For long stretches of roads /highways you could put strings of lights that light up in a manner that sets the speed. If you’re traveling at the speed limit the lights would appear to be stationary. If you travel faster or slower the lights would appear to speed up or slow down.

This would also help traffic jams by allowing the light chase speed to be adjusted before the jam to reduce speeds.

This would probably have an effect on fuel consumption for people that pulse the throttle by giving them a chase light to match speed too.