IMDB says they did block streets. But often they ran past the blocked off area, so some of the shots are in real traffic. I always found Bullitt and French Connection to be very overrated as far as car chases go.
The Ringer ranked Ronin as the #1 car chase. Where De Niro is chasing Natascha McElhone through Paris. That was a classic.
If a car ripping through a city in real traffic is your thing, check out C'était un rendez-vous. It's Paris at 6AM on a Sunday, but it is 100% real traffic. He is up on sidewalks, going the wrong way down one way streets, running red lights. The sound dubbing is sort of bad though. They use the sound to make it appear the car is going much faster than it really is.