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AASHTO — Geometric Design of Highways and Streets (2001 "Green Book") Chapter 4 - Cross Sectional Elements - Lane Widths pp 315-316
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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