Instead of those portable radar signs letting drivers know how fast they are going- this device would link a hidden radar gun to a ball cannon located strategically down the road.

The balls could be low cost play balls- the brightly colored kind you see in stores for $2.00 so losing one would not be cost prohibitive. (unlike my other thought of launching a million dollar cruise missle up the tailpipe of the speeding jerk as he disappears down the road). They should be large enough and have enough mass to create a loud thump for the truly brain dead driver.

When the ball shoots into the road, the driver slams on his brakes and slinks off in a sweaty pool of "what if it were real" once he realizes he has been had.

For drivers who dont stop, the balls could be powder coated to leave a tell tale mark like the dye-packs they drop in money bags during bank robberies.

If the slamming of breaks results in a tailgater running into the back of the target, all the better. If the second car was close enough to hit the front car he was both following too closely and speeding. (if this were not the case, the front car would be pulling away from the back car)

Heart attacks are also a potential side benefit by reducing the bad driver count by one for the length of their recovery time.