If we sincerely want cleaner transportation, then we need other power sources. On a one-lane road, the slowest driver is always in front. This is not a Murphian corrollary, but a kinematic fact. A Humvee is what a Jeep looks like when designed by a military committee. Fighting traffic congestion by widening the road is like fighting obesity by loosening your belt.
Electic motors are compact, powerful, controllable, and quiet.
An appropriately designed electromotive system needs no clutch or multi-ratio transmission, and it can recapture part of a vehicle's kinetic energy while braking, returning that energy to a storage battery as electrical charge. Electric vehicles will need better batteries or other energy storage devices. Fuel cells may be the answer. Solar charging may help in consistently sunny locales. Steam is not as silly as it may seem.