It would be complicated, and you might be right.

Side issue: cars leave when the person driving is willing to leave, which isn't the same thing as being the driver yourself or dealing with mass transit schedules.

I've heard for New York and would find it plausible for other places with good mass transit, that New Yorkers do more walking than people in places with little mass transit.

It might be possible to sort out at least some of the confounding factors-- not every city has good mass transit, for example.