Differentiating between Left-Turn Gap and Lag Acceptance at Unsignalized Intersections as a Function of the Site Characteristics
Publication: Journal of Transportation Engineering
Volume 138, Issue 5
Abstract
Left-turn gap acceptance at an unsignalized intersection is dependent on many factors. The Highway Capacity Manual (HCM) uses a single value of critical gap for all types of intersections; however, this may be oversimplistic and lead to inaccurate estimates of left-turn delay and capacity. Most existing studies also do not differentiate between gap and lag when evaluating gap acceptance. In this paper, binary logit models were developed to estimate the probability of accepting or rejecting a given gap or lag for a left-turning vehicle from a major road at an unsignalized intersection considering a number of potential influencing factors. Gap acceptance behavior was found to be influenced by the type of gap presented to the driver (gap or lag). Gap duration, total wait time, time to turn, distance to next signal downstream, and median type were found to be significant factors in predicting the probability of accepting or rejecting a gap. In the model for lag acceptance lag duration, time to turn, crossing width, speed limit, and distance to next signal downstream were found to be significant. Equations for estimating the critical gap and lag were developed. Critical gap and lag were found to be varying over a wide range of values depending on the type of intersection. The range was smaller for critical gaps than lags. The findings from this study can improve operational analysis of left turns at unsignalized intersections by using different critical gaps for different traffic and geometric conditions.
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Acknowledgments
The data used in this research were from the National Cooperative Highway Research Program (NCHRP) project 3-91, “Left-Turn Accommodations at Unsignalized Intersection.” The research was sponsored by the American Association of State Highway and Transportation Officials (AASHTO) in cooperation with the Federal Highway Administration (FHWA), and was conducted in the National Cooperative Highway Research Program, which is administered by the Transportation Research Board of the National Research Council. The opinions and conclusions expressed or implied in this paper are those of the authors. They are not necessarily those of the Transportation Research Board, the National Research Council, the Federal Highway Administration, the American Association of State Highway and Transportation Officials, or the individual states participating in the National Cooperative Highway Research Program. The authors appreciate the efforts of the numerous staff and student workers who collected and reduced the NCHRP 3-91 data used in this research. The authors would like to thank the anonymous reviewers who helped improve this paper with their valuable comments.
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Information
Published In
Journal of Transportation Engineering
Volume 138 • Issue 5 • May 2012
Pages: 580 - 588
Copyright
© 2012. American Society of Civil Engineers.
History
Received: May 3, 2011
Accepted: Oct 20, 2011
Published online: Oct 24, 2011
Published in print: May 1, 2012
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