Car speed sensor to turn on something
With most modern cars you can attach a computer or microcontroller to the OBD-II diagnostic port and ask the engine management system how fast the car is travelling.

Or you can use one of the shaft speed measurement techniques (eg hall effect, retroreflective) on the prop shaft after the gearbox, and convert that to wheel speed.

Or you can use a GPS unit, although that won't work in tunnels.