Experimental systems
Multicolour auxiliary signals
Some jurisdictions, such as the US states of Oregon and Idaho, permit vehicles to be equipped with auxiliary rear signal systems displaying green light when the accelerator is depressed, yellow light when the vehicle is coasting, and red light when the brake is depressed. Such systems have in the past been sold as aftermarket accessories, but are today seldom seen in traffic.
Research and development
The US National Highway Traffic Safety Administration, among other bodies, has commissioned studies of these and many other vehicle signal systems and configurations, in an effort to determine the most promising avenues and best practices for enhanced crash avoidance via optimised vehicle conspicuity and signal lighting systems.