To recap.... OldMainframeGuy suggested the possibility of making the phone available during an emergency while you're driving. You went on about signal blocking, which is totally unnecessary, and prisons for some reason. I said the cell phone signal would not need to be blocked, and no one mentioned that. (It would actually be safer not to block the cell signal in case of a crash, etc., even if the law allowed it.) Again, the phone knows how fast it's going; most people think about GPS and such for tracking speed, but the accelerometer knows how fast the phone is moving, and every smartphone has one.
So, as a quick hypothetical example, once the phone reaches 25 mph*, it could deny access to everything but the phone app and any maps app. You should be able to use the phone in an emergency, I reckon, while travelling at a decent speed. Sure, most of the time you have an emergency, you're not moving, but some times you are. I remember that one time antspants was chasing me down the highway when he caught me with his first wife.
* : I looked up the metric speed unit, since I didn't know it earlier. It's apparently kiloliters per fortnight.
