In cities? Yes. You start preferencing more space efficient modes of transport, and traffic jams go away, while transport time decreases. If you’re going to put a tunnel with fixed access points under a city, it makes a lot more sense to run trains than private cars.

If you’re going to have a 4 lane road reserve two of the lanes for super high capacity vehicles (busses).

Traffic jams are a byproduct of the limits of physics. Once you reach a certain density it is simply impossible to provide enough private car parking and lane capacity.