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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 3-6 and Figure 3-7. 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 US Customary or Metric to
see a PDF of the image. (click in image to see full-size image) Anchor: #i1023085grtop

Figure 3-7. (US). Cross Sections For Arterial and Collector Multi-Lane Undivided Rural Highways. click US Customary or Metric to see a PDF of the image.

(US). Cross Sections For Multi-Lane Rural
Highways. Click US Customary or Metric to
see a PDF of the image. (click in image to see full-size image) Anchor: #i1023087grtop

Figure 3-8. (US). Cross Sections For Multi-Lane Rural Highways. Click US Customary or Metric to see a PDF of the image.

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 here to
see a PDF of the image. (click in image to see full-size image) Anchor: #i1023137grtop

Figure 3-9. Multi-Lane Rural Highway Intersection. Click here to see a PDF of the image.

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

(ft)1

Deceleration

Length (ft)2

-

Design Turning

ADT (vpd)

Minimum Storage

Length (ft)

30

50

160

-

150

50

35

50

215

-

300

100

40

50

275

-

500

175

45

100

345

-

750

250

50

100

425

-

--

--

55

100

510

-

--

--

60

150

615

-

--

--

65

150

715

-

--

--

70

150

830

-

--

--

75

150

950

-

--

--

80

150

1075

-

--

--

(Metric)

Mainlane Design

Speed (km/h)

Taper Length

(m)1

Deceleration

Length (m)2

-

Design Turning

ADT (vpd)

Minimum Storage

Length (m)

50

15

50

-

150

15

60

15

65

-

300

30

70

30

85

-

500

50

80

30

105

-

750

75

90

30

130

-

-

-

100

45

200

-

-

-

110

45

240

-

-

-

120

45

290

-

-

-

130

45

330

-

-

-

1 For low volume median openings, such as those serving private drives or U-turns, a taper length of 100 ft [30 m] may be used regardless of mainlane design speed.

2 Deceleration length assumes that moderate deceleration will occur in the through traffic lane and the vehicle entering the left-turn lane will clear the through traffic lane at a speed of 10 mph (15 km/h) slower than through traffic. Where providing this deceleration length is impractical, it may be acceptable to allow turning vehicles to decelerate more than 10 mph (15km/h) before clearing the through traffic lane.



Right Turn Deceleration Lane. Right (12 ft [3.6 m] lane with 4 ft [1.2 m] adjacent shoulders) turn lanes provide deceleration or acceleration area for right-turning vehicles. The deceleration length and taper lengths for right turn lanes are the same as for left-turn lanes (See Table 3-13). Adjustment factors for grade effects are shown in Table 3-14.

Acceleration Lanes. Acceleration lanes for right-turning and/or left-turning vehicles may be desirable on multi-lane rural highways. Acceleration distances and taper lengths are provided in Figure 3-10. Adjustments for grade are given in Table 3-14.

(US). Lengths of Right-Turn Acceleration
Lanes. Click US Customary or Metric to see
a PDF of the image. (click in image to see full-size image) Anchor: #i1024981grtop

Figure 3-10. (US). Lengths of Right-Turn Acceleration Lanes. Click US Customary or Metric to see a PDF of the image.

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

(US Customary)

Deceleration Lanes

-

Ratio of Length on Grade to Length on Level*

Design Speed of Roadway (mph)

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

-

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

Design Speed of Roadway (mph)

20

25

30

35

40

45

50

All Speeds

-

3 to 4 % Upgrade

3 to 4% Downgrade

30

----

----

----

----

----

----

----

----

35

----

----

----

----

----

----

----

0.7

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

1.51.55

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

1.55

1.6

1.65

1.7

1.75

1.8

1.9

0.6

80

1.6

1.65

1.7

1.75

1.8

1.9

2.0

0.6

-

5 to 6% Upgrade

5 to 6% Downgrade

30

----

----

----

----

----

----

----

----

35

----

----

----

----

----

----

----

0.6

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

2.2

2.3

2.4

2.65

2.9

3.2

3.5

0.5

80

2.4

2.5

2.6

2.9

2.9

3.6

4.0

0.5

*Ratio in this table multiplied by length of deceleration or acceleration distances in Table 3-13 and Figure 3-10, gives length of deceleration/acceleration distance on grade.



Anchor: #i1065367Table 3-14: Speed Change Lane Adjustment Factors as a Function of a Grade

(Metric)

Deceleration Lanes

-

Ration of Length on Grade to Length on Level*

Design Speed of Roadway (mph)

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

-

Ratio of Length on Grade to Length for Design Speed (km/h of Turning Roadway Curve)*

Design Speed of Roadway (km/h)

40

50

60

70

80

All Speeds

-

3 to 4 % Upgrade

3 to 4% Downgrade

50

----

----

----

----

----

----

60

1.3

1.4

1.4

----

----

0.7

70

1.3

1.4

1.4

1.5

----

0.65

80

1.4

1.5

1.5

1.5

1.6

0.65

90

1.4

1.5

1.5

1.5

1.6

0.6

100

1.5

1.6

1.7

1.7

1.8

0.6

110

1.5

1.6

1.7

1.7

1.8

0.6

120

1.5

1.6

1.7

1.7

1.8

0.6

130

1.5

1.6

1.7

1.7

1.8

0.6

-

5 to 6% Upgrade

5 to 6% Downgrade

50

----

----

----

----

----

----

60

1.5

1.5

----

----

----

0.6

70

1.5

1.6

1.7

----

----

0.6

80

1.5

1.7

1.9

1.8

----

0.55

90

1.6

1.8

2.0

2.1

2.2

0.55

100

1.7

1.9

2.2

2.4

2.5

0.5

110

2.0

2.2

2.6

2.8

3.0

0.5

120

2.3

2.5

3.0

3.2

3.5

0.5

130

2.6

2.8

3.4

3.6

4.0

0.5

*Ratio in this table multiplied by length of deceleration or acceleration distances in Table 3-13 and Figure 3-10, gives length of deceleration/acceleration distance on grade.



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Travel Lanes and Shoulders

Travel Lanes. Travel lanes should be 12 ft [3.6 m] minimum width on rural multilane highways. 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 3-12: Design Criteria For Multilane Rural Highways (Non-controlled Access) (All Functional Classes).

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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. In this regard, design should be rather simple to avoid driver confusion. In addition, adequate sight distance should be provided throughout, especially in maneuver or conflict areas. See Stopping Sight Distance in Chapter 2 for further information regarding sight distance.

Right angle crossings are preferred to skewed crossings, and where skew angles exceed 60 degrees, alignment modifications are generally necessary. Turn Lanes may be provided in accordance with previous discussions.

Chapter 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 and intersection sight distance.

Intersections formed at by-pass and existing route junctions should be designed so as not to mislead drivers. Treatment of an old-new route connection is illustrated in Figure 3-11.

For intersections with narrow, depressed median sections, it may be necessary to effect superelevation across the entire cross section to provide for safer operation at median openings.

For more information on intersection design, See Stopping Sight Distance in Chapter 2.

For more information on border areas, see Borders.

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

Figure 3-11. 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 3-12. Transition geometrics should meet the design criteria 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 [400 m] 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.

(US). Typical Transitions From Two-Lane
To Four-Lane Divided Highways. Click US Customary or Metric to
see a PDF of the image. (click in image to see full-size image) Anchor: #i1031638grtop

Figure 3-12. (US). Typical Transitions From Two-Lane To Four-Lane Divided Highways. Click US Customary or Metric to see a PDF of the image.

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

The Federal Highway Administration will allow the existing alignments to remain in place when existing two-lane roadways are converted to four-lane divided facilities. 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 in the horizontal or vertical alignment of the existing road. Other features such as signing, roadside hardware, safety end treatments, etc., should meet current standards.

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

An accident analysis of the existing two-lane roadway should be conducted. Any specific areas involving high accident frequencies will be reviewed and corrective measures 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 highway or railroad crossings, or to increase safety at accident-prone crossings.

Further information on grade separations and interchanges may be found in Chapter 3, Freeways and Chapter 10 of AASHTO’s A Policy on Geometric Design of Highways and Streets.

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