In GTA5 cops works via someone calling you in. Basically if an NPC sees you do something they will call the police with their cellphone, which sends out a single cop to investigate, and then that cop calls in backup when they find you.

However, they're just way too picky. I don't know if many people remember when GTA3 got popular, but one of the major factors that made it fun is that typically the cops weren't all over you for petty crap. When GTA3 came out, I think the other competitive game at the time was Driver... and cops in that came after you for EVERYTHING. Literally, the game expected you to follow the same laws you do in real life. Drive on the wrong side of the road? Police. Run a stop light... police. GTA3 changed that, for the better.

I'm not saying GTA5 is overbearing, I'd consider it a nitpick... but I just wish every little thing didn't send everyone into a panic of chaos and the cops being called. I also wish that running from the cops didn't raise your level either. That's just stupid. Of COURSE I'm going to run.