Road Tunnels
Roads can now run underground. You'll find the Tunnel portal in the build menu under Ground, right below Street.
Place one portal at each end — press R to turn the entrance — then connect them underground with the road tunnel. The road tile in front of the entrance is added automatically. In the Underground view, a green frame shows you when two portals are truly connected.
A tunnel can be at most 20 tiles long. Once the link is complete, cars and buses drive through on their own. Especially useful where no route is possible above ground — underneath a runway, for example.