NCHRP Synthesis 299

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The objective of this synthesis was to summarize the key findings of various geometric design research efforts published in the 1990s. In the decade following the publication of the 1990 edition of AASHTO’s A Policy on Geometric Design of Highways and Streets (also known as the Green Book), the research community conducted a large amount of research with potentially significant implications on safety and operations. The primary goal of this synthesis was to capture and distribute that information for consideration in the development of the 2004 edition of the Green Book. Regarding auxiliary lanes, the synthesis included criteria for turn lane installations as well as guidance for turn lane length.

The paper presented several approaches for determining the need for a left-turn lane based on traffic volume, but did not include recommendations as to which of the methods were best. In general, the directional roadway volume is more critical than the volume of left-turning traffic. When considerations for the installation of a left-turn lane are made based on safety, an exclusive left-turn lane is typically suggested when the number of crashes per year per direction exceeds three or four. When determining the required length of a left-turn lane, both turning volume and through volume should be considered. At low turning volumes, the possibility of through vehicles blocking the left-turn lane is higher, while with higher turning volumes, the possibility of turning vehicles blocking the through lane is higher. Both scenarios should be considered when deciding on the length of a left-turn lane. The most significant information presented for right-turn lanes is a set of guidelines for installation based on a cost-benefit analysis including number of stops, delay, safety, and fuel consumption, as well as the cost of right-of-way. This method by McCoy et al. (“Guidelines for Right-Turn Lanes on Urban Roadways”, Transportation Research Record 1445, Transportation Research Board, National Research Council, Washington, D.C., 1994, pp. 130-137) is the most comprehensive set of right-turn lane installation guidelines because they account for the effects of right-of-way costs and roadway speed in addition to more standard measures of effectiveness.

Fitzpatrick, K., and M. Woolridge, M., NCHRP Synthesis 299: Recent Geometric Design Research for Improved Safety and Operations, Transportation Research Board, National Research Council, Washington, D.C., (2001) 

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