The quickest path to cleaner air is through drivers' wallets. At least that's the philosophy behind several new efforts to promote eco-conscious travel—and fight gridlock—by making commuters foot the bill. Last September, the Golden Gate Bridge raised its basic toll for cars to $5 from $3, but drivers of certain low-emissions cars pay nothing when heading into San Francisco during peak commuter periods. In Seattle, a federally funded "congestion pricing" test begins this year: 400 families will each receive a monthly stipend to pay for tolls, and their driving routines will be tracked by satellite. If they use heavily trafficked roads, they'll be charged more, but if they drive during off-peak times or on less-congested roads, they can keep the remaining money at the end of the month. Lastly, in February, London became one of the first European cities to make drivers pay to enter the central part of town during peak hours—part of Mayor Ken Livingstone's ambitious plans to reduce traffic and smog.