system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do speed detectors work?
gpt
There are multiple different types of speed detectors. The three common ones are Radar, LIDAR and cameras.

Radar works by sending out radio waves at fixed intervals that hit a car, and the difference in time taken to get to the car and back can calculate the speed of the car. It also takes advantage of something called the Doppler Effect, which is what causes emergency vehicle sirens to switch pitch when driving towards you vs away from you.

LIDAR works basically the same as radar, but using lasers instead of radio waves.

Traffic enforcement cameras work by having sensors in the ground (usually) that tells you how long it takes for a car to go from one point to another.