Learn the basics of radar detectors and radar jammers. Because anything is better than obeying the law.
Everyone has to prioritize when it comes to finances. Some people want to spend their money on online gaming subscriptions with an occasional kick over to their savings account. Some want to toss it to the government in exchange for the privilege of going over the speed limit while telling themselves that this time they won't get caught. Some want to spend just as much cash in order to speed, without knowing that they're helping out with road construction and those littering signs that quote outrageous and never-imposed fines. That last group buys radar detectors.
Radar guns use a principle called Doppler Shift in order to figure out speed. The guns shoot out radio waves at a certain frequency. The waves bounce off a car, and come back to the gun. If the car is stationary, the waves come back at exactly the same frequency they went out at. If the car is moving, they come back at a higher frequency. A good way to picture this is imagining throwing out rubber balls into the darkness at the rate of one ball per second. If the balls come bouncing back at the rate of one per second, they're bouncing off a stationary object. If they bounce back at twice the rate you throw them out, then they're bouncing off something that's rushing towards the thrower. The closer together that they bounce back, the faster it's coming. Radar guns measure the frequency of the waves as they come back to the gun, and use that to calculate the car's speed.
If the police were using balls instead of radar, drivers wouldn't have any problem knowing when they were being scanned. The balls bouncing down the road would be a giveaway. Radio waves can't be sensed by humans. Almost every human does have a radio device that picks up radio waves in their car, it's just that the waves being detected are usually the ones transmitting oldies, or obnoxious political opinions, or if they're very unlucky, wacky morning djs. Radar detectors do the same thing that radios do, they pick up waves at a certain frequency. Radar detectors simply pick up waves at the frequency that police radar guns use, and make a noise when those waves are detected.
Some drivers go a step farther and use radar jammers. These jammers not only detect waves, but send a confusing mish-mash of waves back along with the radar gun's waves. This makes it hard for the gun to calculate the speed of the car.
Police departments change the frequencies of radar that they use, or pick up lidar – which works much the same way that radar does, only using individual pulses of infrared light instead of a continuous beam of radio waves. Radar detector users respond by buying new equipment, or modifying their old stuff. Or, if they've entered the true depths of depravity, becoming smug bicycle riders. But that's too horrible to contemplate.