Hi jackr99,
today's cars are normally equipped with ABS (anti blocking system) which monitor individual wheel rotation to apply brakes according to road conditions.
The ABS sensors could be used to determine car speed.
I do not suggest to adapt speed automatically since the traffic situation is unknown and might be fatal if emergency braking is necessary while the automatic throttle control dictates 20mph.
Driving through Denmark you'll find lots of speed indicators based on radar. They tell you the exact speed you're driving. If you miss slowing down you'll get a new photo of yourself sitting in the car - very expensive.
Every car driver is responsible for his actions and a speeding ticket here and there will be good for the community cash.
Just an over speed warning should suffice.
I once was asked to develop a system which I called "AGDS" (Anti Ghost Driver System) to be used for highways to prevent entry on the wrong lane.
My first suggestion was using IR detectors along the entries and exits with automatic engine shutdown if a highway was entered or left the wrong way.
The suggestion was appreciated but much too expensive to build and maintain - including car modifications.
My second suggestion was accepted. It was nothing but a mechanical solution puncturing all tires going the wrong way.
Boncuk