I'm assuming by that point, you wouldn't have people driving anymore, it would all be automatic. Likely hooked into some sort of flight control system that would allow the vehicles to navigate around each other and avoid collisions.

Plus, look at it this way. Accidents are common now because roads restrict cars into shared paths of travel, requiring drivers to successfully avoid colliding with other people moving very close to them. If you are able to fly, you'd be able to beeline from point A to point B, distributing vehicles across a much broader area of travel. Plus, the added vertical axis means you won't even necessarily collide if your vehicle can just move up or down around potential midair obstacles.