London ? The Hackings, like many of their neighbors, are a two-car family. Every morning, Giles Hacking gets into his Mercedes CL500 in West Kensington and drives to his office across town near London Bridge.

Sarah Hacking piles the three children into the Jeep Cherokee and drops them off at their schools.

Soon, though, multicar families like the Hackings may be wishing all they had to contend with was London’s $8-a-gallon gasoline. In an unusual municipal experiment aimed at fighting global warming where the rubber meets the road, the British capital in October is to begin imposing a $50-a-day carbon emissions fee on every gas-guzzling private vehicle driven in the central city.

The new fee, adopted by the mayor after a long consultation with the public, has prompted threats of a lawsuit from Porsche and anger from many London drivers, some of whom have vowed to make it a central issue in the campaign leading to the mayoral election May 1.

The program has become a test case for major cities around the world. The New York City Council this week voted for a three-year trial program that would impose an $8 charge on vehicles entering Midtown and Lower Manhattan, a plan that still needs approval from the state Legislature.

San Francisco has studied imposing a charge as a way of easing central-city traffic jams; cities in Norway and Sweden also have flirted with congestion pricing.