I think one factor you haven't realized yet is that a planet might not have oil reserves, or might not use them for fuel if it does have them. BT is the future of the 80's after all, so looking at Blade Runner, probably one of the most popular 80's movies that clearly shows the future, we see that we're all living in hyper-dense cities, so it isn't unreasonable to use this as the model for cities in BT, and this model probably doesn't have privately owned vehicles.