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Section 5: Multi-Lane Rural Highways

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Overview

This section includes guidelines on geometric features for multilane rural highways. The guidelines are outlined in Table 3-12, and Figure 3-7 and Figure 3-8. The guidelines apply for all functional classes of roadways.

Anchor: #CHDCADEETable 3-12: Design Criteria For Multilane Rural Highways (Non-controlled Access) (All Functional Classes)

(US Customary)

Type of Facility

Six-Lane Divided

Four-Lane Divided

Four-Lane Undivided1

Design Speed (Arterials)2 (mph)

Min.

Min.

Min.

Flat

703

703

703

Rolling

604

604

604

Lane Width (ft)

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)

10

85

10

85

10

85

Shoulder Inside (ft) for Depressed Medians

10

4

4

4

Not applicable

Min. Structure Widths for Bridges to Remain in place (ft)

Depressed Median

--

42

--

30

--

56

1 Undivided section may be used on two-lane highways to improve passing opportunities. Most appropriate for use in rolling terrain and/or restricted right of way conditions.

2 For multilane collectors, minimum design speed values are 10 mph less than tabulated.

3 60 mph acceptable for heavy betterment under unusual circumstances. Otherwise, 70 mph should be minimum.

4 50 mph acceptable for heavy betterment under unusual circumstances. Otherwise, 60 mph should be minimum for rural design.

5 Applies to collector roads only. On four-lane undivided 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.



Anchor: #i1064733Table 3-12: Design Criteria For Multilane Rural Highways (Non-controlled Access) (All Functional Classes)

(Metric)

Type of Facility

-

Six-Lane Divided

Four-Lane Divided

Four-Lane Undivided1

Design Speed (Arterials)2 (km/h)

Min.

Min.

Min.

Flat

1103

1103

1103

Rolling

1004

1004

1004

Lane Width (m)

3.6

-

Des.

Min.

Des.

Min.

Des.

Min.

-

Median Width (m)

Surfaced

4.8

1.2

4.8

1.2

Not applicable

-

Depressed

-

22.8

14.4

22.8

14.4

-

Shoulder Outside (m)

3.0

2.45

3.0

2.45

3.0

2.45

Shoulder Inside (m) for Depressed Medians

3.0

1.2

1.2

1.2

Not applicable

Min. Structure Widths for Bridges to Remain in place (m)

Depressed Median

--

12.6

--

9.0

--

16.8

1 Undivided section may be used on two-lane highways to improve passing opportunities. Most appropriate for use in rolling terrain and/or restricted right of way conditions.

2 For multilane collectors, minimum design speed values are 20 km/h less than tabulated.

3 100 km/h acceptable for heavy betterment under unusual circumstances. Otherwise, 110 km/h should be minimum.

4 80 km/h acceptable for heavy betterment under unusual circumstances. Otherwise, 100 km/h should be minimum for rural design.

5 Applies to collector roads only. On four-lane undivided highways, outside surfaced shoulder width may be decreased to 1.2 m where flat (1V:10H), sodded front slopes are provided for a minimum distance of 1.2 m from the shoulder edge.



(US). Cross Sections For Arterial and Collector Multi-Lane Undivided Rural Highways. (click in image to see full-size image) Anchor: #i1023085

Figure 3-8. (US). Cross Sections For Arterial and Collector Multi-Lane Undivided Rural Highways.

NOTE: Online users can click here to see this illustration in PDF format. (US Customary)

(M) Cross Sections For Arterial and Collector Multi-Lane Undivided Rural Highways. (click in image to see full-size image)

Figure 3-9. (M) Cross Sections For Arterial and Collector Multi-Lane Undivided Rural Highways.

NOTE: Online users can click here to see this illustration in PDF format. (Metric)

(US). Cross Sections For Multi-Lane Rural Highways. (click in image to see full-size image) Anchor: #i1023087

Figure 3-10. (US). Cross Sections For Multi-Lane Rural Highways.

NOTE: Online users can click here to see this illustration in PDF format. (US Customary)

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

Figure 3-11. (M) Cross Sections For Multi-Lane Rural Highways.

NOTE: Online users can click here to see this illustration in PDF format. (Metric)

References to other applicable criteria are as follows:

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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.

Undivided four-lane roadways have generally been associated with higher accident rates than divided roadways. This higher accident 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.

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Basic Design Criteria

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 main lanes should be in accordance with the TxDOT Access Management Manual.

For multilane highways constructed in developed (or expected to be developed) areas, such as by-passes in close proximity to urban areas, it may be desirable to control access to the main lanes by either purchasing access rights as part of the right-of-way acquisition or by design (i.e., provision of frontage roads). Where desired, control of access by design may be provided either 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, economic conditions, etc.

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

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Medians

The width of the median is the distance between the inside edges of the travel lanes. Insofar as practical, wide (desirably 76 ft [22.8 m]) medians should be used to provide sufficient storage space for tractor-trailer combination vehicles at median openings, reduce headlight glare, provide a pleasing appearance, and reduce the chances of head-on collisions. However, in areas that are likely to become suburban or urban in nature, medians wider than 60 ft [18 m] 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 intersections. The increased time for vehicles to cross the median can lead to inefficient signal operation.

Four-Lane Undivided Highways. Improvement of an existing two-lane highway to a four-lane highway facility preferably should include a median. Undivided highways may be constructed as betterment projects for existing two-lane highways to improve passing opportunities and traffic operations. Undivided highways are sometimes provided in rolling terrain, or where restricted right-of-way conditions and moderate traffic volumes dictate. Table 3-12: Design Criteria For Multilane Rural Highways (Non-controlled Access) (All Functional Classes) and Figure 3-7 include the general geometric features for four-lane undivided highways.

Surfaced Medians. Surfaced medians of 4 ft to 16 ft [1.2 m to 4.8 m] are classified as narrow medians and are used in restricted conditions. Medians 4 ft [1.2 m] wide provide little separation of opposing traffic and a minimal refuge area for pedestrians. Surfaced medians of 14 ft to 16 ft [4.2 m to 4.8 m] offer space for use by exiting traffic turning left, but do not offer protection for crossing vehicles. Surfaced median designs are most appropriate in areas with roadside development.

Wide Medians. Medians 76 ft [22.8 m] wide significantly reduce headlight glare, are pleasing in 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 [18 m] have been found to be undesirable for intersections that are signalized or may be signalized in the design life of the project.

Median Openings. Median openings at close intervals 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; however, as a general rule the minimum spacing should not be less than one-quarter mile [400 m] in rural areas. Spacing often is selected to provide openings at all public roads and at major traffic generators such as industrial sites or shopping centers. Additional openings should be provided so as not to surpass a maximum one-half mile [800 m] spacing.

Left-turn lanes should be provided at all median openings. At intersections with highways or other major public roads, turn lanes for right-turning vehicles entering and exiting the highway are usually provided, as shown in Figure 3-9. 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, platform approaches to the main lanes.

Multi-Lane Rural Highway Intersection. (click in image to see full-size image) Anchor: #i1023137

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

NOTE: Online users can click here to see this illustration in PDF format.

Median opening width should in no case be less than 40 ft [12 m] nor less than crossroad pavement width plus 8 ft [2.4 m]. Turning templates for a selected control radius and design vehicle are often used as the basis for minimum design of median openings, particularly for multilane crossroads and skewed intersections. See 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 provide deceleration and storage area for vehicles making left turns to leave a divided highway. Storage, taper, and deceleration lengths for design are summarized in Table 3-13. Turn lanes shorter than the lengths given in Table 3-13 may be acceptable on some low volume rural highways. Also adjustments for grade are given in Table 3-14.

Anchor: #i1064854Table 3-13: Lengths of Median Turn Lanes Multilane Rural Highways

(US Customary)

Mainlane Design

Speed (mph)

Taper Length