[Inspired by the "Curtain" idea, see link]
Attach to the underside of a bridge a device which includes a radar capable of detecting both the speed, direction, and distance, of approaching vehicles.
When the device has been armed (upon receiving a coded wireless signal from a police officer), it
will begin scanning the roadway passing beneath the bridge for vehicles. When one is detected, the device will calculate precisely when that vehicle will pass beneath.
A specific (fixed) amount of time (based on distance/height between road surface and the device) before the vehicle passes beneath the device, it will deploy itself, dropping a large quantity of custard (or other opaque, sticky substance) on the vehicle passing below.
This will obscure the windshield of the vehicle, preventing the driver from seeing where he's going. If he's not completely stupid, he'll quickly bring his vehicle to a halt.