Large-scale Study in Four U.S. States Found Flashing Yellow Arrow Treatment Can Reduce Crashes Related to Left-turn Movements by 15 to 50 Percent.

The FHWA Conducted an Evaluation on Crash Reduction after Flashing Yellow Arrow Treatments are Implemented at 307 Test Sites.

Date Posted

Safety Evaluation of Flashing Yellow Arrows at Signalized Intersections

Summary Information

This study evaluated the safety effectiveness of the flashing yellow arrow (FYA) treatment at signalized intersections. Previous studies showed that FYA treatment is associated with a reduction in left-turn (LT) crashes when there is no protected LT phasing. However, most of these studies only used data from one state, and the studies that used data from multiple states had limited samples of intersections. This study evaluated 307 treated sites and 438 untreated reference sites from 4 states; Nevada, North Carolina, Oklahoma, and Oregon. The primary target crash types were LT crashes and left turn opposite through (LTOT) crashes. Total crashes, injury and fatal (KABC) crashes, rear-end (RE), and angle (ANG) crashes were also investigated.


Data (number of crashes per year and signal information) was retrieved for the four states, and all were obtained directly from the agency staff. The sites were studied for the year 2015 before FYA was installed and after FYA was installed. The analysis was conducted using an empirical Bayes (EB) methodology for observational before-after studies. Safety performance functions (SPFs) were estimated for the six crash types through a modeling effort. Based on the before-after left-turn phasing schemes and the number of legs at the intersection, the sites were divided into seven categories (treatment groups).

Category Phasing Before FYA Phasing After FYA Number of Legs Number of Sites
1 Traditional PPLT FYA PPLT on one road 3 40
2 Traditional PPLT FYA PPLT on one road 4 136
3 Traditional PPLT FYA PPLT on both roads 4 64
4 Permissive or traditional PPLT FYA permissive on one road 4 25
5 Permissive FYA permissive on one road 4 12
6 At least one protected phase FYA PPLT without TOD operations 4 18
7 At least one protected phase FYA PPLT with TOD operations 4 12


The effect of the FYA was calculated for each category as a crash modification factor (CMF) to investigate the effect of site characteristics on the effectiveness of a particular treatment. Finally, the team conducted a quantitative economic analysis on the effect of FYAs using benefit to cost (B/C) ratio.



  • The economic benefits calculated from the crash reduction were found to be between $72,010 to $152,535 for the first five treatment categories that involved permissive or PPLT phasing in the before period.
    • At an annualized treatment cost of $854 to $1709 per intersection, the mean benefit-cost  ratios ranged from 56:1 to 144:1.
  • Intersections in the five treatment categories involving permissive or PPLT phasing in the before period experienced a reduction in LT and LTOT crashes at the intersection level. The reduction ranged from 15 to 50 percent depending on the treatment category. Alternatively, intersections in categories 6 and 7 that had at least one protected left-turn phase in the before period experienced an increase in LT and LTOT crashes.


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