If your hypothesis is that smoother rides - based on street/vehicle/driver factors - are associated with reduced passenger use of handles or bars to stabilize themselves while standing, I'd say that makes sense. I'm less sure about the between city comparison. I prefer not to touch common surfaces for hygienic reasons, which is pretty influential for me. There are three circumstances in which I'd hold a bar/handle to stabilize myself: 1) It's so crowded that I can't spread my feet far enough apart to stabilize me so I make prolonged use of a bar/handle, 2) I'm exhausted and use the bar/handle for a prolonged period to rest some stabilizer muscles, 3) There's a sharp turn or stop and I proactively or reactively use a bar/handle temporarily for stability until the acceleration's back to normal. Hope that helps :)