It's the cars! So many people have their pet problem to itch of ___ while driving. Driving on your cell, driving and texting, driving drunk - in some US states they even banned driving and talking! They banned driving under 18 with friends in the car. Driving is obviously the common theme here. The fact is a car is between 2000-7000lbs of metal moving 5-20x faster than you can run. Allowing them to become something everyone uses to accomplish a fundamental task like get to work has guaranteed we put fast-moving, fatal machines in the hands of poorly trained people, sometimes in poorly thought-out situations, and in some cases, just plain idiots.
This is especially stupid in London where there are such great alternatives to driving.
In the US, Boston has taken the right approach - make it impossible to drive fast on local roads and bury the highways. This puts the dangerous vehicles out of the way of pedestrians and buildings where they can do damage. The only remaining risk is to the drivers themselves, who put themselves at the peril of everyone else choosing this dangerous method of travel over Boston's excellent subway system. Few sane residents of Boston elect to drive in that environment.
Stop making stupid laws about driving and just bury the expressways in populated areas. Laws saying "Stop that" are the easy, cheap way out, and they'll never work.