Gridlock is a fallacy
Or if not completely a fallacy, it's a state which very, very rarely exists when there are assorted alternatives.
The green brigade regularly trot out "if it wasn't for us all the cities would be gridlocked", and use the phrase as a scare tactic. If it really was so imminent all the major cities would have shut down long ago.
What actually happens is people have a tolerance for how far they commute. I worked on a project at Motorola where we spent two months understanding how people commute and their attitudes towards it.
The magic figure is 70 minutes.
Below that people think it's a fair door to door travelling time, beyond that it's unacceptable and they will look for a different method or even job. The perfect commute is one you don't remember, and the ideal is a ten minute walk. Second best is a ten minute drive. Introducing a need to rely on other people to create the transport (taxi, train, bus) adds frustration.
As an aside we did this work in 2002 to took at what technology we might want to sell commuters. As most people spend at least 40 minutes of their commute sitting on a bus or train we thought there might be something in these "tablet" things but the product management people dismissed the idea.
Back to the point, the reason we don't get gridlock is that before we get to that stage we get journeys regularly taking so long they pass the 70 minute pain point at which time people find alternatives.
So we end up with a situation where cities are often on the verge of gridlock but never actually get there.