I was part of this programme as the cyclist riding around Bath http://www.bbc.co.uk/programmes/b08h4m30

The findings were that the intensity of the exposure was less of a problem. It was the length of time you were exposed. So sitting in a car, even with car filters, pretty much put you at greatest risk. Someone on a bicycle just moved through the congestion quicker. Sadly the walker dropped out of filming. I would suspect walking along busy routes should be avoided due to the speed at which you are traveling, but is still better than sitting in a car.