A friend of mine who is a civil engineer suggested that to start, it might be easier to use in-pavement car elecromagnetic sensors like those cut into many left turn lanes that trigger stoplights to change. Lay two of them out in series before the bump and calculate a car's speed by the time difference between the two sensors. This way you don't have to mess with radar.

Also, I thought another version of these speed bumps could stay down most all of the time and only raise up when a too-fast car is detected. If the car is going just a little too fast, it gets a little bump, if it is going a lot too fast, a bigger bump. This way the driver can feel exactly what speed is expected and adjust accordingly over a series of bumps.

I like speed bumps. They discourage vandalism and reckless driving in congested areas like apartment parking lots. Ne'er-do-wells dislike them, and go elsewhere to smash car windows and threaten old people.