What affect will it have on the environment?

Studies have so far not conclusively proven either a positive or negative effect on the environment: driving at 20 mph causes some emissions to rise slightly and some to fall. The greatest environmental benefit from the change will come from unlocking the potential for walking or cycling short distances instead of driving. There are 45,000 car trips to work each day in Bristol that are under 3 miles in distance. Adopting a smoother driving style can also achieve lower emissions.