Hitzemeyer, E., Wilde, H., and Ellenberger, D., "What Color Should Rear Turn Signals Be?," SAE Technical Paper 770812, 1977, doi:10.4271/770812.
Once optional, flashing turn signals have become standard equipment on motor vehicles around the world. However, distinctness of rear turn signals is often lost when combined in the same compartments with stop lamps of the same size, intensity, color and location. In the U.S., a proposed solution is physically separating the function, while other countries effect separation by color. The SAE is investigating more realistic red/yellow lamp outputs that would permit use of universal lamps, while in Germany, manufacturers have surveyed many experienced American and Canadian drivers to determine preference for the color of these lamps.