United States Department of Transportation
i
Assessment of Multiple Left-Turn Phasing Strategies
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2003-12-01
[PDF-1.61 MB]
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TRIS Online Accession Number:970451
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Edition:Final Report, August 2002–December 2003
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Abstract:Left-turn movements at an intersection affect the capacity of that intersection. As the left-turning volume at an intersection continues to grow, the green time required to meet the left-turn demand increases. This, in turn results in longer cycle lengths. Also with the increase in the left-turn volume, the queues lengthen resulting in greater storage length requirements. The combinations of these effects tend to increase delay at the intersection and lower the level of service. Installation of multiple left-turn lanes (dual and triple) can result in the reduction of vehicle queue lengths, delays and left-turn storage length. In Missouri, multiple left-turns are gaining popularity. However, the installation of these multiple left-turns raises questions for which the Missouri Department of Transportation (MoDOT) has not yet developed answers. Specifically, MoDOT seeks guidance on: criteria for determining when to install double and triple left-turns; the type of phasing to be used for dual and triple left-turn lanes; whether to use "Dallas" or permitted lead-lag phasing for any left-turn lanes; and where to begin reducing the number of receiving lanes downstream of an intersection with multiple left-turn lanes. Based on the current practices being followed by various state DOTs and from the review of literature, the following recommendations are being made: (1) Capacity analysis should be used to determine the set of conditions for upgrading left-turn lanes from single to dual and dual to triple; (2) If it is not feasible to perform capacity analysis due to a lack of resources, the following rules of thumb may be used for determining the point of upgrade: when left-turning volume is greater than or equal to 300 vph, upgrade from single to dual left-turn lane, and when left-turning volume is greater than or equal to 600 vph, upgrade from dual to triple left-turn lane; (3) Protected only phasing should be used for dual and triple left-turn lanes; (4) "Dallas" phasing should be used instead of lead-lag protected + permissive phasing for single left-turn lanes along with R10-12a (combined R10-10L and R10-12) sign to avoid confusion to the adjacent through traffic; and (5) For downstream lane drop distance, consider the solution by Shen as shown in Table 2 of this report.
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