It's the supposed realism that the film is shot in, that helps to bring the ruthless character of Hackman's character. On second thought, there are many surreal elements in there, that make the detective look more like a caricature.
My favorite little piece was when he overtook a girl on a bike wearing red boots with his car. The next thing is we find him in bed with her. I do own a bike and I also have a drivers license which I have set to use in many countries around the world. I have to sadly admit that I seem to not to possess enough skills on either vehicle to pull off something to this effect.
The chase sequence worked so well in my opinion, because the film managed to lock me in with Hackman's determination - the chase itself is not very high speed compared to the hackfests of nowaday with two to three edits per second.
Another interesting little detail is that the films easily works in two languages (French/English). Something that seems to have disappeared from most main-streamish movies. But it works fantastically and I'd love to see more of it. People can read subtitles, maybe even better than in the 70s where you could get around without Twitter and texting.