Generally, the busiest streets are pedestrianised; you just can't move that volume of people in cars.

(Actually, the _very_ busiest streets are probably major bus gates (where, again, there are no cars), those can have fairly ridiculous density at peak times.

Why not? A lot of streets often seem busy simply due to the incredible space inefficiency of cars. What I like to do sometimes, when I wait at a traffic light with my bicycle, is to count the actual number of cars that cross during a phase. The amount feels large, but when you count, it's often not more than the amount of cyclists that cross in the next phase, and they take up just a tiny fraction of what cars need.