We all have choices to make about how we travel. Most people who own a car and have free or cheap parking available at either end of a journey are highly likely to drive by default. A large proportion of trips in towns are less than two miles, a distance readily suited to cycling, and for the shortest trips, walking. Buses also offer a viable option on some routes. If we can persuade drivers to switch to other modes for at least some journeys, we can reduce congestion and pollution. To do this, we need to make these other modes at least as attractive as car travel, and to discourage car travel for short trips. Did you know that over a quarter of traffic between 8 and 9am is on the school run? That is why traffic disappears at half term holidays.
In technical terms, traffic is a non-linear phenomenon. A road or junction will flow freely until it reaches about 85-90% capacity. from this point on congestion starts to develop, and rapidly so once 100% is reached. What this means in reality is that reducing the number of vehicles using congested roads at peak times by a relatively small amount can achieve disproportionate benefits in reducing congestion and air pollution.
A famous example of this phenomenon was the Stockholm congestion charge, explained in entertaining fashion in this TED talk on solving traffic jams.
Somerset Air Quality 