Well I for one is unabashedly in love with my city and its sexy infrastructure, so I would definitely drive in it. I frequently click a vehicle and let it drive me wherever it want to go in 1st perpective, and I like what I see.
I agree that the current lack of collision detection between objects is probably the major hurdle in the way of realizing this feature in a decent manner. I don't know if collision will ever be implemented as it is somewhat outside the paradigm of the game. It wouldn't be impossible to implement collision in the game I think if they wanted to, and they might, you never know... It could be useful to have collision physics if they ever wanted to implement interesting disaster scenarios for instance. I can imagine random traffic accidents would be nice addition to the standard fire outbreak, or vehicles floating about in flooded streets. The water physics begs to be used in some kind of destructive manner.
It should be doable without collision physics too I think. They could perhaps simply set up stricter rules that disallow driving outside of roads and even block access to the points on the roads currently occupied by other agents. The agents in the game are "aware" of other agents (the game keeps track of where they are), and usually they will stop and not drive though one another, but of course occasionally they do because it's not really important (or they get the go'ahead at the exact same time or something).