The site told me that walking from my flat to work at a medium pace would burn 326 calories, and that the same journey in a car would output 1.90 kg of carbon emissions.
From the site, "We think walking in and around town can 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.".
It's London-only at the moment, which makes sense because the city is more complex than US cities (none of this block system for us), so it's harder to get from A to B by foot. In a city where a 400 metre, one way journey can cost £4 ($8), I think that this site will soon gain a lot of users.