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Publication Number: FHWA-HRT-13-069
Date: August 2013
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During late-night flash (LNF) mode (from late night to early morning hours), traffic signals flash yellow for one road (typically, the major road), requiring caution but no stopping, and flash red for the other road (typically, the minor road), requiring drivers to stop and then proceed through the intersection after yielding to the traffic on the major road. The intent of LNF is to reduce energy consumption and delay during periods of low traffic demand. However, in recent years, many agencies have begun replacing LNF with normal phasing operation because of safety concerns.
The safety impacts of replacing LNF with normal phasing operation have been studied since the 1980s. Gaberty and Barbaresso analyzed crash data at 59 four-leg intersections in Oakland County, MI, where the nighttime flash mode was replaced with normal phasing operation.(1) Results indicated a 91-percent reduction in angle crashes and a 95-percent reduction in injury right-angle crashes. However, it was not clear whether high-crash locations were selected for the change and whether the results may have been biased due to regression to the mean (RTM). Similarly, Polanis evaluated the safety of removing LNF from 19 sites in Winston-Salem, NC, using a naïve before–after method and concluded that nighttime right-angle crashes decreased by 78 percent.(2) Srinivasan et al. conducted a before–after evaluation of LNF conversion using the empirical Bayes (EB) method based on a small sample of 12 intersections in Winston-Salem, NC.(3) The EB method was used to specifically address the possible bias due to RTM. The authors concluded that nighttime crashes decreased by 35 percent, and nighttime angle crashes decreased by 34 percent. More recently, Murphy conducted an evaluation of 67 intersections in North Carolina using a before–after EB method but without using data on traffic volumes.(4) Murphy found that for sites where LNF was discontinued, there was a 27-percent reduction in nighttime crashes, a 23-percent reduction in injury and fatal crashes, and a 48-percent reduction in frontal-impact crashes.(4)
It is clear that while all the previous studies seem to indicate that removing LNF (and replacing it with normal phasing operation) will reduce crashes at night, each study had at least one limitation—possible bias due to RTM was not explicitly addressed, the sample was small, or traffic volumes were not considered. The objective of this effort was to evaluate the effect of eliminating LNF operations at signalized intersections using state-of-the-art methods and to address the noted limitations. The goal was to include an adequate sample of locations for which traffic volume data were available.
Topics: research, safety
Keywords: research, safety, Light-Emitting Diodes, crash modification factor, CMF, SPF, LED, empirical Bayes
TRT Terms: research, Safety and security, Safety, Transportation safety