Why not built a receiver into every car that would receive situations such as approaching a STOP sign, a yellow or red traffic light, a railway intersection, a school crossing, a traffic condition ahead or you name it and warn the driver of the upcoming situation inside the car? A stop sign or a pedestrian crossing will just transmit a "I am here!" monotonous broadcast while other traffic "devices" will continuously transmit their current condition such as a traffic light etc.
In the future the on board system could also apply brakes if necessary or even engage an automated driver system to avoid an accident...