I think you may be missing some of the key elements of Musk's vision. The tunnels would be for autonomous, electric vehicles only. Congestion wouldn't exist, ea

I think you may be missing some of the key elements of Musk's vision. The tunnels would be for autonomous, electric vehicles only. Congestion wouldn't exist, each vehicle would travel along at the same speed. This is the same vision above ground, but harder to implement fully for two main reasons: special cases/driving situations exist in many many places, and it's hard to get 100% compliance, as plenty of people aren't ready to give up their manual driving freedom. In these tunnels, it would be standardized and electric only, solving both of these challenges and accelerating adoption.