He wants to limit his speed

Someone online told me he'd bought a radar detector to control his own speeding.

To me it seems that the regular speed signaling devices in cars are perfect for that, but he's not a very visual oriented guy.
I know. My memory stores images far better than sounds, but he always remembered exactly what was said and had problems remembering the texts of books. I've read some boring studybooks out loud for him and a blind student.

The detector gives a sound when it's in reach of a police speeding radar and when the car moves too fast.

Not everywhere radar detectors are allowed. When you go abroad always inform yourself well when you're using such device. In Greece the fine is as high as 2000 euro! In many countries you'll get a fee, see your detector confiscated, and you'll be faced with extra penalties like loss of license for a while.

The main reason is that it's thought that radar detector users work against the police by trying not to be caught for speeding, and that they speed when they're out of reach of the police radar.

In 2001 the results of the MORI study were publised and it turned out that detector users posed a lesser risk on the roads. They caused 28% less risk.

Isn't that interesting!