Installing speed rely in every car might be unpractical and expensive. How about install road sensors and displays flow speed on the side of the road? Current

Installing speed rely in every car might be unpractical and expensive. How about install road sensors and displays flow speed on the side of the road? Currently people use brake light to signal and detect slowdown ahead, but sometimes people slow down without hitting the brakes. Displaying flow speed every 500 or 1000 feet on the side of the road would give fore warning on traffic slowdown ahead.