While brake lights are a fine and necessary signal, I find myself wishing that cars would display speed & acceleration deltas compared to the vehicle in front. Brake lights do not convey how much deceleration is occurring, so it's not immediately obvious how much time there is to impact.

As an example, I see lots of drivers do minor braking on US highways, where their small braking corrections will cause their tail lights to flash for a brief moment. These minor corrections become noise that drowns out the signal of significant, safety-related braking. If my vehicle could warn me that their rate of deceleration puts our vehicles on a collision course... now that would be very useful.