Some people might say that snow or rain or other forms of bad weather would be the easiest way to cause people to drive slower and safer, but in reality a police officer sitting on the side of the road is the easiest way to make everyone slow down. You will never see a more drastic change in people's driving behavior. A person could be going upwards of 80 mph but the second they realize their is a police officer, they immediately slow down usually to below the speed limit to guarantee they don't get pulled over for speeding. The radar guns police use, uses physics to help find out if the driver is going too fast. As the police officer aims the radar gun at cars passing by, the gun sends out radio waves toward the car. Then, the radio waves hit the car and bounce back toward the gun. The gun then measures the frequency of the returning waves, so the faster you are going toward the police radar gun, the higher frequency the waves will be. This concept uses a lot of physics including radio waves, frequency and also the Doppler effect. Since the car is moving toward the gun, the frequency of the returning radio waves will be much higher.