Streets can serve many access points. It is easy to get on to and off of the street. Additional access points can be added with little disruption. It is hard to tell how many cars are actually using each street. PPP, on the other hand, would be comparable to a railway.
Travel is generally between two well defined points.
You can't get on and off anywhere. It is relatively easy to count and monitor passengers. You need a ticket to board. In fact, yes, it is. That is what tramways do.