You could replace highways with a magical teleporter and you'd still have traffic and congestion. Most traffic happens at the interface between the highway and

You could replace highways with a magical teleporter and you'd still have traffic and congestion. Most traffic happens at the interface between the highway and the rest of the road network, as cars can only join the flow of traffic at whatever rate its capacity frees up - and cars entering the city compete for that capacity with cars leaving parking spots. Cities are already crammed full of street capacity, so the only way to increase capacity is to increase the number of people per car. And that solution, taken to its logical conclusion, is a train. Subways face similar problems to highway interchanges - passengers have to rejoin the flow of (foot) traffic. This is easier than with cars, since people are smaller and most of them can use stairs, but the cost of stations ends up being the larger portion of cost, not the cost of tunnels (which can be built more cheaply using cut & cover than they can with boring machines anyway). I'd like to see Musk throw his resources behind cheaper ways to make stations, instead of a problem that's better solved by digging a ditch.