Do Curve Warning Signs Actually Make Roads Safer?

A surprising number of signs, pavement markings, and other traffic control devices have been designed and implemented on a hunch in an attempt to improve roadway safety.  Many of them have never been rigorously studied to determine if they do indeed enhance roadway safety.

About ten years ago we worked on a research project with the University of Minnesota staff to dig into the effectiveness of different curve warning systems.  In digging through my files, I realized I never shared this on the blog.  This practical, prove/disprove assumptions research is my favorite type of academic work, so here’s a quick update on curve warning signs.

Background

In 2009, the Federal Highway Administration studied a host of low cost safety devices to determine whether or not these strategies actually work.  The hope was that through rigorous study, we can spend our limited funds on the safety measures with the highest benefit to cost ratios.  With this goal in mind, the effectiveness of providing more conspicuous signing and lane markings at roadway curves was analyzed in FHWA Publication Number FHWA-HRT-09-046.

Crash data for horizontal curves on rural, two lane roads was collected in Connecticut and Washington.  Connecticut upgraded their existing curve warning signs to fluorescent yellow sheeting as well as adding fluorescent yellow curve warning signs to locations that previously had no curve warning signs.  Washington focused on installing more chevron curve delineation signs (type W1-8), either adding chevrons to previously unsigned curves or adding more chevrons to previously signed curves.  Crash data was collected on roadways with the improved curve warning signs and compared with crash data from similar roadways where the improved curve warning signs had not yet been installed.

Results

The most significant safety improvements were related to crash reductions during dark conditions and the reduction of injury plus fatal crashes.  Crashes on curves during dark conditions were reduced by 35.3% in Connecticut and 24.5% in Washington.  Injury plus fatal crashes were reduced by 25.2% in Connecticut and 18.0% in Washington.  These findings were found to be significant at the 95-percent confidence level, meaning there is a strong correlation in the statistical data between installing curve warning signs and reducing these types of crashes.

An economic analysis was also done on improving curve delineations with warning signs.  It was found that installing curve warning signs on rural, two lane roads is very cost effective with a benefit-cost ratio exceeding 8:1.  A ratio of 2:1 or better is typically considered a good investment.  In short, installing curve warning signs on rural roads is an excellent way to spend our safety improvement funds.

Recommendations

Those responsible for rural, two lane roads should develop plans to ensure adequate warning signs are in place along their horizontal curves.  Plans should be prioritized to add warning signs to curves with the highest traffic volumes, smallest curve radii, and most hazardous roadsides.  Those are the curves that will see the greatest reductions in crashes.

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