Safety Effect of U-Turn Conversions in Tehran: Empirical Bayes Observational Before-and-After Study and Crash Prediction Models
Publication: Journal of Transportation Engineering
Volume 139, Issue 1
Abstract
Directional midblock median openings with left-turn lanes and loons were installed on major highways in Tehran, Iran, to prohibit direct left turns from major and minor roadways and to prevent through movements of minor roadways. Therefore, traffic lights were removed and intersections were converted to U-turns. The safety effect of this treatment was estimated by an empirical Bayesian method that indicated there was an increase of approximately 13.22% in crashes. Moreover, accident prediction models for crash types and frequency were developed to examine the geometry and traffic factors contributing to the crashes at U-turn conversions and to explain the reason some sites showed a decrease in the number of crashes while others showed a large increase. The results of data analysis showed that the increase in weaving length and median opening length resulted in the decrease in angle crashes. Also, the increase in the turn radius resulted in the decrease in angle and total crashes. Total crashes and crash types increased with the average daily traffic (ADT) increase on major roadways and U-turns. Consequently, the result of safety analysis with close examination of each site demonstrated that it is necessary to mention geometric specifications in order to improve the efficiency of the treatment. Therefore, it can be concluded with no reasonable doubt that with a certain range of traffic volume and right geometric specification, the treatment may be safe.
Get full access to this article
View all available purchase options and get full access to this article.
Acknowledgments
This paper was derived from Master of Science dissertation conducted at the School of Civil Engineering, Iran University of Science and Technology by Azizi, L under the supervision of Dr. Sheikholeslami, A. We would like to specially thank Professor Hauer, E for his great guidance in applying the EB method. We are also indebted for other people, all of whom were very patient with regard to our numerous questions and requests.
References
El Esawey, M., and Sayed, T. (2011). “Operational performance analysis of the unconventional median U-turn design using micro-simulation.” 90th Annual Meeting of Transportation Research Board, Washington, DC.
Greibe, P. (2003). “Accident prediction models for urban roads.” Accid. Anal. Prev., 35(2), 273–285.
Hauer, E. (1997). Observational before-after studies in road safety, Pergamon Press, Elsevier Science, Oxford, UK.
Hauer, E. (2004). “Statistical road safety modeling.” Transportation Research Record 1897, Transportation Research Record, Washington, DC.
Hauer, E., Harwood, D. W., Council, F. M., and Griffith, M. S. (2002). “Estimating safety by the empirical Bayes method: A tutorial.” Transportation Research Record 1784, Transportation Research Board, Washington, DC.
Liu, P., Chen, H., and Lu, J. (2008). “Impacts of the separation distances between driveway exits and downstream U-turn locations on the safety performance of right-turns followed by U-turns.” 87th Annual Meeting of the Transportation Research Board, Washington, DC.
Lu, J., Dissanayake, S., Xu, L., and Williams, K. (2001). “Safety evaluation of right turns followed by U-turns as an alternative to direct left turns—Crash data analysis.” University of South Florida, Tampa, Transportation Research Information Services, Tampa, FL.
Persaud, B. N., and Lyon, C. (2007). “Empirical Bayes before-after safety studies: Lesson learned from two decades of experience and future directions.” Accid. Anal. Prev., 39(3), 546–555.
Persaud, B. N., Retting, R. A., Garder, P. E., and Lord, D. (2001). “Safety effect of roundabout conversions in the United States: Empirical Bayes observational before-after study.” Transportation Research Record 1751, Transportation Research Board, Washington, DC.
Potts, I. B., Harwood, D. W., Torbic, D. J., Richard, K. R., Gluck, J. S., and Levinsin, H. S. (2004). “Safety of U-turns at unsignalized median opening.”, National Cooperative Highway Research Program, Transportation Research Board, Washington, DC.
Sayed, T., El Esawey, M., and Pump, J. (2007). “Evaluating the safety impacts of improved signal visibility at urban signalized intersections.” Transportation Research Record 2019, Transportation Research Board, Washington, DC.
Shahi, J., and Choupani, A. (2009). “Modelling the operational effects of unconventional U-turns at a highway intersection.” Transportmetrica, 5(3), 173–191.
SAS Institute. (2002). User guide and help for SAS.STAT version 9.1, Cary, NC.
Information & Authors
Information
Published In
Journal of Transportation Engineering
Volume 139 • Issue 1 • January 2013
Pages: 101 - 108
Copyright
© 2013 American Society of Civil Engineers.
History
Received: Sep 26, 2011
Accepted: Jun 13, 2012
Published online: Aug 22, 2012
Published in print: Jan 1, 2013
Authors
Metrics & Citations
Metrics
Citations
Download citation
If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download.