Three years after unveiling his plan for U.S. energy independence, which won praise from environmentalists for its reliance on wind power, Texas oilman T. Boone Pickens is back with a proposal to convert the U.S. trucking fleet to natural gas. But as his new plan gains traction, questions arise over how green it really is.

Electric cars are finally coming to market in the U.S., but what is the future potential for this much-touted technology? A good way to find out would be to launch demonstration projects in selected U.S. cities to determine if, given incentives and the proper infrastructure, the public will truly embrace plug-in vehicles.

In recent months, several conservative governors have rejected federal funds to begin constructing high-speed rail lines in their states. But a high-speed rail advocate argues that such ideologically driven actions are folly, as other U.S. states and countries around the world are moving swiftly to embrace a technology that is essential for competitive 21st-century economies.

Will electric cars one day become part of a network of rechargeable batteries that can help smooth out the intermittent nature of wind and solar power? Many experts believe so, pointing to programs in Europe and the U.S. that demonstrate the promise of vehicle-to-grid technology.

Flying dwarfs any other individual activity in terms of carbon emissions, yet more and more people are traveling by air. With no quick technological fix on the horizon, what alternatives — from high-speed trains to advanced videoconferencing — can cut back the amount we fly?

For years, the promise and hype surrounding electric cars failed to materialize. But as this year’s Detroit auto show demonstrated, major car companies and well-funded startups — fueled by federal clean-energy funding and rapid improvement in lithium-ion batteries — are now producing electric vehicles that will soon be in showrooms.

Green rankings in the U.S. don’t tell the full story about the places where the human footprint is lightest. If you really want the best environmental model, you need to look at the nation’s biggest — and greenest — metropolis: New York City.

The number of vehicles worldwide is expected to reach two billion in the next two decades. Surprisingly, China where the demand for cars has been skyrocketing just may offer the best hope of creating a new, greener transportation model.

Even as they debut the next generation of hybrids and battery-powered cars, auto company executives are not confident that the American public will buy them.