You can do both. The web site is where all the work is done. The app on your phone is a portal to the web site. But it also streamlines reporting back to the site.
As you're driving it displays google maps (on iPhone at least) and will beep and point out where a cop is. If it's a green warning, it means it's just been reported and if you see a cop there too you give it a thumbs up, and it goes yellow. If it's yellow and you see a cop you give it a thumbs up and it goes red and means that at least three people have seen and reported that a cop is there.
If you don't see a cop there you give it a thumbs down and after a while if enough people do that then it will be removed.
It keeps the maps and app live.
I have my iPhone mounted in a TomTom cradle so it's clear to see and quick to report or thumb.
The app is free to download and use, apart from your phone data costs.