This one-hour program is divided into six chapters. 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.

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.

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.

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.

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.

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.