I like to think of PT as placing an "upper bound" on how bad traffic can get. If PT is faster than sitting in a car, people will take PT instead, alleviating co

I like to think of PT as placing an "upper bound" on how bad traffic can get. If PT is faster than sitting in a car, people will take PT instead, alleviating congestion. This more or less results in traffic getting worse until it reaches the speed of PT, at which point people start switching transport modes and it stops getting worse. If the train took the same amount of time as driving I'd take the train every time - driving sucks, whereas I can read or watch videos on the train. This works because (grade-separated) PT can (at least relative to roads) take on an effectively unlimited amount of extra commuters without getting slower, as pretty much all of its costs and transport times are fixed, whereas roads just continually get slower with additional traffic. This is part of the problem with busses and the like - they get caught up in traffic, so unless they get their own dedicated lanes you won't get the same traffic-reducing benefits as you would with heavy rail and the like.