New U.S. research by UMTRI suggests vehicles with amber rear turn signals are up to 28% less likely to be involved in certain kinds of crashes than similar vehicles with red rear turn signals. The research, conducted at the behest of NHTSA (the U.S. National Highway Traffic Safety Administration), was the first comprehensive study of the question of rear turn signal colour since a 1981 Canadian study was unable to draw significant conclusions because of insufficient data.
International regulations outside North America have long required rear turn signals to emit amber light, but U.S. and Canadian regulations permit amber or red with no preference either way.
The researchers note that unanswered questions remain, particularly with respect to the effect of turn signal intensity on safety.
In accord with the response curve of the human visual system to light of different colours, American regulations require amber turn signals to be 62% more intense, at minimum, than red turn signals or photometrically-identical red brake lamps.
The study can be downloaded free from NHTSA.