In my opinion developing this project is a must for any good ecologist as you. Indeed cycling around the city would further contribute to preserve environment if we find an alternative use of consumed or punctured tires.
This is not an original idea, recently you can buy these belts in fairs and markets, and six years ago opendesignclub wrote how to make tread belts (you also could read his instructable).
I used some thinner bicycle tires for my project, the one you use on your technical racing bicycle, or on your stylish fixed gear one. Usually these tires are also coloured and they could have stripes along all the tread, so you can obtain very trendy belts. Don't worry if the tread is not in good conditions, that's exactly the purpose of this project, and only if you're so lucky to cut your tire right away with a glass you will have a brand new belt. In this instructable I describe how I made four different belts from one very consumed coloured tire, two pierced black ones almost new, and a wrecked mountain bike tire.