Oxford St wasn't made for cars (horse carriages rather) and will likely be closed to traffic in a few years. There were some projects in the 70s-80s to make the city more car-friendly (mainly via one-way roads) but they're about to reverse that to slow down traffic. One of the busiest junctions in the city (bank) has already been closed to all traffic except buses during the day.

Having slow traffic helps pedestrians and increases the incentive to use the tube instead of a cab or car.