The University of Michigan Transportation Research Institute, widely considered North America’s foremost automotive lighting research body, is studying the effect on traffic safety of vehicles’ rear turn signal colour. For many years, US regulations have allowed rear turn indicators to produce red or yellow light. European, Japanese, Australian and other regulations outside North America have long required only yellow, but nobody’s produced scientific data sufficient to persuade the US to mandate yellow. Sporadic study of the matter within North America — the last major attempt was in 1981 — has heretofore yielded inconclusive results. The UMTRI study, which is to be released soon, likely will quantify the relative safety merits of the two colours.