Roadway Design Manual: Urban Streets

Section 2: Urban Streets

Overview

“Urban Streets,” as used in this chapter refers to roadways in developed areas that provide access to abutting property as well as movement of vehicular traffic. Access to these facilities is controlled through driveway locations, medians, and intersections with other roadways. The TxDOT Statewide Planning Map provides guidance on area types.

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Level of Service

Urban streets and their auxiliary facilities should be designed for Level of Service (LOS) B as defined in the Highway Capacity Manual. Densely developed urban areas may necessitate the use of LOS D. The functional class of urban facility according to the Statewide Planning Map should be used to determine the appropriate LOS. For more information regarding LOS as it relates to facility design, see Service Flow Rate under subheading Traffic Volume in Chapter 2, Section 3.

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Basic Design Features

This subsection includes information on the following basic design features for urban streets:

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Geometric Design Criteria for Urban Streets;

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Medians;

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Median Openings;

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Clear Zones;

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Borders;

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Berms;

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Grade Separations and Interchanges;

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Right-of-Way Width;

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Intersections;

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Speed Change Lanes;

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Horizontal Offsets; and

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Bus Facilities.

Table 3-1 shows tabulated basic geometric design criteria for urban arterial, collector, and local streets. The basic design criteria shown in this table reflect minmum and desirable values applicable to new location, reconstruction or major improvement projects. See Chapter 2 for additional guidance on choosing an appropriate design speed.

Anchor: #CIHBBACGTable 3-1: Geometric Design Criteria for Urban Streets
Item	Functional Class	Desirable	Minimum
Design Speed (mph)	All	Up to 60	30
Horizontal Radius	All	See Table 2-3, Table 2-4, and Table 2-5
Maximum Grade (%)	All	See Table 2-9
Stopping Sight Distance	All	See Table 2-1 and Figure 2-3
Width of Travel Lanes (ft)	Arterial Collector Local	12 12 12	11¹ 11² 11^2,3
Curb Parking Lane Width (ft)	Arterial Collector Local	12 10 9	10⁴ 8⁵ 8⁵
Shoulder Width (ft), Uncurbed Urban Streets^{12, 13}	Arterial Collector Local	10 8 8	4⁶ 3⁶ 2⁶
Width of Speed Change Lanes (ft)	Arterial and Collector Local	11-12⁷ 10-12⁷	10 9
Offset to Face of Curb (ft)	All	2	1
Median Width	All	See Medians
Border Width (ft)	Arterial and Collector Local	20	15
Border Width (ft)	Arterial and Collector Local	10⁸
Right-of-Way Width	All	Variable ⁹
Clear Sidewalk Width (ft)¹¹	All	6-8¹⁰	5
On-Street Bicycle Lane Width	All	See Chapter 6, Bicycle Facilities
Superelevation	All	See Chapter 2, Superelevation Rate, Superelevation Transition Length, Superelevation Transition Placement,Superelevation Transition Type
Vertical Clearance at New Structures	All	See Table 2-11
Clear Zone Width	All	See Table 2-12
Turning Radii	All	See Chapter 7, Minimum Designs for Truck and Bus Turns
Notes: In highly restricted locations or locations with less than 5% trucks and speeds less than or equal to 40 mph, 10-ft permissible. Engineering judgement should be exercised when determining if 10-ft is acceptable and must be approved by the district. In industrial areas 12-ft usual, and 11-ft minimum for restricted ROW conditions. In non-industrial areas, 10-ft minimum. In residential areas, 9-ft minimum. Where there is no demand for use as a future through lane, 8-ft minimum. In non-commercial and non-industrial areas, 7-ft minimum. Where only minimum width is provided, it should be fully surfaced. Where desirable width is provided, partial (not less than minimum width) surfacing or full width surfacing may be provided at the option of the designer. See AASHTO's A Policy on Geometric Design of Highways and Streets for further discussion on appropriate lane width for different conditions. Where available right-of-way is limited and in areas of high right-of-way costs, an absolute minimum border width of 2-ft may be acceptable where there is no sidewalk and adequate sight distance is provided. Right-of-way width is a function of roadway elements as well as local conditions. Wider than 6-ft is applicable for commercial areas, school routes, or other areas with concentrated pedestrian traffic. Cross slopes, ramps, and sidewalks must be in compliance with the Americans with Disabilities Act Accessibility Guidelines and the Texas Accessibility Standards. See Chapter 7, Section 3, Curb and Curb and Gutters and Sidewalks and Pedestrian Facilities for additional information. 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. Where right turn lanes are present on uncurbed facilities, a 4-ft fully surfaced shoulder must be provided.

For minor rehabilitation projects where no additional lanes are proposed, existing curbed cross sections should be compared with the design criteria in Table 3-1 to determine the practicality and economic feasibility of minor widening to meet the prescribed standards. Where only minimal widening is required to conform with a standard design, it is often cost effective to retain the existing street section, thereby sparing the cost of removing and replacing concrete curb and gutter and curb inlets. For these type projects, Resurfacing, Restoration, and Rehabilitation (3R) guidelines are usually applicable, see Chapter 4.

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Medians

Medians are desirable for urban streets with four or more traffic lanes. The primary functions of medians are to provide the following:

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Storage space for left-turning vehicles;

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Separation of opposing traffic streams; and

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Access control to/from minor access drives and intersections.

Medians used on urban streets include the following types:

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Raised;

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Flush; and

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Two-way left-turn lanes.

Raised Medians. A raised median is used on urban streets where it is desirable to control or restrict mid-block left-turns and crossing maneuvers. Installing a raised median can result in the following benefits:

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Restricting left-turn and crossing maneuvers to specific locations or certain movements;

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Improving traffic safety;

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Increasing throughput capacity and reducing delays; and

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Providing pedestrian refuge areas.

A raised median design should be considered where:

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ADT exceeds 20,000 vehicles per day;

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New development is occurring, and volumes are anticipated to exceed 20,000 vehicles per day; or

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There are operational concerns for mid-block turns.

For these conditions, a raised median may improve safety by separating traffic flows and controlling left-turn and crossing maneuvers. The use of raised medians should be discouraged where the roadway cross-section is too narrow for U-turns.

For median left turn lanes at intersections, a median divider width of 4-ft (measured to face of curb) is recommended to accommodate a single left turn lane. Where a pedestrian refuge is needed, a median divider width of 6-ft is required (measured minimum to face of curb, desirable to back of curb). See Chapter 7, Pedestrian Facilities for additional guidance. To prevent recurring damage to the divider, the divider should be at least 2-ft wide (measured to face of curb). If pedestrians are expected to cross the divider, then the divider should be a minimum of 5-ft wide (normal to direction of pedestrian travel) to accommodate a cut-through landing or refuge area that is at least 5-ft x 6-ft. See Dual Left-Turn Lanes for additional median width discussion.

See Appendix E, Median U-Turn Intersection (MUT) for information on alternate intersection design for roadways with raised medians.

Flush Medians. Flush medians are medians that can be traversed. Although a flush median does not permit left-turn and cross maneuvers, it cannot physically prevent these maneuvers because the median can be easily crossed. Therefore, for urban arterials where access control is desirable, flush medians should not be used.

A flush median design should include the following:

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Delineation from through lanes using double yellow stripes and possibly a contrasting surface texture or color to provide visibility; and

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Flexibility to allow additional left-turn bay storage if necessary.

Two-Way Left-Turn Lanes. Two-way left-turn lanes (TWLTL) are flush medians that may be used for left turns by traffic from either direction on the street. The TWLTL is appropriate where there are operational concerns for mid-block turns, such as areas with (or expected to experience) moderate or intense strip development. Used appropriately, the TWLTL design can improve the safety and operational characteristics of streets as demonstrated through reduced travel times and crash rates.

Recommended median lane widths for the TWLTL design are as shown in Table 3-2. When applying these criteria to new location projects or on reconstruction projects where widening necessitates the removal of exterior curbs, the median lane width should not be less than 12-ft, and preferably the corresponding desirable value shown in Table 3-2. Minimum values shown in Table 3-2 are appropriate for restrictive right-of-way projects and improvement projects where attaining the desirable width would necessitate removal and replacement of exterior curbing to gain a small amount of roadway width.

Anchor: #i1600118Table 3-2: Median Lane Widths for Two-Way Left-Turn Lanes
Design Speed (mph)	Width of TWLTL (ft)
	Desirable¹	Minimum
≤ 40	14	11
45 or 50	14	12
> 50	16	14
Note: Maximum width of TWLTL should not exceed 16-ft to avoid driver confusion

A site can be considered suitable for the use of a TWLTL when an urban street meets the following criteria:

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Future ADT volume of greater than 3,000 vehicles per day for an existing two-lane urban street, 6,000 vehicles per day for an existing four-lane urban street, or 10,000 vehicles per day for an existing six-lane urban street; and

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Side roads plus driveway density of 20 or more entrances per mile.

When the above two conditions are met, the site should be considered suitable for the use of a TWLTL.

All cross sections should be evaluated for pedestrian crossing capabilities. See Chapter 7, Pedestrian Facilities for additional guidance.

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Median Openings

Median openings should only be provided for street intersections or at intervals for major developed areas. Spacing between median openings must be adequate to allow for introduction of left-turn lanes and to prevent false calls at signal detection loops. Directional openings, pictured in Figure 3-1, can be used to limit the number and type of conflicts.

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Figure 3-1. Types of Directional Openings.

The positive offset design shown in Figure 3-2 (c) can improve the sight distance for left-turning vehicles where there are opposing left-turn lanes. This design has also been found to substantially reduce the frequency of left-turn crashes compared to negative (a) or no offset (b) designs and is desirable for use where practical. Further discussion and examples of offset left-turn lanes can be found in AASHTO's Policy on Geometric Design of Highways and Streets.

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Figure 3-2. Examples of Left-Turn Lanes with Negative, Zero, and Positive Offset. Source: AASHTO's A Policy on Geometric Design of Highways and Streets

An important factor in designing median openings is the shape of the median end or median nose. The median end shape can directly alter the effective turning path the design vehicle can make. The shape of a median nose should be designed to accommodate the turning path of the design vehicle. One form of a median end at an opening is a semicircle, which is a simple design that is satisfactory for median widths less than 10-ft wide. One alternate median end design that fits the paths of design vehicles is a bullet nose. The bullet nose is formed by two symmetrical arcs with a small radius to round the nose. Consider the use of local design standards for median noses where available. In the absence of a local standard refer to Chapter 9 of AASHTO's A Policy on Geometric Design of Highways and Streets for more information regarding the design of median openings.

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Clear Zones

Table 2-12 presents the general clear zone guidelines for urban roadways.

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Borders

The border is the area between the roadway and right-of-way line that accommodates sidewalks, provides sight distance, and utility accommodation, and separates traffic from privately owned areas. Every effort should be made to provide wide borders to serve functional needs, reduce traffic nuisances to adjacent development, and for aesthetics. Minimum and desirable border widths are as listed in Table 3-1.

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Berms

There are two different types of berms typically used on urban streets. The first, constructed as a narrow shelf or path, is typically used to provide a flush grade behind a curb to accommodate the possible future installation of sidewalks. The width of the berm should accommodate a buffer between the curb and the sidewalk, the sidewalk, and any needed buffer on the backside of the sidewalk. The second type of berm is constructed as a raised mound to facilitate drainage or for landscaping purposes. A raised mound berm should be placed outside of the clear zone, when practical. If it cannot be placed outside of the clear zone, care should be taken to ensure that the slopes and configurations of the berm meet the clear zone requirements as discussed in Slopes and Ditches in Chapter 2.

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Grade Separations and Interchanges

Although uncommon on urban streets, grade separations and interchanges may sometimes be the only means available for providing sufficient capacity at critical intersections. However, single or multiple grade separations do not usually improve the throughput of a roadway when other intersections are signalized along the street. With the exception of grade separations at railroads, grade separations on urban streets are usually diamond interchanges with four legs. Locations considered include high-volume intersections and where terrain conditions favor separation of grades. See Appendix E and FHWA's Intersection Control Evaluation (ICE) for additional information on optimal geometric configurations and traffic control solutions for interchanges.

Where feasible the width of the entire roadway approach, including parking lanes or shoulders, should be carried across or under the separation. Interchange design elements may have slightly lower dimensional values as compared to freeways due to the lower speeds involved. For example, the length of diamond ramps may be controlled by the minimum distance to overcome the elevation difference at suitable gradients.

In some instances, it may be feasible to provide grade separations or interchanges at all major crossings for a long section of arterial street. The urban street then assumes the operating characteristics and appearance of a freeway. In these instances, it may be appropriate to eliminate the remaining at-grade crossings and control access by design (i.e., provide continuous frontage roads) where right-of-way availability permits. It is not desirable to intermix facility types by providing intermittent sections of fully controlled and non-controlled access facilities.

For additional discussion on grade separations and interchanges, see Grade Separations and Interchanges and Chapter 10 of AASHTO's A Policy on Geometric Design of Highways and Streets.

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Right-of-Way Width

Right-of-Way width is the area necessary to accommodate the various cross-sectional elements, including widths of travel and turning lanes, bicycle lanes, shoulders or parking lanes, median, borders, sidewalks, sidewalk offsets, slopes and provide ramps or connecting roadways where interchanges are involved.

The width of right-of-way for urban streets is influenced by the following factors:

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Traffic volume requirements;

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Land use;

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Availability and cost; and

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Extent of expansion.

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Intersections

The number, design, and spacing of intersections influence an urban street’s capacity, speed, and safety. Capacity analysis of signalized intersections is one of the most important considerations in intersection design. Traffic volumes, operational characteristics, and traffic control measures closely influence the dimensional layout or geometric design considerations of intersections.

Space limitations and lower operating speeds on urban streets reduce the curb or edge of pavement radii for turning movements as compared to rural highway intersections. Smaller radii are also advantageous for pedestrian safety as they may encourage reduced speeds for turning movements. Effective turning radii (i.e. edge line pavement markings) of 15-ft to 25-ft permit passenger cars to negotiate right turns with little to no encroachment on other lanes. Where heavy volumes of trucks or buses are present, increased effective turning radii of 30-ft to 50-ft may expedite turns to and from through lanes. To optimize the actual curb or edge of pavement radii for combination tractor-trailer units and buses, refer to Minimum Designs for Truck and Bus Turns, Chapter 7.

In general, intersection design should be simple and free of complicated channelization to minimize driver confusion. Sight distance is an important consideration even in the design of signalized intersections since, during the low volume hours, flashing operation may be used (see discussion in Intersection Sight Distance, Chapter 2). For information on the design of Alternative Intersections, reference Appendix E.

Figure 3-3 illustrates lines of sight for a vehicle entering an intersection.

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Figure 3-3. Entering Intersection Lines of Sight.

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Speed Change Lanes

Speed Change Lanes are defined as acceleration or deceleration lanes for left or right turns, exit or entrance acceleration or deceleration lanes, or climbing lanes. A design waiver is required for speed change lanes that do not meet minimum length and width criteria.

On urban streets, speed change lanes generally provide space for the deceleration and optional storage of turning vehicles. The length of speed change lanes for turning vehicles consists of the following two components:

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Deceleration length; and

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Storage length.

Left-Turn Deceleration Lanes. Figure 3-4 illustrates the use of left-turn lanes on urban streets. A short symmetrical reverse curve taper or straight taper may be used. For median left-turn lanes at intersections, a median divider width of 4-ft (measured to face of curb) is recommended. Where pedestrian refuge is needed, a median divider width of 6-ft is required (measured minimum to face of curb, desirable to back of curb). If pedestrians are expected to cross the divider, then the divider should be a minimum of 5-ft wide (normal to direction of pedestrian travel) to accommodate a cut-through landing or refuge area that is at least 5-ft x 6-ft. For illustrations of these two cases, see Chapter 7 Pedestrian Facilities for additional guidance.

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Figure 3-4. Left-Turn Lanes on Urban Streets.

Table 3-3 provides recommended taper lengths, deceleration lengths, and storage lengths for left-turn lanes. These guidelines may also be applied to the design of right-turn lanes.

Anchor: #i1628354Table 3-3: Lengths of Single Left-Turn and Right-Turn Lanes on Urban Streets ¹
Design Speed (mph)	Deceleration Length²(ft) Speed Differential³			Taper Length (ft)	Minimum^{6, 7} Storage Length (ft)
Design Speed (mph)	None	5-mph⁴	10-mph⁵	Taper Length (ft)	Minimum^{6, 7} Storage Length (ft)
30	150	105	70	50	100
35	205	150	105	50	100
40	265	205	150	50	100
45	340	265	205	100	100
50	415	340	265	100	100
55	505	415	340	100	100
60	600	505	415	100	100
Notes: The minimum length of a left-turn lane is the sum of the deceleration length plus queue storage. In order to determine the design length, the deceleration plus storage length must be calculated for peak and off-peak periods, the longest of these two lengths will be the minimum design length. Based on 6.5 ft/s2 deceleration to stopped condition throughout the entire length. Larger deceleration rates may be used when deceleration lengths based on 6.5 ft/s2 are impractical. Speed differential = the difference between the assumed speed of a turning vehicle at the moment when it arrives at the taper and the design speed of the roadway. Based on 6.5 ft/s2 deceleration from 5 mph less than design speed to stopped condition throughout the entire length. Based on 6.5 ft/s2 deceleration from 10 mph less than design speed to stopped condition throughout the entire length. See Storage Length Calculations discussion. For right-turn lanes the minimum queue storage is 30-ft. The minimum storage length applies when: (1) the required queue storage length calculated is less than the minimum length, or (2) there is no rational method for estimating the left-turn volume. A design waiver will be required only if the 100 ft minimum storage length cannot be provided.

Deceleration Length. Deceleration length, with no speed differential, as shown in Table 3-3 assumes that deceleration starts at the beginning of the taper and continues to a stopped condition. Where providing this deceleration length is impractical, it may be acceptable to assume that turning vehicles will begin decelerating prior to arriving at the taper and clearing the through traffic lane. Using this assumption, see Table 3-3 for 5 mph and 10 mph speed differential deceleration lengths.

Storage Length Calculations. The required storage may be obtained using an acceptable traffic model such as the latest version of the Highway Capacity Manual software (HCS), SYNCHRO, or VISSIM or other acceptable simulation models. Where such model results have not been applied, the following formulas may be used:

Signalized:

Where:

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L = storage length, ft

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V = left-turn volume per hour, vph Consider multiple turn lanes when V >150.

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N = number of cycles per hour Recommend between 20 and 25 cycles per hour for peak period operations if unknown.

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2 = a factor that provides for storage of all left-turning vehicles on most cycles A value of 1.8 may be acceptable on collector streets.

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S = queue storage length, in feet, per vehicle

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% trucks	S (ft)
<5	25
5-9	30
10-14	35
15-19	40

Unsignalized:

Where:

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L = storage length in feet

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V = left-turn volume per hour, vph

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2 = a factor that provides for storage of all left-turning vehicles on most cycles A value of 1.8 may be acceptable on collector streets.

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S = queue storage length, in feet, per vehicle

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% trucks	S (ft)
<5	25
5-9	30
10-14	35
15-19	40

Dual Left-Turn Deceleration Lanes. For major signalized intersections where high peak hour left-turn volumes exceeding 150 vehicles per hour are expected, dual left-turn lanes should be considered. As with single left-turn lanes, dual left-turn lanes should include lengths for deceleration, storage, and taper. Table 3-4 provides recommended lengths for dual left-turn lanes.

Anchor: #i1624976Table 3-4: Lengths of Dual Left-Turn and Right-Turn Lanes on Urban Streets ¹
Design Speed (mph)	Deceleration Lengths²(ft)	Taper Length (ft)	Minimum^{3, 4} Storage Length (ft)
30	150	100	100
35	205	100	100
40	265	100	100
45	340	150	100
50	415	150	100
55	505	150	100
60	600	150	100
Notes: The minimum length of a left-turn lane is the sum of the deceleration length plus queue storage. In order to determine the design length, the deceleration plus storage length must be calculated for peak and off-peak periods, the longest total length will be the minimum design length. Based on 6.5 ft/s² deceleration to stopped condition throughout the entire length. Larger deceleration rates may be used when deceleration lengths based on 6.5 ft./s² are impractical. See Storage Length Calculations discussion. The minimum storage length shall apply when: (1) the required queue storage length calculated is less than the minimum length, or (2) there is no rational method for estimating the left-turn volume. A design waiver will be required only if the 100 ft minimum storage length cannot be provided.

Right-Turn Deceleration Lanes. Figure 3-5 illustrates a right-turn deceleration lane. The length of a single right-turn deceleration lane is the same as that for a single left-turn lane (Table 3-3). However, the minimum queue storage is 30 ft for right-turn lanes. The length for a dual right-turn lane is the same for a dual left-turn lane (Table 3-4). Refer to the TxDOT Access Management Manual for guidelines as to when to consider a right-turn deceleration lane.

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Figure 3-5. Lengths of Right-Turn Deceleration Lanes.

Right-Turn and U-Turn Acceleration Lanes. Acceleration lanes are generally not used on urban streets (See Figure 3-6(A)). See Table 3-13 for acceleration distances and taper lengths if an acceleration lane is necessary. Right turn lanes into acceleration lanes that end must be yield controlled. This also applies to U-turn lanes at grade separated interchanges (See Figure 3-6(B)).

Free Right-Turn Lanes. When an acceleration lane does not end, it can be considered a free right turn lane where stop or yield control may not be provided (See Figure 3-6(C)). It is recommended not to have down-stream driveways where free right turn lanes are provided. Refer to the TxDOT Access Management Manual for driveway spacing requirements.

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Figure 3-6. Types of Right Turn Treatments at Intersections.

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Drainage Horizontal Offsets

For low-speed streets, cross drainage culvert ends should be offset at least 4-ft from the back of the curb or 4-ft from the outside edge of the shoulder. The designer should make the best use of the available border width to obtain wide clearances. Sloped open ends may be used effectively as safety treatments for small culverts. Consideration should be given to future sidewalk needs.

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Bus Facilities

Urban areas benefit from the effective bus utilization of downtown and radial arterial streets and the effective coordination of transit and traffic improvements. To maintain and increase bus patronage, bus priority treatments on arterial streets may be used to underscore the importance of transit use. Possible bus priority treatments on non-controlled access facilities include measures designed to separate car and bus movements and general traffic engineering improvements designed to expedite overall traffic flow.

This subsection includes the following topics:

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Bus lanes; and

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Bus streets.

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Bus Lanes

Bus lanes are usually used exclusively by buses; however, in some instances carpools, taxis, or turning vehicles may share the lane. Bus lanes may be located along curbs or in medians and may operate with, or counter to, automobile flow. For more information on bus lanes, refer to Bus Lanes/Bus Rapid Transit Systems on Highways: Review of the Literature, California Partners for Advanced Transit and Highways, Berkeley (Miller, 2009).

Curb Bus Lanes (Normal Flow). Curb bus lanes in the normal flow direction generally convert existing lanes into bus-only lanes during peak periods. They are often implemented in conjunction with removal of curb parking so that there is little adverse effect on existing street capacity. It can be challenging to enforce the bus-only lanes during those hours and therefore may produce only marginal benefits to bus flow. In addition, this type of bus lane causes conflict between right-turning vehicles and buses.

Median Bus Lanes. Median bus lanes are in effect for the duration of the day. Wide medians are required to provide refuge for bus patrons, and passengers are required to cross active streets to reach bus stops. Additionally, left-turn traffic must be prohibited or controlled to minimize interference between transportation modes.

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Bus Streets

Reserving entire streets for the exclusive use of buses represents a major commitment to transit and generally is not feasible due to adverse effects on abutting properties and businesses, including parking garages or lots, drive-in banks, etc.