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June 7, 2013 18:53
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AASHTO — Geometric Design of Highways and Streets (2001 "Green Book") Chapter 4 - Cross Sectional Elements - Lane Widths
pp 315-316
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LANE WIDTHS | |
The lane width of a roadway greatly influences the safety and comfort of driving. Lane | |
widths of 2.7 to 3.6 m [9 to 12 ft] are generally used, with a 3.6-m [12-ft] lane predominant on | |
most high-type highways. The extra cost of providing a 3.6-m [12-ft] lane width, over the cost of | |
providing a 3.0-m [10-ft] lane width is offset to some extent by a reduction in cost of shoulder | |
maintenance and a reduction in surface maintenance due to lessened wheel concentrations at the | |
pavement edges. The wider 3.6-m [12-ft] lane provides desirable clearances between large | |
commercial vehicles traveling in opposite directions on two-lane, two-way rural highways when | |
high traffic volumes and particularly high percentages of commercial vehicles are expected. | |
Lane widths also affect highway level of service. Narrow lanes force drivers to operate their | |
vehicles closer to each other laterally than they would normally desire. Restricted clearances have | |
much the same effect. In a capacity sense the effective width of traveled way is reduced when | |
adjacent obstructions such as retaining walls, bridge trusses or headwalls, and parked cars restrict | |
the lateral clearance. Further information on the effect of lane width on capacity and level of | |
service is presented in the Highway Capacity Manual (HCM) (4). In addition to the capacity | |
effect, the resultant erratic operation has an undesirable effect on driver comfort and crash rates. | |
Where unequal -width lanes are used, locating the wider lane on the outside (right) provides | |
more space for large vehicles that usually occupy that lane, provides more space for bicycles, and | |
allows drivers to keep their vehicles at a greater distance from the right edge. Where a curb is | |
used adjacent to only one edge, the wider lane should be placed adjacent to that curb. The basic | |
design decision is the total roadway width, while the placement of stripes actually determines the | |
lane widths. | |
Although lane widths of 3.6 m [12 ft] are desirable on both rural and urban facilities, there | |
are circumstances where lanes less than 3.6 m [12 ft] wide should be used. In urban areas where | |
pedestrian crossings, right-of-way, or existing development become stringent controls, the use of | |
3.3-m [11-ft] lanes is acceptable. Lanes 3.0 m [10 ft] wide are acceptable on low-speed facilities, | |
and lanes 2.7 m [9 ft] wide are appropriate on low-volume roads in rural and residential areas. For | |
further information, see NCHRP Report 362, Roadway Widths for Low-Traffic Volume Roads (5). | |
In some instances, on multilane facilities in urban areas, narrower inside lanes may be utilized to | |
permit wider outside lanes for bicycle use. In this situation, 3.0-m to 3.3-m [10- to 1 1-ft] lanes are | |
common on inside lanes with 3.6-m to 3.9-m [12- to 13-ft] lanes utilized on outside lanes. | |
Auxiliary lanes at intersections and interchanges often help to facilitate traffic movements. | |
Such added lanes should be as wide as the through-traffic lanes but not less than 3.0 m [10 ft]. | |
Where continuous two-way left-turn lanes are provided, a lane width of 3.0 m to 4.8 m [10 to | |
16 ft] provides the optimum design. | |
It may not be cost-effective to design the lane and shoulder widths of local and collector | |
roads and streets that carry less than 400 vehicles per day using the same criteria applicable to | |
higher volume roads or to make extensive operational and safety improvements to such very low- | |
volume roads. AASHTO is currently evaluating alternative design criteria for local and collector | |
roads and streets that carry less than 400 vehicles per day based on a safety risk assessment. |
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