Long-term, self-driving cars will resolve a lot of our traffic problems today. The roads we have already have PLENTY of capacity. However, every human mind fighting for their position in the roadway leads to vast inefficiencies. Eventually, every car on the roadway will talk to one another and algorithms will ensure everyone gets where they need to go as quickly as possible. Traffic will cease to exist, and cars on highways will be zooming by at speeds much higher than they do today as every movement is coordinated therefore elimating the need for a speed limit. The speed limit on our highway is 60 MPH. If it weren't for stupid humans it could be much higher. However, there are many hills and people don't step on the accelerator when they're going up, so that causes slow downs. People often step on the brakes for new mergers because the ramp is so small. There could be some community clean-up and idiots will slow to a crawl to observe. All of this and more will be solved once self-driving cars enter the mainstream, and the wheel is taken away from stupid people.
If I say so myself, public transit will be outdated soon enough as cars retake their place as the fastest, most efficient form of land-based transport.