It’s a really simple and obvious idea and I wish more people would do it: build more tunnels, tunnels are great. It’s just a hole in the ground, it’s not that hard. But if you have tunnels in cities you would massively alleviate congestion and you could have tunnels at all different levels; you could probably have 30 layers of tunnels and completely fix the congestion problem in high-density cities. So, I strongly recommend tunnels.