Report numberRA-2006-79
TitleTraffic signals with permissive or protected left turn phasing. Effects on traffic safety.
SubtitlePart 2: Data analysis in Flanders, Belgium.
AuthorsAn Dreesen
Erik Nuyts
Published byPolicy Research Centre for Traffic Safety 2002-2006
Number of pages44
Document languageDutch
Work packageOther: Infrastructure and space

Intersections are places in the traffic network where the potential for conflicts is high. Specific infrastructural measures can be taken to organise and bring structure at these points. One of these measures is the installation of traffic signals.


Signalisations at intersections provide a separation of traffic flows in time. Different types of signalisation can be distinguished what the phasing for left turning vehicles is concerned. An intersection with a permissive left turn phase means there is no exclusive phase for left turning traffic, which must use gaps in oncoming traffic to turn left. At a fully protected left turn, vehicles turning left get an exclusive phase to turn left. There is no oncoming traffic that has to be given way. Partly protected signals have a fully protected left-turn for at least one intersection approach, but not for all of them.


In this report the effects of two types of traffic signals on the number of accidents are calculated: fully protected left turn signals and partly protected left turn signals, compared to permissive left turn signals. By using a comparison group, we corrected both for the general trend of the accidents and for regression to the mean. To get a more general estimate of the effect of protected left turn signals, the results of the locations were grouped in a meta-analysis.


For the installation of fully protected left turn signals, we found a significant decrease of
-30% for all injury accidents (95% confidence interval [-42%, -16%]). For accidents with severely or deadly injured victims, a significant decrease of -45%, [-66%, -11%] was found.


The installation of partly protected left turn signals resulted in a non-significant decrease of the injury accidents of -10% (95% confidence interval [-33%, +22%]). For accidents with severely or deadly injured victims, a significant decrease of -54% ([-77%, -5%]) was found.


The effects on distinct types of accidents were different: both for fully and partly protected signals the number of rear-end accidents increased and the number of left-turn accidents decreased.


These results on Flemish data are comparable with the effects presented in international literature.


A lognormal regression revealed no variables that influenced the effect of fully protected left turn signals.

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The Policy Research Centre for Traffic Safety carries out policy relevant scientific research under the authority of the Flemish Government. The Centre is the result of a

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