Cars with internal combustion engines create high costs for subterraneanly streets for the necessary air exchange to bring the fumes out
Plug in hybrids - minimum 50km electric
Even a standing car with combustion engine on needs 100 times more air than a sitting human. In the worst case is in the tunnel a traffic jam and nobody switches of the comustion engine. In this case uses a car as much air as 100 humans, but a car uses only 6m * 2,5m place, an area where it's hard to place 100 humans.
Construction and operating costs for the airing are a big expense at every kind of subterraneanly traffic. Communal politicians will love the plug-in hybrid. When it's able to drive 50km electric, combustion engines while driving through a tunnel are unnecessary. Simple put a sign at the entrance, combustion engines forbidden.
Bounds less by the price
The bounds for underground streets will be less by costs than in the oil age when the combustion engines of cars had been always turned on. Underground will be more a question where is is usefull and where it's uncomfortable to be banned in the car from the surface.
The new freedom of planing demands a responsible dealing with it.