The "stop" lights on the back of a car should change color from red to green depending on whether the car is decelerating or accelerating. This way they can function as both "stop" lights and "go" lights. This is an idea that was also recently mentioned on Should Exist. However, I would like to add that the lights have an additional feature -- they should blink with a frequency proportional to the rate of change of acceleration or deceleration of the vehicle. The faster a vehicle slows down the faster its "stop" light blinks red. The faster a vehicle speeds up, the faster its "go" light blinks green. So for example, when a vehicle slows down rapidly, drivers behind it can see how fast it is slowing down by the rate of red flashing of its tail lite. If it speeds up they can tell how quickly it is speeding up by the rate of green flashing of its tail lite.
This could be an important safety modification for automobiles that would prevent many freeway traffic accidents by enabling drivers to better guage the rate of deceleration and acceleration of cars in front of them. To make this system even smarter, there could be a similar (but smaller) light on the front bumper of cars as well, so that when looking in your rear-view mirror you could tell how fast the car behind you was speeding up or slowing down.