Got it, what we need is a
Holographic projection of a tractor
Then every car is fitted with a speaker. The speaker emits a high pitched screech once a driver is within 400m of a cyclist up ahead. The driver must drop below 20 mph for this screeching to stop. Once within 100m they must drop their speed to be only 5mph greater than that if the cyclist. It gets louder and louder the closer they get if they ignore the sound. Within 50m and still too fast the car will slam on the brakes, turn the engine off, and refuse to start for 30 mins. Like Pavlovs dogs they will be trained to slow the fark down.