Protected Left Turn movement

Left-turning traffic is over-represented in conflicts at intersections. An estimated 27% of all intersection-related crashes in the United States are associated with left turns, with more than two-thirds of these crashes occurring at signalized intersections (U.S. Department of Transportation, 2009).

When a driver has a protected left turn signal, a solid green arrow gives the driver the complete right of way in a left-turn lane, with no pedestrian movement or vehicular traffic conflicting with the left turn. Protected left turn signals are introduced to eliminate conflict between left-turning vehicles and crossing pedestrians as well as through traffic. When a protected left turn signal is used, the time to provide that phase must be taken from the through phase or the cycle length must be extended. Criteria that are frequently used to implement left turn phasing include traffic volume of left-turning vehicles, delays to left-turning vehicles, and left-turn collision experience.

Zegeer et al., (2004) cited research results from Zegeer et al., (1982) that found that exclusive phasing for left turns and pedestrians was associated with approximately a 50% reduction in motor vehicle–pedestrian crashes as compared to standard timing where pedestrians crossed and vehicles turned concurrently. The authors also cited the findings of a study undertaken by the Insurance Institute for Highway Safety (1997), which found that early release of pedestrians reduced the vehicle-pedestrian conflict rate by up to 95% in St. Petersburg, Florida.

Srinivasan et al. (2008) conducted a before-after evaluation using empirical Bayes methodology to evaluate the effect of changing permissive or permissive/protected phasing to protected left-turn phasing at signalized intersections in Winston-Salem, North Carolina. The results of the treatments indicated that changing permissive/protected to protected only left-turn phasing eliminated 100% of angle collisions and replacing permissive with protected only left-turn phasing eliminated 97.9% of angle crashes at treated intersections. However, the authors found that the total number of crashes at the treated sites remained relatively unchanged, which suggested that other types of less severe crashes increased.

A U.S. Department of Transportation Federal Highway Administration Issue Brief on traffic signals provided research based estimates of crash reductions that could be expected if specific countermeasures are implemented with respect to traffic signals (Federal Highway Administration, 2009). Converting permissive or permissive/protected to protected only left-turn signals is expected to reduce all left-turn crashes by 99%; converting permissive to protected left-turn phase on multiple approaches (1 to 4) is expected to reduce all crashes by 6% to 22% depending on the number of approaches that are treated; providing a protected left-turn phase at an intersection in an urban area is expected to reduce fatal/injury left-turn and right angle crashes by 17% and 25%, respectively; and providing a protected left-turn phase at an intersection is expected to reduce all crashes, left-turn crashes, right-angle crashes, and rear-end collisions by 27%, 48%, 63%, and 31%, respectively.

Ozmen et al. (2009) conducted a study in Las Vegas, Nevada to evaluate the impact of converting a protected/permitted left-turn signal control to protected-only left turns during low traffic volume time periods by suppressing the permitted green ball and yellow ball displays. Before and after studies were carried out using crash data from 10 intersections that had implemented the periodic protected left turn display. The results of the analysis indicated that an increase in crashes did not occur, which led the authors to conclude that there were no safety concerns due to the use of the special display.

Chen et al. (2012) evaluated the safety impacts of changing left-turn signal phasing from permissive to protected/permissive or protected-only at 68 intersections in New York City between 1995 and 2009. Changes in the total number of crashes, multiple-vehicle crashes, and left-turn crashes after the introduction of protected/permissive or protected-only signal phasing for a treatment group and a comparison group were observed. The change from permissive-only phasing to protected phasing resulted in a 55% reduction in crashes at the treatment group sites. The authors suggested that factors such as safety, delay, geometry traffic flows, and operations need to be considered before implementing left-turn phasing as an intersection control.

Scope of the Problem

• U.S. Department of Transportation (2009) Permissive/Protected Left-Turn Phasing, Intersection Safety Case Study (Report No. FHWA-SA-09-015). Federal Highway Administration, Washington, D.C. Retrieved from http://safety.fhwa.dot.gov/intersection/resources/casestudies/fhwasa09015/pp_article.cfm

Evidence

• Zegeer, C.V., Stutts, J., Huang H., Cynecki, M.J., Van Houten, R., Alberson, B., Pfefer, R., Neuman, T.R., Slack, K.L., and Hardy, K.K. (2004). NCHRP Report 500 Guidance for Implementation of the
• AASHTO Strategic Highway Safety Plan Volume 10: A Guide for Reducing Collisions Involving Pedestrians. Transportation Safety Board, Washington, D.C. Retrieved from http://safety.fhwa.dot.gov/intersection/resources/fhwasa09027/resources/NCHRP%20Report%20500%20Vol%2010%20A%20Guide%20for%20Reducing%20Collisions%20Involving%20Pedestrians.pdf
• Insurance Institute for Highway Safety (1997). Three-Second Head Start Gives Pedestrians Advantage at Intersections. Status Report, Vol. 32, No. 7. Retrieved from http://www.usroads.com/journals/aruj/9802/ru980202.htm
• Zegeer, C.V., Opiela, K.S., and Cynecki, C.J. (1982). Effect of Pedestrian Signals and Signal Timing on Pedestrian Accidents. Transportation Research Record 847, pp. 62-72, TRB, National Research Council, Washington, D.C. Retrieved from http://safety.fhwa.dot.gov/intersection/resources/casestudies/fhwasa09015/pp_article.pdf
• Harkey, D., Srinivasan, R., Baek, J., Council, F. M., Eccles, K., Lefler, N., Gross, F., Persaud, B., Lyon, C., Hauer, E., & Bonneson, J. A. (2008) Crash Reduction Factors for Traffic Engineering and ITS Improvements, NCHRP Report No. 617. Retrieved from http://onlinepubs.trb.org/Onlinepubs/nchrp/nchrp_rpt_617.pdf
• Federal Highway Administration (2009). Issue Brief, Traffic Signals. U.S. Department of Transportation, (Report No. FHWA-SA-10-005). Retrieved from http://safety.fhwa.dot.gov/intersection/resources/fhwasa10005/docs/brief_5.pdf
• Ozmen, O., Tian, Z.Z. & Gibby, R. (2009). Safety of Special Time-of-Day Protected/Permitted Left-Turn Signal Control Display. TRB 88th Annual Meeting Compendium of Papers CD-ROM, #09-2074. Washington, D.C. Retrieved from http://trid.trb.org/view.aspx?id=881541
• Chen, L., Chen, C. & Ewing, R. (2012). Left-turn Phase: Permissive, Protected, or Both? Paper presented at the 91st Annual Meeting of the Transportation Research Board, January 22-26, 2012, Washington, DC. Retrieved from http://www.cmfclearinghouse.org/study_detail.cfm?stid=284