I've been playing around with the what this website, http://www.walkit.com/, can provide. For example when I walk 1.4 miles to the Open University regional office in Portwall Lane (BS1 6ND) from home (BS4 2HX) to deliver a tutorial for work the website shows that I've avoided emitting 0.45kg of carbon dioxide car pollution (or 0.2 kg of bus pollution) - click picture to see all figures. Take a look and see how much carbon you could save or are saving by walking (the site is also useful for route planning, journey times, calories burnt!!).
The site's about section says: We think walking in and around town can often be a smart choice. No timetables to keep to, no journey delays, no overcrowding, healthy, green, free, direct, access to services (and sunlight!) en route.
We don’t in any way pretend that you can substitute walking for all the urban car/bus/taxi/tube trips you make. We do, however, want to at least help you make more informed decisions about whether you choose to walk for all, or part, of any given journey.