Study Details

Study Title: Evaluating the Safety Effects of Signal Improvements

Authors: Schultz et al.

Publication Date:JAN, 2014

Abstract: A large percentage of fatal and injury crashes on roadways occur at intersections. To aid in reducing these severe crashes traffic signals are often implemented. To truly understand whether the signal installations are helping to reduce severe crashes, there is a need to evaluate the effectiveness of the traffic signal improvements through the development of Crash Modification Factors (CMFs). Recent research has shown that traditional safety evaluation methods have been inadequate in developing CMFs. In recent years, Bayesian statistical methods have been utilized in traffic safety studies to more accurately analyze the effectiveness of safety improvements. The hierarchical Bayesian method is an advanced statistical technique that has the capability to account for the shortcomings of traditional methods and to more fully reflect the effectiveness of safety improvements. This report uses a hierarchical Bayesian model to analyze the effectiveness of new traffic signal installations and modifications to existing traffic signals from permissive only phasing to protected/permissive left turn phasing through the development of CMFs for multiple scenarios. A benefit-to-cost (B/C) analysis was also performed for each improvement to determine how long it would take to recover the cost of installation. The results showed that there was a slight increase in overall and non-severe crashes and a decrease in severe crashes for both new signal installations and modifications to existing signals. The B/C analysis indicated that there is a benefit to both improvements and that new signal installation costs can be recovered in approximately 5 years, while the installation of a left-turn signal modification can be recovered in approximately 9 weeks.

Study Citation: Schultz, G.G., M.Saito., A.L.Dowell., and C.S.Reese., Evaluating the Safety Effects of Signal Improvements. Presented at the 93rd Annual Meeting of the Transportation Research Board, Washington, D.C., (2014).


CMFs Associated With This Study

Category: Intersection traffic control

Countermeasure: Change permissive left-turn phasing to protected/permissive

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
1.11-111 StarAllAllNot specified
101 StarAllMinor ArterialNot specified
1.26-261 StarAllPrincipal Arterial Other Freeways and ExpresswaysNot specified
0.54461 StarAllAllNot specified
1.02-21 StarLeft turnAllNot specified
0.42581 StarHead onAllNot specified
0.67331 StarRear endAllNot specified
0.13871 StarSideswipeAllNot specified
0.56441 StarAllAllNot specified
0.7301 StarAllAllNot specified
0.51491 StarAllMinor ArterialNot specified
0.67331 StarAllPrincipal Arterial Other Freeways and ExpresswaysNot specified
1.19-192 StarsAllAllNot specified
1.15-152 StarsAllAllAllNot specified
1.55-552 StarsLeft turnAllAllNot specified
0.22782 StarsHead onAllAllNot specified
1.29-292 StarsRear endAllAllNot specified
1.41-412 StarsSideswipeAllAllNot specified
1.28-281 StarAllAllAllNot specified
1.06-61 StarAllAllAllNot specified
0.9731 StarAllAllMinor ArterialNot specified
1.24-241 StarAllAllPrincipal Arterial Other Freeways and ExpresswaysNot specified
1.61-612 StarsLeft turnAllNot specified
0.21792 StarsHead onAllNot specified
1.32-322 StarsRear endAllNot specified
1.38-382 StarsSideswipeAllNot specified
1.33-331 StarAllAllNot specified

Countermeasure: Install a traffic signal

CMF CRF(%)QualityCrash TypeCrash SeverityRoadway TypeArea Type
1.16-161 StarAllAllNot specified
1.14-142 StarsAllMinor ArterialNot specified
1.71-712 StarsAllPrincipal Arterial Other Freeways and ExpresswaysNot specified
0.56442 StarsAllAllNot specified
0.52481 StarLeft turnAllNot specified
0.64361 StarHead onAllNot specified
1.43-432 StarsAllAllAllNot specified
0.64361 StarSideswipeAllNot specified
0.5502 StarsAllAllNot specified
0.9461 StarAllAllNot specified
0.62381 StarAllMinor ArterialNot specified
0.58421 StarAllPrincipal Arterial Other Freeways and ExpresswaysNot specified
1.36-362 StarsAllAllAllNot specified
1.33-332 StarsLeft turnAllAllNot specified
0.43572 StarsHead onAllAllNot specified
2.16-1162 StarsRear endAllAllNot specified
1.15-152 StarsSideswipeAllAllNot specified
1.15-151 StarAllAllAllNot specified
1.1-102 StarsAllAllMinor ArterialNot specified
1.63-632 StarsAllAllPrincipal Arterial Other Freeways and ExpresswaysNot specified
1.42-422 StarsAllAllNot specified
1.35-351 StarRear endAllNot specified
1.632 StarsLeft turnAllNot specified
0.42582 StarsHead onAllNot specified
2.18-1182 StarsRear endAllNot specified
1.23-232 StarsSideswipeAllNot specified
1.52-522 StarsAllAllNot specified