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HITTING THE ROAD Projections suggest that by 2050 there will be two billion vehicles on the world's roads, two and a half times as many as there are today. The "Car Talk" duo, brothers Tom and Ray Magliozzi, hit the road in search of the car of the future.

running time 8:16 HYDROGEN FUEL CELLS Iceland, with a thousand times fewer people and cars than the U.S., has introduced the world's first hydrogen-fueled public buses in an effort to curb dependence on imported oil.

running time 6:40 TOWARDS BIOFUELS Tom and Ray look at possible new ways to make ethanol. Such "biofuels" have much lower greenhouse-gas emissions but, critics say, come at a cost.

running time 6:34 FRONTIERS OF EFFICIENCY Engines have become more fuel-efficient over the years, but that efficiency has been channeled into more powerful vehicles rather than improving fuel economy.

running time 11:13 PLUGGING IN Can the next generation of hybrid cars cause an energy revolution by allowing us to plug into the grid? Vehicles powered from power plants could substantially reduce greenhouse-gas emissions.

running time 10:22 TAKING ACTION To make the switch to more Earth-friendly vehicles, we need more than visionary thinkers and eco-conscious consumers, experts say. We need concerted government action.

running time 7:07