Safety Evaluation Of Discontinuing Late-Night Flash Operations at Signalized Intersections.

摘要

During late-night flash (LNF) mode (from late night to early morning hours), traffic signals flashnyellow for one road (typically, the major road), requiring caution but no stopping, and flash red fornthe other road (typically, the minor road), requiring drivers to stop and then proceed through thenintersection after yielding to the traffic on the major road. The intent of LNF is to reduce energynconsumption and delay during periods of low traffic demand. However, in recent years, manynagencies have begun replacing LNF with normal phasing operation because of safety concerns.nThe 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 na?ve 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 onntraffic 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.