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HFG RURAL ENVIRONMENTS Version 1.0
PASSING LANES
Introduction
A passing lane is a lane added in one or both directions of travel on a two-lane, two-way highway to improve passing
opportunities. This definition includes passing lanes in level or rolling terrain, climbing lanes on grades, and short four-
lane sections (1). Passing lanes have been used mostly to allow drivers to bypass vehicles that are unable to maintain
normal highway speeds on grades, usually called climbing lanes. Potts and Harwood (2) found that the primary benefit of
passing lanes is the improvement of overall traffic operations on two-lane highways. This improved operation has direct
implications for driver behavior because a driver stuck behind a slow-moving vehicle may be more likely to experience
time delays and frustration, which could lead drivers to increase speeds to unsafe levels to pass a slow-moving vehicle.
Design Guidelines
RECOMMENDED VALUES OF LENGTH AND SPACING BY AVERAGE DAILY TRAFFIC (ADT) AND TERRAIN (6)
ADT (vpd) Recommended
Recommended
Distance between
Level Terrain Rolling Terrain Passing Lane Length (mi)
Passing Lanes (mi)
1950 1650 0.8-1.1 9.0-11.0
2800 2350 0.8-1.1 4.0-5.0
3150 2650 1.2-1.5 3.8-4.5
3550 3000 1.5-2.0 3.5-4.0
TYPICAL PASSING LANE SIGNAGE AND MARKINGS (3 )
Based Primarily on Based Equally on Expert Judgment Based Primarily on
Expert Judgment and Empirical Data Empirical Data
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HFG RURAL ENVIRONMENTS Version 1.0
Discussion
Two-lane highways with passing lanes provide a definite improvement in level of service over those without passing
lanes (2). In particular, at medium and high volumes, a roadway with continuous alternating passing lanes can provide
improvement by two levels of service over conventional two-lane highway without passing lanes. Similarly, a two-lane
highway with less frequent passing lanes typically provides an improvement of one level of service over a conventional
two-lane highway (2). Comparable improvements in service provided by passing lanes were found to reduce driver
frustration and improve overall quality of service and the benefits of the passing lane extend beyond the confines of the
added lane itself (4). Harwood, Hoban, and Warren (3) found that passing lanes improve the percentage of time drivers
spend following other cars on those roads by 10% to 31% in comparison to a conventional two-lane highway without
passing lanes. Passing lanes are generally well received by drivers; one study conducted in Kansas found that 93% of all
respondents were positive about passing lanes and indicated a higher acceptance and satisfaction of the concept. Also,
46% of these drivers thought the passing lanes were just right in length, while 53% thought the passing lanes were too
short (5). Mutabazi, Russell, and Stokes (5) also found that 88% of drivers agree more passing lanes are needed.
Highway engineers typically provide passing lanes with a primary objective of dispersing platoons and hence reducing
travel time, with safety as a secondary objective. However, drivers view safety as the main benefit accrued from passing
lanes (5). In the Kansas survey, 93% of drivers thought passing lanes improve safety, while 8% believed it encourages
speeding. These driver perceptions are consistent with crash data analyses, which indicate that the installation of a passing
lane on a two-lane highway reduces crash rates by approximately 25% (3). Harwood et al. (3) also found that crash
frequency per mile per year within passing lanes sections on two-lane highways is 12% to 24% lower than for
conventional two-lane highway sections.
Signage: Warning signs should be used to give drivers a preview of an upcoming passing lane and to warn drivers that the
passing lane is ending. The safety and convenience benefits of passing lanes are reduced if passing lanes are not
adequately signed. Clearly defined and well-maintained lane markings provide a similar function that can reduce the
likelihood of drivers' selecting an oncoming lane in an attempt to enter or remain in a passing lane. In a survey of
passing-lane signs, Wooldridge et al. (6) found that 61% of motorists prefer the wording "Left Lane for Passing Only"
versus 29% who prefer "Keep Right Except to Pass." When surveyors reviewed the sign "Passing Lane Ahead 2 Miles,"
61% of motorists would wait 2 mi to pass while the rest would pass when ready. This advance signing is useful because it
also informs the driver of the repetitive nature of the passing lane design, allowing the driver to understand the purpose
and nature of the roadway's characteristics. The sign should be used if the distance to the next passing lane is less than
12 mi. The sign "Right Lane End" is recommended to be located at a distance that will provide adequate notice that the
passing lane is terminating.
Design Issues
Length: The effective length of the passing lane is defined as the physical length of the passing lane plus the distance
downstream to the point where traffic conditions return to a level similar to that immediately upstream of the passing
lane (5). Through computer stimulation, Harwood et al. (3) found the effective length to range between 4.8 km (3 mi) and
12.8 km (8 mi), depending on the physical length of the passing lane, traffic flow, traffic composition, and downstream
passing opportunities.
Width and lane drop: Rinde (7) found the minimum width considered adequate for a two-lane road with a passing lane to
be 40 ft. In the opinion of Rinde (7), passing should not be allowed for vehicles traveling in the single lane of three-lane
roadways at traffic volumes above 3000 AADT. Also, the use of an appropriate lane-addition transition on the upstream
end of a passing lane is needed for effective passing lane operations (2). The recommended length of this transition area is
half to two-thirds of the length of the lane-drop taper (2).
Cross References
Signing Guidelines, 18-1
Marking Guidelines, 20-1
Key References
1. Mutabazi, M.I., Russell, E.R., and Stokes, R.W. (1999). Location and configuration of passing lanes. Transportation Research Record, 1658, 25-33.
2. Potts, I.B., and Harwood, D.W. (2004). Benefits and Design/Location Criteria for Passing Lane. Jefferson: Missouri Department of Transportation.
3. Harwood, D.W., Hoban, C.J., and Warren, D.L. (1988). Effective use of passing lanes on two-lane highways. Transportation Research Record, 1195,
79-91.
4. Hoban, C.J. and Morrall, J.F. (1986). Overtaking Lane Practice in Canada and Australia (ARR 144). Victoria: Australia Road
Research Board.
5. Mutabazi, M.I., Russell, E.R., and Stokes, R.W. (1998). Drivers' attitudes, understanding and acceptance of passing lanes in Kansas. Transportation
Research Record, 1628, 25-33.
6. Wooldridge, M.D., Messer, C.J., Heard, B.D., Raghupathy, S., Parham, A.H., Brewer, M.A., and Lee, S. (2001). Design Guidelines for Passing
Lanes on Two-Lane Roadways (Super 2) (FHWA/TX-02/4064-1, TTI: 0-4064). College Station: Texas Transportation Institute.
7. Rinde, E.A. (1977). Accident Rates vs. Shoulder Width: Two-Lane Roads, Two-Lane Roads with Passing Lanes (CA-DOT-TR-3147-1-77-01).
Sacramento: California Department of Transportation.
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