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Section 5: Multi-Lane Rural Highways

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Overview

The term “multi-lane rural highway” as used in this chapter, refers to roadways in un-developed areas that have more two or more lanes of traffic in each direction. Access to these facilities is controlled through driveway locations and intersecting roadways. Multi-lane rural highways can be undivided or divided with a depressed median or a surface mounted median.

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Basic Design Features

This subsection includes guidelines on geometric features for multilane rural highways. The guidelines are outlined in Table 3-11, Figure 3-10 and Figure 3-11. These guidelines apply for all functional classes of roadways.

Table 3-11 shows minimum design speed for rural multilane highways. See Chapter 2 for definition of level and rolling terrain as well as guidance on choosing an appropriate design speed.

Anchor: #CHDCADEETable 3-11: Design Criteria for Multilane Rural Highways (Non-controlled Access) (All Functional Classes)

Type of Facility

Six-Lane Divided

Four-Lane Divided

Four-Lane Undivided

Design Speed (Arterials) (mph)

Min.

Min.

Min.

Level

701

701

701

Rolling

602

602

602

Design Speed (Collector) (mph)

Min.

Min.

Min.

Level

60

60

60

Rolling

50

50

50

Lane Width (ft)6

12

12

12

-

Des.

Min.

Des.

Min.

Des.

Min

Median Width (ft)

Surfaced

16

4

16

4

Not Applicable

-

Depressed

76

48

76

48

-

Shoulder Outside (ft)4,5

10

8

10

8

10

83

Shoulder Inside (ft) for Depressed Medians5

10

4

4

4

Not applicable

Vertical Clearance, New Structures

Table 2-11

Minimum Horizontal Radius

Table 2-4 and Table 2-5

Stopping Sight Distance

Table 2-1

Maximum Grade (%)

Table 2-9

Clear Zone

Table 2-12

Superelevation

See Chapter 2, Superelevation Rate, Superelevation Transition Length, Superelevation Transition Placement, Superelevation Transition Type

Turning Radii

See Chapter 7, Minimum Designs for Truck and Bus Turns

Notes:

  1. 60 mph acceptable when conditions warrant and are documented through a design exception.
  2. 50 mph acceptable when conditions warrant and are documented through a design exception.
  3. On four-lane undivided collector highways, outside surfaced shoulder width may be decreased to 4-ft where flat (1V:10H), sodded front slopes are provided for a minimum distance of 4-ft from the shoulder edge. Arterials must have a fully surfaced shoulder.
  4. 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.
  5. Where left or right turn lanes are present on uncurbed facilities, a 4-ft fully surfaced shoulder must be provided.
  6. See Ch. 3 Section 3 for TWLTL criteria.


Cross Sections for Arterial and Collector
Multi-Lane Undivided Rural Highways. (click in image to see full-size image) Anchor: #i1023085grtop

Figure 3-10. Cross Sections for Arterial and Collector Multi-Lane Undivided Rural Highways.

Cross Sections for Multi-Lane Rural Highways. (click in image to see full-size image) Anchor: #i1023087grtop

Figure 3-11. Cross Sections for Multi-Lane Rural Highways.

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Level of Service

Rural arterials and their auxiliary facilities should be desirably designed for Level of Service B in the design year as defined in the Highway Capacity Manual.

Generally, undivided four-lane roadways have been associated with higher crash rates than divided roadways. This higher crash rate has frequently been attributed to the lack of protection for left-turning vehicles. Therefore, if an undivided facility is selected for a location, the impact of left-turning vehicles should be examined.

For more information regarding level of service as it relates to facility design, see Service Flow Rate in the sub section titled Traffic Volume of Chapter 2, Section 3.

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Basic Design Features

This subsection includes information on the following basic design features for multi-lane rural highways:

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Access Control

The installation of all access driveways along multilane facilities from adjacent property connecting to the mainlanes should be in accordance with the TxDOT Access Management Manual.

For multilane highways constructed in developed or developing areas, it may be desirable to control access to the mainlanes through right-of-way acquisition or by design (i.e., provision of frontage roads). Designed access control may be provided solely in the interchange areas or continuously throughout a section of highway, depending on traffic volumes, the degree of roadside development, availability of right-of-way, and economic conditions.

All frontage road development must be in accordance with the rules contained in 43 Texas Administrative Code (TAC) §15.54. The Project Development Process Manual can also be referenced for additional information.

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Medians

The width of the median in a multi-lane rural highway is the distance between the inside edges of the opposing travel lanes. If practical, wide medians (approximately 76-ft) should be used to provide sufficient storage space for tractor-trailer combination vehicles at median openings, reduce headlight glare, provide a pleasing appearance, reduce the chances of head-on collisions and provide a sheltered storage area for crossing vehicles, including tractor-trailer combinations. Wide medians should generally be used whenever feasible but median widths greater than 60-ft have been found to be undesirable for intersections that are signalized or may be signalized in the design life of the project.

In areas that are likely to become suburban or urban in nature, medians wider than 60-ft should be avoided at intersections except where necessary to accommodate turning and crossing maneuvers by larger vehicles. Wide medians may be a disadvantage when signalization is required at future intersections. The increased time for vehicles to cross the median can lead to inefficient signal operation.

Four-Lane Undivided Highways. Conversion of a two-lane highway to a four-lane highway facility should include a median when possible. If an existing two-lane highway has rolling terrain or restricted right-of-way conditions, conversion to a four-lane undivided highway may be considered to improve passing opportunities and traffic operations. Table 3-11 and Figure 3-10 include the general geometric features for four-lane undivided highways. In cases where a median is being proposed and the existing roadbed will remain in place, Non-Freeway Rehabilitation (3R) alignment criteria may be applied to the existing roadbed as described in Chapter 4. However, 4R criteria must be applied to the new roadbed.

Surfaced Medians. Surfaced median designs are most appropriate in areas with roadside development. Surfaced medians of 4-ft to 16-ft are classified as narrow medians and are used in restricted conditions. Medians 4-ft wide provide little separation of opposing traffic and a minimal refuge area for pedestrians. Surfaced medians of 14-ft to 16-ft offer space for use by exiting traffic turning left, but do not offer protection for crossing vehicles.

Median Openings. Closely spaced median openings on divided highways can cause interference between high-speed through-traffic and turning vehicles. The frequency of median openings varies with topographic restrictions and local requirements As a general rule, the minimum spacing should be one-quarter mile or greater in rural areas. It is typical to provide median openings at all public roads and at major traffic generators such as industrial sites or shopping centers. Additional openings should be provided to maintain a maximum one-half mile spacing.

As shown in Figure 3-12, left-turn lanes should be provided at all median openings and right-turn deceleration and acceleration lanes should be considered at intersections with highways or other major public roads with significant turning movements. See the Access Management Manual, “Auxiliary Lanes” section and related table for additional considerations and warranting thresholds for right-turn deceleration and acceleration lanes as well as left-turn deceleration lanes at median openings.

For divided highways with independent main lane alignment, particular care should be exercised at median openings to provide a satisfactory profile along the crossover with flat approaches to the main lanes.

Multi-Lane Rural Highway Intersection. (click in image to see full-size image) Anchor: #i1023137grtop

Figure 3-12. Multi-Lane Rural Highway Intersection.

Median openings should be at least 40-ft wide or the width of the crossroad pavement width plus 8-ft. Design vehicles are often used as the basis for minimum design of median openings, particularly for multilane cross roads and skewed intersections. See Chapter 7, Section 7, Minimum Designs for Truck and Bus Turns for additional information.

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Turn Lanes

Turn lanes, or speed change lanes, should generally be provided wherever vehicles must slow to leave a facility or accelerate to merge onto a facility.

Median Turn Lane (Left-Turn Lane). Median turn lanes with 4-ft adjacent shoulders provide deceleration and storage area for vehicles making left turns to leave a divided highway. Storage, taper, and deceleration lengths for design are illustrated in Figure 3-13 and summarized in Table 3-12. Taper lengths shorter than those in Table 3-12 may be acceptable on some low volume rural highways. Also, adjustments for grade are given in Table 3-14

Left Turn Lanes on Multilane
Rural Highways. (click in image to see full-size image) Anchor: #RTPBTHWSgrtop

Figure 3-13. Left Turn Lanes on Multilane Rural Highways.

Anchor: #i1662250Table 3-12: Lengths of Median Turn Lanes Multilane Rural Highways

Mainlane Design

Speed (mph)

Taper Length

(ft)1

Deceleration

Length (ft)2

30

50

150

35

50

205

40

50

265

45

100

340

50

100

415

55

100

505

60

150

600

65

150

700

70

150

815

75

150

935

80

150

1,060

Notes:

  1. For low volume median openings, such as those serving private drives or U-turns, a taper length of 100-ft may be used regardless of mainlane design speed.
  2. 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.


Storage Length Calculations. For storage length calculations on multi-lane rural highways, the storage length calculations in Urban Streets apply.

Right Turn Lane. Right turn lanes (12-ft lane with 4-ft adjacent shoulders) provide deceleration or acceleration areas for right-turning vehicles. The deceleration length and taper lengths for right turn lanes are the same as for Median Turn lanes (see Table 3-12). Adjustment factors for grade effects are shown in Table 3-14.

Acceleration Lanes. Acceleration lanes for right-turning or left-turning vehicles may be desirable for vehicles entering on multi-lane rural highways. Examples of both tapered and parallel accelerations lanes are shown in Figure 3-14. Recommended acceleration lengths are shown in Table 3-13. Adjustments for grade are given in Table 3-14.

Examples of Tapered and Parallel Acceleration
Lanes. (click in image to see full-size image) Anchor: #i1024981grtop

Figure 3-14. Examples of Tapered and Parallel Acceleration Lanes.

Notes:

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  1. La is the recommended acceleration length as shown in Table 3-13 or as adjusted by Table 3-14.
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  3. Point A is the feature that controls speed on the acceleration lane. La should not start back on the curvature of the ramp unless the radius equals 1,000-ft or more.
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  5. Lg is the recommended gap acceptance length. Lg should be a minimum of 300 to 500-ft depending on nose width. (Nose width 2'-10')
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  7. The value of La or Lg, whichever produces the greater distance downstream from where the nose equals 2-ft, is suggested for use in the design of the acceleration lane distance.

Anchor: #i1758998Table 3-13: Minimum Acceleration Lane Lengths for Entrance Terminals with Flat Grades of Less Than 3%

Design Speed of Controlling Feature on Ramp (mph)

Highway Design Speed (mph)

Stop Condition

15

20

25

30

35

40

45

50

30

180

140

-

-

-

-

-

-

-

35

280

220

160

-

-

-

-

-

-

40

360

300

270

210

120

-

-

-

-

45

560

490

440

380

280

160

-

-

-

50

720

660

610

550

450

350

130

-

-

55

960

900

810

780

670

550

320

150

-

60

1,200

1,140

1,100

1,020

910

800

550

420

180

65

1,410

1,350

1,310

1,220

1,120

1,000

770

600

370

70

1,620

1,560

1,520

1,420

1,350

1,230

1,000

820

580

75

1,790

1,730

1,630

1,580

1,510

1,420

1,160

1,040

780

80

2,000

1,900

1,800

1,750

1,680

1,600

1,340

1,240

980



Anchor: #CHDBAAFBTable 3-14: Speed Change Lane Adjustment Factors as a Function of a Grade

(US Customary)

Deceleration Lanes

Design Speed of Roadway (mph)

Ratio of Length on Grade to Length on Level1

3 to 4 % Upgrade

3 to 4 % Downgrade

5 to 6% Upgrade

5 to 6% Downgrade

All

0.9

1.2

0.8

1.35

Acceleration Lanes

Design Speed of Roadway (mph)

Ratio of Length on Grade to Length on Level1 for Design Speed (mph) of Turning Roadway Curve

20

25

30

35

40

45

50

All Speeds

3 to 4 % Upgrade

3 to 4% Downgrade

40

1.3

1.3

1.3

1.3

----

----

----

0.7

45

1.3

1.3

1.35

1.35

----

----

----

0.675

50

1.3

1.35

1.4

1.4

1.4

----

----

0.65

55

1.35

1.4

1.45

1.45

1.45

1.45

----

0.625

60

1.4

1.45

1.5

1.5

1.5

1.55

1.6

0.6

65

1.45

1.5

1.55

1.55

1.6

1.65

1.7

0.6

70

1.5

1.55

1.6

1.65

1.7

1.75

1.8

0.6

75

1.6

1.65

1.7

1.75

1.8

1.9

2.0

0.6

80

1.7

1.75

1.8

1.9

2.0

2.05

2.1

0.6

 

5 to 6% Upgrade

5 to 6% Downgrade

40

1.5

1.5

1.5

1.6

----

----

----

0.6

45

1.5

1.55

1.6

1.6

----

----

----

0.575

50

1.5

1.6

1.7

1.8

1.9

2.0

----

0.55

55

1.6

1.7

1.8

1.9

2.05

2.1

----

0.525

60

1.7

1.8

1.9

2.05

2.2

2.4

2.5

0.5

65

1.85

1.95

2.05

2.2

2.4

2.6

2.75

0.5

70

2.0

2.1

2.2

2.4

2.6

2.8

3.0

0.5

75

2.15

2.25

2.35

2.58

2.8

3.03

3.25

0.5

80

2.3

2.4

2.5

2.75

3.0

3.25

3.5

0.5

Note:

  1. Ratio in this table multiplied by length of deceleration or acceleration distances in Table 3-3 and Table 3-13, gives length of deceleration/acceleration distance on grade.


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Travel Lanes and Shoulders

Travel Lanes. Travel lanes should be provided with widths as shown in Table 3-11. The Highway Capacity Manual should be consulted to determine the number of lanes to be used in the design.

Shoulders. Shoulders should be provided with widths as shown in Table 3-11.

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Intersections

In the design of intersections, careful consideration should be given to the appearance of the intersection from the driver’s perspective. Design should be kept simple to avoid driver confusion. In addition, adequate sight distance should be provided, especially in maneuver or conflict areas. See Stopping Sight Distance and Intersection Sight Distance in Chapter 2, Section 4 for further information regarding sight distance. For guidance on Alternative Intersections and Interchanges, see Appendix E.

Right angle crossings are preferred to skewed crossings. Alignment modifications are generally necessary where skew angles exceed 75 or 105 degrees. Turn Lanes may be provided in accordance with previous discussions in this manual.

Chapter 7, Section 7 Minimum Designs for Truck and Bus Turns provides information regarding the accommodation of various types of truck class vehicles in intersection design. AASHTO’s A Policy on Geometric Design of Highways and Streets should be consulted for further information on intersection design. Additional information can be found in this manual regarding Sight Distance in Chapter 2, Section 4. Intersections with narrow, depressed median sections, may require superelevation across the entire cross section to provide for safer operation at median openings.

Intersections formed due to route relocation should be designed so as not to mislead drivers. Treatment of an old-new route connection is illustrated in Figure 3-15.

Treatment of Old-New Route Connection at
Point Where Relocation Begins. (click in image to see full-size image) Anchor: #i1031624grtop

Figure 3-15. Treatment of Old-New Route Connection at Point Where Relocation Begins.

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Transitions to Four-Lane Divided Highways

Typical transitions from a two-lane to a four-lane divided highway are shown in Figure 3-16. Transition geometric design criteria is based on the highest design speed of the two roadways. The transition should be visible to the driver approaching from either direction and median openings should not be permitted within one-quarter mile of the transition area. Transition areas should be located so that obstructions such as restrictive width bridges or underpasses or other fixed objects are not within the no-passing zone of the two-lane highway approach.

Typical Transitions from Two-Lane to Four-Lane
Divided Highways. (click in image to see full-size image) Anchor: #i1031638grtop

Figure 3-16. Typical Transitions from Two-Lane to Four-Lane Divided Highways.

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Converting Existing Two-Lane Roadways to Four-Lane Divided Facilities

When converting an existing two-lane roadway to a four-way divided facility, the Federal Highway Administration (FHWA) allows existing alignments to remain in place. Specifically, the new roadbed will be constructed to full current standards. When the existing lanes are converted to one-way operations, no changes are required to the horizontal or vertical alignment of the existing road as long as it meets 3R criteria (see Chapter 4). Other features such as signing, roadside hardware, and safety end treatments, should meet current standards.

Existing structures with substandard widths on the existing lanes may remain if that width meets minimum rehabilitation (3R) requirements for multi-lane facilities.

Before converting the existing two-lane roadway, a crash analysis of should be conducted to identify specific areas with high crash frequencies so that corrective measures can be taken where appropriate.

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Grade Separations and Interchanges

Grade separations or interchanges on multilane rural highways may be provided at high-volume or high-crash rate highway or railroad crossings.

For additional discussion on grade separations and interchanges, see Grade Separations and Interchanges, see Section 6 Freeways and Chapter 10 of AASHTO’s A Policy on Geometric Design of Highways and Streets.

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