GPS isn't accurate enough to keep a car in the right lane. You need maybe 0.5m resolution for that, and GPS only gives you 3m or so.

Sorry, mixed up. Somehow thought he/she was talking about the signs and such.

Gotcha. Cameras are good for those too. You can get speed limits and such from databases, but they may be out of date. A camera is guaranteed to see the latest every time. For an extreme example, the interstate closest to me has dynamic speed limits at peak times. My car can pick up the current limit with the camera, but a database lookup obviously couldn't.