[NikolaeVarius]

Well sadly, roads have a bad habit of being confined by physical time and space and cannot just be arbitrarily widened.

[bluGill]

You can go up and down though.

Again, I'm not making a value judgement here. Transit is a valid option that could be better

[ryukafalz]

Up is expensive and rather unpleasant for those walking nearby on the surface. Down is even more expensive.

Don't forget that no matter how many lanes you add to a highway heading into a city, eventually that highway ends up... in the city. Too many cars in a city makes for a loud and dangerous-to-navigate environment for those who live there.