future Traffic Volume of

0-400 ADT

400-1,500 ADT

1,500-2,000 ADT

2,000 or more ADT

LANES (ft.)

20

10

10

11

12

25

10

10

11

12

30

10

10

11

12

35

10

10

11

12

40

10

11

11

12

45

10

11

11

12

50

10

11

12

12

55

10

11

12

12

60

11

11

12

12

65

11

11

12

12

70

11

11

12

12

75

11

12

12

12

80

11

12

12

12

SHOULDERS (ft)4,5

Each Shoulder

Two-Way Operation

23

4

8

8 – 10

Inside Shoulder

One-Way Operation

23

23

4

44

Outside Shoulder

One-Way Operation

23

4

8

8 – 10

Notes:

  1. May retain existing paved width on a reconstruction project if total paved width is 24 ft. and operating satisfactorily
  2. Use rural collector criteria (Table 3-7) for determining minimum design speed
  3. At locations where roadside barriers are provided, use minimum 4-ft. offset from travel lane edge to barrier face
  4. If the one-way frontage road section contains three or more travel lanes, then minimum inside shoulder width is 8-ft to 10-ft.
  5. A 5-ft minimum clear space for bicyclists should be provided on bridges being replaced or rehabilitated Off-system Bridges, with current ADT greater than 400 ADT, where this addition may represent an unreasonable increase in cost may be excepted from the bicycle clear space requirement. See Ch. 6 Section 1 for specific off-system bridge requirements for current ADT of 400 or less.


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Conversion of Frontage Roads from Two-Way to One-Way Operation

In some areas, existing frontage roads are operating as two-way facilities. Such two-way operation has the following disadvantages:

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  • Higher crash rates due to the risk of head-on collisions at the ramp terminals;
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  • Increased potential for wrong-way entry to the mainlanes;
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  • Complicated intersections requiring left turns from the arterial onto the frontage road from both directions;
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  • Restriction from using signal phasing and sequencing options normally available at signalized diamond interchanges; and
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  • Limited capacity when compared to one-way operations.

Existing two-way frontage roads should be converted to one-way operation when one or more of the following conditions exist:

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  • Queuing on the frontage road approach that routinely backs up from the arterial intersection to within 100-ft of a freeway entrance or exit ramp gore;
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  • The Level of Service (LOS) of a signalized intersection of the frontage road and the arterial drop below LOS C;
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  • Queuing in the counter-flow direction, which would not exist if the frontage road were one-way, routinely backs up from the stop line at a freeway entrance or exit ramp to within 100-ft of the arterial street;
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  • Crash rates are above the statewide average crash rate for two-way frontage roads; or
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  • Major freeway reconstruction or rehabilitation is occurring in a developed or developing area.

Conversion of two-way frontage roads in urbanizing rural areas, with relatively long distances between crossover interchanges, will require consideration of additional crossovers to minimize the distance traveled for adjacent residents and business patrons. Existing local street systems in the area will facilitate traffic circulation and minimize the travel time impact of converting frontage roads from two-way to one-way operation.

The simple conversion of two-way to one-way frontage roads will be accomplished with ramp and terminal design based on reconstruction criteria shown in Chapter 3, Section 6 Ramps and Direct Connectors. Other existing frontage road lanes may retain dimensions that meet rehabilitation criteria shown in Chapter 4, Section 4 Frontage Roads. However, if the frontage roads are being reconstructed, then reconstruction design criteria shown in Chapter 3, Section 6 Freeways will be applicable throughout the section.

Major reconstruction or rehabilitation of existing two-way frontage roads shall require approval from the Design Division if the two-way operation is to remain. Any new frontage road must be designed and constructed for one-way operation unless prior approval from Design Division is obtained. Two-way frontage roads should only be considered in rural areas where:

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Grade Separations and Interchanges

The ability to accommodate high volumes of traffic safely and efficiently through intersections depends largely on the arrangement of the intersection. Grade separations offer the greatest efficiency, safety, and capacity for intersecting roadways. A grade separation refers to the crossing of two roadways by a physical separation so that neither roadway interferes with the other. An interchange is a system of interconnecting roadways in conjunction with one or more grade separations that provides for the movement of traffic between two or more roadways or highways on different levels.

The decision to provide a fully access-controlled facility becomes the warrant for providing highway grade separations or interchanges at the most important intersecting roadways (usually arterials and some collectors) and railroads. No at-grade intersections are allowed along the mainlanes of a freeway.

Effect on Community. An interchange or series of interchanges on a freeway through a community may affect large continuous areas or even the entire community. For this reason, interchanges must be located and designed so that they will provide the best possible traffic service. Drivers who have exited from a freeway expect to be able to re-enter in the same vicinity; therefore, partial interchanges that do not serve all desired traffic movements should be avoided.

Classifications. Interchanges are generally classified according to the number of approach roadways or intersection legs, as three-leg, four-leg and multi-leg interchanges. Three-leg interchanges are commonly referred to as Tee (Trumpet) interchanges. Four-leg interchanges are often referred to as Cloverleaf or Diamond interchanges. Directional interchanges have three or more legs including direct connectors.

The following subsections include a brief description and some of the advantages and disadvantages of each of the types of interchanges.

For information on the design of Diverging Diamond Interchanges, refer to Appendix E.

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Three-Leg Interchanges

Three-leg interchanges provide connection for three intersecting approach roadways. These interchanges should only be used after careful consideration because expansion to include a fourth leg is usually very difficult. If the potential exists that a fourth leg will ultimately be included, another type of interchange may be more appropriate.

Trumpet. The most widely used 3-leg interchange is the trumpet, as shown in Figure 3-21. This type of interchange is particularly suitable for the connection of a major facility and a freeway. Preference should be given to the major turning movements so that the directional ramps carry the higher traffic volume and the lower traffic volumes use the loop ramps.

Trumpet Three Leg Interchange. (click in image to see full-size image) Anchor: #i1040463grtop

Figure 3-21. Trumpet Three Leg Interchange.

Direct. Direct three-leg interchanges are those in which all movements are provided without the use of loops. These interchanges should be used only where all movements have high traffic volumes. Direct three-leg interchanges require more than one structure or, alternatively, a three-level structure. Both variations are illustrated in Figure 3-22.

Directional Three-Leg Interchange. (click in image to see full-size image) Anchor: #i1040465grtop

Figure 3-22. Directional Three-Leg Interchange.

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Four-Leg Interchanges

Four-leg interchanges can take a wide variety of forms. The choice of interchange is generally established after careful consideration of dominant traffic patterns and volumes, right-of-way requirements, and system considerations. The three primary types of four-leg interchanges are Diamond, Cloverleaf, and Directional.

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Diamond Interchanges

The diamond interchange is the most common interchange, especially in urban areas, since it requires less area than any other type. The diamond interchange is used almost exclusively for major-minor crossings since left-turn movements are made at-grade across conflicting traffic on the minor road. Separation between frontage road intersections in diamond interchanges in urban or suburban conditions should desirably be 300-ft as shown in Figure 3-23. Provide proper traffic signal timing if the 300-ft cannot be met.

Typical Interchange for At-Grade Portion
of Diamond Interchange in Urban or Suburban Areas. (click in image to see full-size image) Anchor: #i1040491grtop

Figure 3-23. Typical Interchange for At-Grade Portion of Diamond Interchange in Urban or Suburban Areas.

The diamond interchange may have several configurations, as discussed in the following paragraphs and shown in Figure 3-24:

Common Diamond Interchanges. (click in image to see full-size image) Anchor: #ECNHYCMSgrtop

Figure 3-24. Common Diamond Interchanges.

Conventional diamond without frontage roads. The conventional diamond (Figure 3-24(A)) is the most common application of a diamond interchange. Traffic exits in advance of and close to the cross street. Entering vehicles quickly access the freeway just past the cross street. This design can result in vehicles backing up onto the freeway when long queues form on the ramp.

Conventional diamond with frontage roads. The conventional diamond with frontage roads (Figure 3-24(B)) is a common variation of a diamond interchange. Traffic exits in advance of and near the cross street. Entering vehicles quickly access the freeway past the cross street. However, exiting vehicles can sometimes back up onto the freeway when long queues form on the ramp or frontage road, and most vehicles must go through the intersection to gain access to most frontage road property.

Reverse diamond or X- pattern. The reverse diamond or "X" interchange pattern (Figure 3-24(C)) is used in locations with significant development along the frontage road. It provides access between interchanges and prevents exiting vehicles from backing up onto the freeway. However, entering vehicles may have to accelerate on an upgrade and exiting maneuvers occur just beyond the crest vertical curve where weaving also takes place. The "X" ramp pattern also encourages frontage road traffic to bypass the frontage road signal and weave with the mainlane traffic. The "X" ramp pattern may cause some drivers to miss an exit located well in advance of the cross street when the exit location is unexpected.

Spread diamond. The spread diamond (Figure 3-24(D)) moves the frontage roads outward to provide better intersection sight distance at the cross street. This design also provides improved operational characteristics with signalized intersections due to the separation between intersections. The additional right-of-way required may limit its usage.

Stacked diamond. Sometimes access to and from the mainlanes is needed on two closely-spaced cross streets. Insufficient distance for consecutive entrance and exit ramps can be resolved by using grade separated or “braided” ramps, resulting in a “stacked diamond” (Figure 3-24 (E)).

Split diamond. In some locations, it may be feasible and desirable to “split” the diamond by having one-way streets for the arterial movement (Figure 3-24 (F)). This is especially true near central business districts where one-way street systems are common. However, the split diamond can also be used to accommodate two closely-spaced two-way arterial roadways crossing a freeway.

Three level diamond. In urban areas, where the cross street carries a high volume of traffic, the three-level diamond interchange, illustrated in Figure 3-25, may be warranted. The through movements of both the controlled access facility and the cross street flow is uninterrupted with only the turning movements requiring regulation by stop signs or traffic lights. This type of interchange is not usually recommended for use as the ultimate design at the crossing of two controlled access facilities because it requires left-turn interchanging traffic to negotiate three traffic signals or stop controls. However, as stage construction for a fully directional interchange between two controlled access facilities, the three-level diamond can be effective. See AASHTO's A Policy on Geometric Design of Highways and Streets for design details.

Three Level Diamond Interchange. (click in image to see full-size image) Anchor: #i1040495grtop

Figure 3-25. Three Level Diamond Interchange.

Single point diamond. A special type of freeway-to-arterial interchange has received attention during recent years and is worthy of discussion. AASHTO’s A Policy on Geometric Design of Highways and Streets refers to it as a “single point diamond” (SPDI) or “single point urban” (SPUI) interchange. The primary features of an SPDI are that all four turning movements are controlled by a single traffic signal and opposing left turns operate to the left of each other. In this type of interchange, the freeway mainlanes may go either over or under the crossing arterial and the turn movements occur at-grade on the arterial, as illustrated in Figure 3-26. This type of interchange has application only in specialized locations. Traffic operations and signalization must be carefully modeled prior to final design selection of the single point urban interchange.

Single Point Diamond Interchange. (click in image to see full-size image) Anchor: #i1040497grtop

Figure 3-26. Single Point Diamond Interchange.

Three level stacked diamond. The three-level stacked diamond interchange is also an interchange requiring only one signalized intersection. In a sense, it is a three-level version of the “single point diamond” configuration, as illustrated in Figure 3-27. This design grade separates both roadways, and accommodates turning movements with signal operations requiring only one signalized intersection. The two-phase signal operation at the intersection typically provides a level of throughput on the turning movements between a conventional diamond interchange and a fully directional interchange. Furthermore, it works best at separating high arterial cross-street and freeway traffic. It has the same shortcomings as the “single point diamond” in the way it brings the left turn movements together.

Three Level Stacked Diamond Interchange 
(see Figure 3-28 for At-Grade Portions of the Interchange). (click in image to see full-size image) Anchor: #i1040499grtop

Figure 3-27. Three Level Stacked Diamond Interchange (see Figure 3-28 for At-Grade Portions of the Interchange).

As indicated in Figure 3-28, vehicles enter the intersection with oncoming vehicles to the right in contrast to the left as is the case on conventional diamond interchange intersections. Also, the design is less attractive with continuous frontage roads.

Three Level Stacked Diamond At-Grade Interchange. (click in image to see full-size image) Anchor: #i1040501grtop

Figure 3-28. Three Level Stacked Diamond At-Grade Interchange.

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Cloverleaf Interchanges

Cloverleaf interchanges are very common in many states. These types of interchanges were popular in the early era of freeway construction but are usually no longer considered preferable for freeway to freeway movement, especially when interchange volumes are high. However, in some instances they may be appropriate when a freeway interchanges with a non-controlled access facility in a location away from an urban or urbanizing area. Cloverleafs should not be used where left-turn volumes are high, exceeding 1200 pcph, since loop ramps are limited to one lane of operation and have restricted operating speeds.

Primary disadvantages of the cloverleaf design include the following:

When used, cloverleaf designs should include collector-distributor roads to provide more satisfactory operations as further noted in the section on Collector-Distributor Roads.

Full cloverleaf. The four-quadrant, full cloverleaf, illustrated in Figure 3-29, eliminates all left-turn conflicts through construction of a two-level interchange.

Full Cloverleaf Interchange. (click in image to see full-size image) Anchor: #i1040515grtop

Figure 3-29. Full Cloverleaf Interchange.

Partial cloverleaf. A cloverleaf without ramps in all four quadrants, illustrated in Figure 3-30, is sometimes used when site controls, such as railroads or streams running parallel to the cross street, limit the number of loops. Alternatively, this can be used when the left-turn conflicts caused by the absence of one or more loops are within tolerable limits. With such an arrangement, left-turn conflicts at the ramp intersections require that satisfactory approach sight distance be provided. Several variations on partial cloverleafs are also discussed in the AASHTO’s A Policy on Geometric Design of Highways and Streets.

Partial Cloverleaf Interchange. (click in image to see full-size image) Anchor: #i1040517grtop

Figure 3-30. Partial Cloverleaf Interchange.

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Directional Interchanges.

Interchanges that use direct or semi-direct connections for one or more left-turn movements are called “directional” interchanges (Figure 3-31). When all turning movements travel on direct or semi-direct ramps or direct connections, the interchange is referred to as “fully directional”. These connections are used for important turning movements instead of loops to reduce travel distance, increase speed and capacity, reduce weaving and avoid loss of direction in traversing a loop. “Fully directional” interchanges are usually justified at the intersection of two freeways.

Four Level Fully Directional Interchange
without Frontage Roads. (click in image to see full-size image) Anchor: #i1040531grtop

Figure 3-31. Four Level Fully Directional Interchange without Frontage Roads.

Four level without frontage roads. The four-level directional interchange as depicted in Figure 3-31 includes direct connections for all freeway-to-freeway movements, without continuation of any frontage roads through the interchange.

Five level with frontage roads. In some instances, it may be desirable to continue the frontage roads through the interchange at the first or second level, producing a five-level directional interchange. Where frontage roads are made continuous through the interchange, the lower three levels are a three-level diamond configuration. Where stage construction is desired, the three-level diamond will adequately serve moderate traffic volumes until the upper two levels of direct connections are constructed to complete the five-level interchange. Figure 3-32 depicts a five level interchange with frontage roads.

Five Level Fully Directional Interchange
with Frontage Roads. (click in image to see full-size image) Anchor: #i1040533grtop

Figure 3-32. Five Level Fully Directional Interchange with Frontage Roads.

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Interchange Spacing

Interchange spacing can have a major influence on freeway operations. Appropriate spacing of interchanges is impacted by several items including interchange type, lane configuration, weaving volumes, signing, signal progression, and lengths of speed-change lanes.

The minimum interchange spacing in rural areas is 2 miles. In urban areas, the desirable interchange spacing is 1-mile. Where closer spacing is desired or required, the use of braided ramps or collector-distributor roads is recommended. Braided ramps are ramps that cross over each other and are vertically separated. These ramps separate incoming and exiting traffic by having one ramp pass over the other, thereby eliminating traffic weaving, improving safety and easing congestion.

Traffic operational analysis will be required where lower values of interchange spacing are used.

Interchange spacing is measured between cross streets as shown in Figure 3-33.

Interchange Spacing as Measured
between Successive Cross Streets. (click in image to see full-size image) Anchor: #WASMHLVKgrtop

Figure 3-33. Interchange Spacing as Measured between Successive Cross Streets.

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Auxiliary Lanes

At interchanges, an auxiliary lane is a full width travel lane that is developed to facilitate traffic operation. Auxiliary lanes are most often used to:

Continuous auxiliary lanes should be constructed between the entrance and exit terminals of interchanges where the distance between the entrance ramp and the exit ramp is short. An auxiliary lane is recommended between the acceleration and deceleration lanes when the distance between successive painted gore noses is less than 2,000-ft.

An auxiliary lane may be introduced as a single exclusive lane or in conjunction with a 2-lane entrance. See AASHTO's A Policy on Geometric Design of Highways and Streets for more information concerning the termination of auxiliary lanes. The design of auxiliary lanes should be based on the traffic volumes for the exiting, entering and through movements.

Shoulder and lane widths for auxiliary lanes are shown in Table 3-18.

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Ramps and Direct Connectors

This subsection discusses ramps and direct connectors and includes information on the following topics:

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General Information

All ramps and direct connectors should be designed for one-lane operation with provision for emergency parking; however, if the anticipated volume exceeds the capacity of one freeway lane, two-lane operation may be provided with consideration given to merges and additional entry lanes downstream.

Once ramp locations have been identified on a schematic layout have been exhibited at a public hearing or have otherwise become a matter of public record, extreme caution should be exercised in making any subsequent changes in ramp location to better serve areas that may have developed after the original design was determined. In all cases, proposed changes should be submitted to the Design Division, and another public hearing may be required.

Right-side ramps are markedly superior in their operational characteristics and safety to those that leave or enter on the left side. With right-side ramps, merging and diverging maneuvers are accomplished into or from the slower moving right travel lane. Since a high majority of ramps are right-side, there is an inherent expectancy by drivers that all ramps will be right-side, and violations of driver expectancy may adversely affect operation and safety characteristics.

Direct access to and from ramps can seriously impair safety and traffic operations and, therefore, should not be permitted. Direct access to an interstate interchange connector is prohibited. Direct access to a freeway ramp without a frontage road not access controlled by designation is discouraged and, if allowed, the location should meet TxDOT guidelines and be a safe and sufficient distance from the mainlane ramp gore.

For Interstate ramps, access along the ramp to or from a private drive, business or street is prohibited. The ramp is considered an extension of the mainlanes. 23 CFR 625.3 and 625.4 require adherence to the policies in AASHTO's manual “A Policy on Design Standards-Interstate System, May 2016”. Specifically, access to the Interstate system, including ramps, shall be fully controlled. Access control shall extend the full length of ramps and ramp terminals at the crossroad or frontage road.

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Design Speed

All ramps and connections should be designed to enable vehicles to leave and enter the traveled way of the freeway at 85 percent (desirable) to 70 percent (usual) of the freeway's design speed, rounded up to the nearest 5 mph increment, and limiting the speed differential to 10 mph on the upper range and 20 mph for the mid-range. The upper and mid-range design speeds are not always practical (e.g. constrained right-of-way or retrofit to existing frontage roads, etc.) and lower design speeds may be selected upon review from Design Division, but they should not be less than the lower range presented in Table 3-20. However, every effort should be made to meet the desirable ramp/connector design speed. These speeds do not pertain to ramp terminals, which should be properly transitioned and provided with speed-change facilities adequate for the highway speed involved.

Where the ramp joins a frontage road, the design speed over the length of the ramp may vary, with the portion of the ramp closer to the frontage road being designed to the frontage road speed (i.e. the lower speed). The independent design speed of the ramp should normally be assumed to be from the end of one physical nose to the beginning of the opposite physical nose. The design speed for a ramp should not be less than the design speed on the intersecting frontage roads. AASHTO's A Policy on Geometric Design of Highways and Streets provides additional guidance on the application of the ranges of ramp design speed shown in Table 3-20:

Anchor: #i1066203Table 3-20: Guide Values for Ramp/Connection Design Speed as Related to Highway Design Speed1

Ramp/Connector Design Speed2

(mph)

Highway Design Speed (mph)

30

35

40

45

50

55

60

65

70

75

80

Upper Range (85%)

25

30

35

40

45

50

50

55

60

65

70

Mid-Range (70%)

20

25

30

30

35

40

45

45

50

55

60

Lower Range (50%)

15

20

20

25

25

30

30

35

35

40

40

Notes:

  1. For corresponding minimum radius, see Table 2-3 and Table 2-4.
  2. Upper-range values of design speed generally are not attainable on loop ramps. For highway design speeds above 50 mph, loop design speed should be no less than 20 mph.


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Ramp Geometrics

Entrance Ramp Connections. The horizontal geometry of entrance ramps will depend on the design speeds of the ramp, the connecting frontage road, and the connecting freeway. The departure from the frontage road should be a smooth transition appropriate for the design speed and speed classification (low speed/high speed) of the frontage road. The pavement elevation and cross slope of the departing ramp should match the pavement elevation and cross slope of the connecting frontage road up to the physical nose. Once the ramp is independent of the connecting frontage road, the ramp horizontal geometry and cross slope should be set to the design speed of the ramp. Where the ramp connects to the freeway, beyond the physical nose, the pavement elevation and cross slope of the entering ramp should match the pavement elevation and cross slope of the connecting freeway.

Exit Ramp Connections. The horizontal geometry of the exit ramps will depend on the design speeds of the ramp, the connecting freeway, and the connecting frontage road. The departure from the freeway may be either tangent or parallel. The pavement elevation and cross slope of the departing ramp should match the pavement elevation and cross slope of the connecting freeway up to the physical nose. Once the ramp is independent of the connecting freeway, the ramp horizontal geometry and cross slope should be set to the design speed of the ramp. Where the ramp connects to the frontage road, beyond the physical nose, the pavement elevation and cross slope of the exiting ramp should match the pavement elevation and cross slope of the connecting frontage road.

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Gores

The term “gore” indicates an area at the junction of entrance or exit ramps with the mainlane or frontage roads which is illustrated in Figure 3-34.

Typical Gore Area Characteristics. (click in image to see full-size image) Anchor: #FGQLFCRXgrtop

Figure 3-34. Typical Gore Area Characteristics.

For an exit ramp junction at the mainlane, the physical nose is a point downstream from the painted nose of the gore, having some dimensional width that separates the roadways. The painted nose is a point, with no dimensional width, occurring where the travel ways intersect for the mainlane and ramp. The neutral area refers to the triangular area between the painted nose and physical nose.

The geometric layout of these is an important part of exit ramp terminal design. It is the decision point area that should be clearly seen and understood by approaching drivers and it should have a geometric shape appropriate for the likely speeds at that point. In a series of interchanges along a freeway, the gores should be uniform and have the same appearance to drivers.

The gore area, and the unpaved area beyond, should be kept as free of obstructions as practical to provide a clear recovery area. The unpaved area beyond the physical nose should be graded to be as nearly level with the roadways as practical so that vehicles inadvertently entering will not be overturned or abruptly stopped by steep slopes.

Grades and Profiles. Design controls for crest and sag vertical curves on ramps and direct connectors may be obtained from Figure 2-6 and Figure 2-7. Longer vertical curves with increased stopping sight distances should be provided wherever possible. Refer to Chapter 2 for guidance on grade breaks without a vertical curve.

The tangent or controlling grade on ramps and direct connectors should be as flat as possible, and preferably should be limited to 4 percent or less. Certain geometric constraints or topographic conditions permit the use of the following grades in Table 3-21.

Anchor: #i1796731Table 3-21: Desirable Maximum Grades for Ramps

Ramp Design Speed

(mph)

Max. Grade

(%)

25 to 30

7

35 to 40

6

Greater than or equal to 45

5



The ramp should have an independent profile from physical nose to physical nose. The profile grade within the neutral area should be designed as a projection of the connecting road profile and cross slope. Figure 3-35 shows typical characteristics of ramp profile limits.

Typical Ramp Profile Characteristics. (click in image to see full-size image) Anchor: #MCGVBFHEgrtop

Figure 3-35. Typical Ramp Profile Characteristics.

In some cases, it may be impractical to match the projected roadway profile and cross slope when designing the ramp proper profile. For these situations, a separated median area, with mountable curb or barrier, that warps the neutral area to join the projected profile of the connecting roadway and ramp proper may be considered. If a raised median is not preferred, surfaced mounted delineators may be used within the warped neutral area to discourage drivers from traversing this part of the ramp. See Figure 3-36 for illustrations of a warped neutral area and Table 3-22 for maximum algebraic differences in cross slope.

Warped Neutral Area. (click in image to see full-size image) Anchor: #OSMMADQNgrtop

Figure 3-36. Warped Neutral Area.

Anchor: #i1797139Table 3-22: Maximum Algebraic Differences in Pavement Cross Slope at Connecting Roadway Terminals

Design Speed of Exit or Entrance Curve

(mph)

Maximum Algebraic Difference in Cross Slope at Crossover Line

(%)

Less than or equal to 20

5.0 to 8.0

25 to 30

5.0 to 6.0

Greater than or equal to 35

4.0 to 5.0



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Cross Section and Cross Slopes

Superelevation rates, as related to curvature and design speed of the ramp or direct connector, should be based on values provided in Table 2-3 or Table 2-4. Table 2-3 may be used for low speed urban facilities and when connecting to frontage roads or cross streets that are also governed by the low speed urban criteria. If the frontage road or cross street are governed by the high-speed non-urban Table 2-4, then the entire ramp should be similarly designed.

The cross section of a ramp or direct connector is a function of the following variables:

Lane and Shoulder Widths. Lane and shoulder widths for ramps and direct connectors are shown in Table 3-18.

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Sight Distance

Sufficient sight distance should be provided for the probable vehicle speeds on ramps and direct connectors, taking into consideration the grade, vertical curve, alignment, and lateral and corner obstructions to vision. Sight distance and sight lines are especially important at merge points for ramps and mainlanes or between individual ramps. Table 2-1 shows recommended stopping sight distances for ramps and direct connections.

The sight distance on a freeway preceding the approach nose of an exit ramp should exceed the minimum stopping sight distance for the freeway design speed, preferably by 25 percent or more. The Decision Sight Distance, as discussed in Chapter 2, is a desirable goal.

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Ramp Terminal Design

The design of ramp terminals for exits from and entrances onto freeways and frontage roads has a significant impact to safety, operation, and capacity of the freeway facility. The following ramp terminal design features are discussed:

Entrance Ramp Acceleration Lane to Freeway. The acceleration lane associated with entrance ramps to freeway begins where the driver transitions from the ramp curvature to the flatter geometry of the speed change lane (SCL). The lane allows for acceleration and checking for gaps in the freeway traffic. The length of the acceleration lane is determined on the basis that merging vehicles should enter the through lane at a speed approximately equal to the running speed of the freeway. The acceleration lane ends at the beginning of the taper. The taper section begins where the width of the SCL becomes less than 12-ft and ends at the point where the SCL has fully merged with the freeway through lane. The taper section is not included in the length of the SCL. The factors for the calculation of the length of an acceleration lane are: (a) the speed at which drivers merge with through traffic, (b) the speed at which drivers enter the acceleration lane, and (c) the manner of acceleration.

Table 3-13 provides minimum acceleration lane lengths (La) for entrance terminals, applicable to both taper- and parallel-type entrances; adjustment factors for grade effects are shown in Table 3-14. Point A (shown in Figure 3-37) is the point where La is measured from and controls speed on the ramp. La should not start back on the curvature of the ramp unless the radius of the ramp equals 1,000-ft or more. Before La is extended back onto the ramp past the physical nose of the gore, the actual vertical and horizontal alignments of the ramp should be analyzed to determine if acceleration above the ramp design speed can reasonably begin there.

Gap acceptance length (Lg) is defined as beginning where the traveled ways of the mainlane and ramp roadways are a minimum of 2 lateral feet apart and ending where the right edge of the ramp is 12-ft from the right edge of the through lane of the freeway. The minimum gap acceptance length is 300-ft. The value of La or Lg, whichever produces the greater distance, is suggested for use in the design of the ramp entrance.

The two general forms of entering a freeway are through a taper type and parallel type speed change lanes.

Taper-Type Entrance Ramp to Freeway. A taper-type entrance shown in Figure 3-37 merges into the freeway with a long, uniform taper. A desirable rate of taper is approximately 50:1 to 70:1 (Design Speed:1 or DS:1) between the outer edge of the acceleration lane and the edge of the through traffic lane. The rate of taper is measured against the alignment of the mainlanes.

The geometrics should be designed in a way that drivers attain a speed that is within 5 mph of the operating speed of the freeway by the time they reach the point where the left edge of the acceleration lane joins the traveled way of the freeway.

Parallel-Type Entrance Ramp to Freeway. The parallel-type entrance ramp to freeway shown in Figure 3-37 provides an added lane of sufficient length to enable a vehicle to accelerate to near-freeway speed prior to merging. A taper is provided at the end of the added lane.

Desirably, a ramp curve with a baseline radius of 1,000-ft or more and length of at least 200-ft should be provided in advance of the added lane. If a radius less than 1,000-ft is used drivers tend to drive directly onto the freeway without using the acceleration lane. The taper at the downstream end of a parallel-type acceleration lane should be a suitable length to guide the vehicle gradually onto the through lane of the freeway. A minimum taper length of 300-ft is required.

The length of a parallel-type acceleration lane is generally measured from the point where the left edge of the ramp travel way joins the traveled way of the freeway to the beginning of the downstream taper, Lg. Acceleration usually takes place downstream from this point. However, a part of the ramp may also be considered in the acceleration length, La, provided the ramp curve approaching the acceleration lane has a radius of 1,000-ft or more and the motorist on the ramp has an unobstructed view of traffic on the freeway to the motorist's left.

An acceleration length, La, of at least 1,200-ft plus the taper is desirable where the ramp-freeway volume is nearing 4,600 vehicles per hour at the ramp-freeway junction.

Single-Lane Entrance Ramp Terminals
on a Tangent. (click in image to see full-size image) Anchor: #QQEJTBMPgrtop

Figure 3-37. Single-Lane Entrance Ramp Terminals on a Tangent.

Entrance Ramp to Freeway on a Curve. Connecting taper-type entrance ramps to freeways in a horizontal curve will need to be independently evaluated depending on the radius of the mainlanes as well as any superelevation and/or direction of the cross slope of the mainlanes, if any. See AASHTO's A Policy on Geometric Design of Highways and Streets for guidance on developing the alignment of a speed change lane on curves.Figure 3-38 through Figure 3-40 show different cross slope and curvature combinations of an entrance ramp connecting to the mainlanes. In all scenarios the rate of taper (DS:1) is measured relative to the alignment of the mainlanes. If the mainlanes are superelevated, the cross-slope transition should occur in the ramp prior to the physical nose. In the case where ramp curves are reverse of mainlanes or ramp cross slope exceed mainlane cross slope, sufficient length of tangent or a larger compound curve should be provided on the ramp to accommodate cross slope transitions prior to the physical nose. When a taper-type entrance ramp is used in a curve, the taper should be evenly distributed about the curve to meet the desirable taper length.

Entrance Ramp to a Freeway Curving
In The Direction Of The Ramp. (click in image to see full-size image) Anchor: #UNULKXGRgrtop

Figure 3-38. Entrance Ramp to a Freeway Curving In The Direction Of The Ramp.

Entrance Ramp To A Freeway Curving
Away From The Direction Of The Ramp. (click in image to see full-size image) Anchor: #QJJPQAFJgrtop

Figure 3-39. Entrance Ramp To A Freeway Curving Away From The Direction Of The Ramp.

Entrance Ramp to a Superelevated
Freeway Curving Away from the Ramp. (click in image to see full-size image) Anchor: #YLTYPODUgrtop

Figure 3-40. Entrance Ramp to a Superelevated Freeway Curving Away from the Ramp.

Exit Ramp Deceleration Lane from Freeway. The deceleration lane associated with exit ramps from freeways is sometimes preceded by a tapered section of roadway. It begins where the speed change lane is 12-ft wide and ends at Point A. The deceleration lane provides a means for vehicles to decelerate from the freeway speed to a speed commensurate with the controlling feature of the ramp. The deceleration lane length (La) is based on three factors: (a) the speed at which drivers maneuver onto the auxiliary lane, (b) the speed at the end of the deceleration lane, and (c) the manner of deceleration.

Taper-Type Exit Ramp from Freeway. A taper-type exit ramp, as shown in Figure 3-41, begins with an abrupt outer edge alignment break providing a clear indication of the point of departure from the through lanes. The divergence angle is usually 2 to 5 degrees. The angular break may be rounded by an appropriate radius at the design engineer's discretion. The divergence angle resulting from the designed alignment should be calculated and compared to the typical 2 to 5 degrees.

Parallel-Type Exit Ramp from Freeway. A parallel-type exit terminal usually begins with a taper, followed by an added lane that is parallel to the traveled way as shown in Figure 3-41. In locations where both the mainlane and ramp carry high volumes of traffic, the deceleration lane provided by the parallel-type exit provides storage for vehicles that would otherwise undesirably queue up on the through lane or on a shoulder. The length of a parallel-type deceleration lane is usually measured from the point where the added lane attains a 12-ft width to the point where the alignment of the ramp roadway departs from the alignment of the freeway. Where the ramp between the gores is curved it is desirable to provide a transition at the end of the deceleration lane.

Minimum deceleration lengths for various combinations of design speeds for the highway and for the ramp roadway are given in Table 3-23. Grade adjustments are given in Table 3-14. The values in Table 3-23 for minimum deceleration lane length on exit ramps do not account for any deceleration in the through lanes, therefore providing a conservative estimate for design. The designer should assume that all deceleration takes place in the speed-change lane when determining the minimum deceleration lane length.

Typical Single-Lane Exit Ramp
Terminals. (click in image to see full-size image) Anchor: #OAEWUYTLgrtop

Figure 3-41. Typical Single-Lane Exit Ramp Terminals.

Anchor: #i1799976Table 3-23: Minimum Deceleration Lane Lengths for Exit Ramps with Flat Grades of Less Than 3 Percent

Deceleration Lane Length, La (ft) for Design Speed of Controlling Feature on Ramp, V’ (mph)

Highway Design Speed, V (mph)

Stop Condition

15

20

25

30

35

40

45

50

30

235

200

170

140

-

-

-

-

-

35

280

250

210

185

150

-

-

-

-

40

320

295

265

235

185

155

-

-

-

45

385

350

325

295

250

220

-

-

-

50

435

405

385

355

315

285

225

175

-

55

480

455

440

410

380

350

285

235

-

60

530

500

480

460

430

405

350

300

240

65

570

540

520

500

470

440

390

340

280

70

615

590

570

550

520

490

440

390

340

75

660

635

620

600

575

535

490

440

390

80

705

680

665

645

620

580

535

490

440



Entrance Ramp from Frontage Road. The geometry of ramps connecting from frontage roads will differ significantly depending on whether they are connecting to a one-way or two-way frontage road. Connection from one-way frontage roads is recommended, as this reduces the chance of wrong-way traffic on a ramp. The horizontal geometry of the ramp connecting from a frontage road should be compatible with the horizontal geometry of the frontage road. If the frontage road is designed based on low-speed urban criteria, then the ramp connection should be similarly designed. If the frontage road is designed on high-speed rural criteria, then the ramp connection should be similarly designed.

While treatment of pavement edges may differ depending on whether the frontage road is shouldered or curbed, the path of the traffic remains the same. The traffic path should follow a smooth horizontal curve tangent to both the inside yellow stripe of the frontage road through to the inside yellow stripe of the ramp. This applies whether or not the traffic is entering from the through lanes on the frontage road or a dedicated auxiliary lane. Geometry guidance for entrance ramps from frontage roads is shown in Figure 3-42 and Figure 3-43. For more geometry and access control information on entrance ramps from frontage roads, refer to Figure 3-18.

Taper-Type Entrance Ramp from
a One-Way Frontage Road. (click in image to see full-size image) Anchor: #UPWEPSSPgrtop

Figure 3-42. Taper-Type Entrance Ramp from a One-Way Frontage Road.

Taper-Type Entrance Ramp from
a Two-Way Frontage Road. (click in image to see full-size image) Anchor: #QUGVAHHMgrtop

Figure 3-43. Taper-Type Entrance Ramp from a Two-Way Frontage Road.

Anchor: #i1801642

Exit Ramps to Frontage Roads

Ramps should connect to the frontage road at the minimum distance specified in Table 3-16. Greater distances are desirable to provide adequate weaving length, space added for vehicle storage and turn lanes at the cross street. For more geometry and access control information on exit ramps to frontage roads, refer to Figure 3-17.

Anchor: #i1801626

Ramp Spacing

Concerning ramp spacing, the term 'ramp' includes all types, arrangements, and sizes of turning roadways that connect two or more legs at an interchange, including direct connectors and freeway to freeway connections.

The minimum acceptable distance between ramps is dependent upon the merge, diverge and weaving operations that take place between ramps, as well as distances required for signing. The minimum distance is measured from painted nose of gore to painted nose of gore. The distance between successive ramps must be determined with the analysis procedures outlined in the Highway Capacity Manual (HCM). During the HCM analysis it may be determined that an auxiliary lane is needed to meet the appropriate LOS. See Level of Service to determine the appropriate LOS for a facility.

The minimum spacing values shown in Figure 3-44 represent a reasonable starting point during planning and early design. Ramp configurations shown in Figure 3-44 are relative to the facility being traveled on and assume no auxiliary lane is provided. When the distance between the successive painted noses of an entrance-exit (EN-EX) ramp combination is less than 2,000-ft on a freeway or 1,500-ft on a collector-distributor or frontage road, the speed-change lanes should be connected to provide an auxiliary lane. When the minimum distance cannot be achieved, consideration should be given to consolidating or braiding the ramps in lieu of requesting a design waiver.

A design waiver will be required if the minimum lengths specified in Figure 3-44 (without an auxiliary lane) or sufficient LOS is not met.

Arrangements for Successive Ramps. (click in image to see full-size image) Anchor: #UTAQNGUAgrtop

Figure 3-44. Arrangements for Successive Ramps.

Anchor: #CHDEJABH

Metered Ramps

Where ramps are currently expected to accommodate metering, or expected to in the future, the geometric design features shown in AASHTO’s Design Criteria for Ramp Metering should be considered. Ramp metering, when properly designed and installed, has been shown to have benefits for the operation of the mainlanes. However, since ramp meters are installed to control the number of vehicles that are allowed to enter the mainlanes, an analysis of the entire roadway network area should be done to determine adverse operational impacts to other roadways. It is suggested that the analysis specifically include both frontage road and adjacent cross street operations of through traffic, turning movements, and queue lengths. See AASHTO's A Policy on Geometric Design of Highways and Streets for additional discussion and design details on ramp metering.

Anchor: #CHDCFJEE

Collector-Distributor Roads

A collector-distributor road is a road that parallels and connects the main travel lanes and frontage roads or entrances ramps. This is sometimes referred to as an "Intersection Bypass". A collector-distributor is similar to an exit ramp but is typically longer than a ramp. A collector-distributor road is used to meet the following goals:

Collector-distributor roads are a component at all cloverleaf interchanges. Where there is considerable demand for frequent ingress and egress, as in and near the business districts of large cities, a collector-distributor road, continuous through several interchanges, should be provided.

Collector-Distributor Design Criteria:

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  • Design Speed: Table 3-20 (ramp criteria);
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  • Roadway Width: Table 3-18 (ramp criteria); and
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  • Capacity analysis and basic lane determination should be performed for the overall system rather than for the separate roadways.
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Frontage Road Turnarounds and Intersection Approaches

Turnaround lanes are to be provided at all interchanges with major arterials in urban and suburban areas where the freeway lanes are flanked by one-way frontage roads. Turnaround lanes are not to be provided where two-way frontage roads are used. In urban and suburban areas, overpasses should be arranged so that turnarounds may be added in the future. This includes provisions for end spans and vertical clearance for future turnarounds at overpasses. Underpasses should also allow for vertical clearances on future elevated turnarounds.

When the cross street overpasses the freeway, the resulting turnarounds will be on bridge structures. In these cases, sight lines and distances should be carefully evaluated with respect to any bridge railing sight obstructions. Similarly, sight lines and distances for cross street underpasses must also be evaluated for bridge header and retaining wall obstructions.

Figure 3-45 shows a typical example of a diamond interchange with frontage roads and turnarounds.

Typical Diamond Interchange with Frontage
Road. (click in image to see full-size image) Anchor: #i1047580grtop

Figure 3-45. Typical Diamond Interchange with Frontage Road.

Results from field-based observations, numerous simulations of site improvements with the potential to improve U-turn operations, and a full safety investigation of factors contributing to crashes at interchanges have produced the following guidance on the planning, design, and operation of turnaround lanes: Reference TTI Study No. 06894.

    Anchor: #RNLVYTJO
  • Turnaround lanes should be considered for future interchanges with a projected (20-year) peak-hour volume of at least 2,000 vph, or roughly 20,000 ADT. For existing interchanges, turnaround lane implementation should be considered when total interchange traffic volume reaches 4,000 vph, or approximately 40,000 ADT;
  • Anchor: #KRFSKLSN
  • Turnaround lane design should include an approach bay with a minimum length of 525 ft. In rural areas, this length primarily provides stopping sight distance on the U-turn approach for higher-speed operations. In urban areas, the bay length requirement is designed to allow U-turning vehicles to avoid interference from left-turn queues in the adjacent lane;
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  • Operations are improved if the turnaround lane departure features either a full added lane or an acceleration lane (minimum 100-ft length) with taper. Turnaround lane departures featuring stop or yield control, or those that terminate with only a taper transition into a frontage road lane, should only be used where geometric constraints or low-volume conditions exist;
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  • Unless trucks are specifically prohibited turnaround design should provide adequate turn radii to accommodate heavy vehicles;
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  • To minimize delay and queuing in the turnaround lane and to minimize the potential for crashes on the frontage road, consider closing driveways within the distances specified in the TxDOT Access Management Manual;
  • Anchor: #NDKXMRTW
  • If closing adjacent driveways is not feasible, consider the use of traffic control devices and/or channelization (e.g., pavement markings, flexible delineators, and/or raised curbs) to improve turnaround lane departure operations. Field observation and simulation studies have verified the benefits (e.g., reduced delay and fewer crashes) of constraining weaving maneuvers from turnaround lanes to adjacent downstream driveways;
  • Anchor: #YJTCPBRQ
  • Consider right-turn accommodations at the interchange and their impacts on operations and safety. Safety improvements have been observed when right turning traffic yields at the intersection through frontage road traffic (i.e., no right-turn bays or right-turn acceleration lanes were present);
  • Anchor: #FJQUEKCB
  • Signal timing can be used as an interchange management tool to support U-turn operation. Both cycle length and split adjustments have been successfully demonstrated to reduce frontage road queue length and average delay on frontage road approaches. Shorter queue lengths reduce the likelihood of a left-turn queue blocking access to a turnaround lane; and
  • Anchor: #OWCJXTPS
  • Consider the use of dotted line markings to improve interchange operations. Dotted lines to extend lane lines into the intersection and guide drivers through the appropriate turning path have shown reduced delay for turnaround lane movements under medium- to high-volume interchange operations. Directing cross street left-turn vehicles to the middle and/or right frontage road lanes provides gaps in the frontage road for vehicles using the turnaround lane.
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