But it's not just Oxford Street, is it? I live on a big road in London which is one of the many, many streets which come up on survey maps as subject to very high pollution. People don't just use it to jog (they will go into a park, or on the back streets) but to get from A to B.
Same road, come evening rush hour, is gridlocked, full of cars with only one person in them. (And yes, some will totally NEED to use cars for whatever reason, but not all of them.) My DS comes home from school on the bus on the same route, and recently it's been quicker for him to walk, the traffic is so bad. (It's not a short walk, and he's carrying a stupidly heavy school bag after a long school day, and with homework in front of him.)