If your car has ever broken down at the road side you may have been subjected to beeping from passing vehicles. Did you know that with a bit of practice and a musical ear you can work out the speed of the vehicles using the Doppler Effect? ** Please do this at a safe location off road well away from everyone else, remember it is an offence to use your horn when stationary (except when another road user poses a danger) it is an offence to use your horn when driving in a built up area between the hours of 11:30pm and 7:00am.** Find a long straight road and a place you can sit safely at the side of the road, ask a friend to drive passed 3 times at 20mph, 30 and 40, each time get the driver to blow the horn for a few seconds as they pass. You should be able to notice the difference in the sound especially the pitch which will go higher when the car was approaching than after it has passed. This is called the Doppler Effect, which is also the basis on which the police speed guns work (although they use radio waves instead of sound waves) Once you have heard all 3 passes you should be able to tell the speed your friend goes past you again at a random speed between 20mph and 40mph fairly accurately.