Would it be possible to make it possible to pause physics for each car? If this is possible, then traffic could all be paused cars that are just being moved along roads as things would be moved when moving something in the World Editor. These paused cars could be unpaused if the player's car is within a certain radius of a traffic car. Don't know if unpausing physics for a few cars at once would cause a hitch or anything like that though. Maybe each car is simulated at 50% of realtime and then set to realtime when the player's car is within the radius?
the problem with this is that you'd still have to invent some way of actually running the cars along the road, and having them pointed in the right direction, etc. as currently all of that is still handled by basic physics.
Why would you want that if the current simplified traffic cars are literally almost as simple as props? At this point it's not the physics being resource intensive but the 3D models of them probably
