Could the No. 1 choice in the continuing search for an alternative vehicle fuel actually be No. 2?

Idaho definitely believes so.

The state, now the United States' No. 3 milk producer with 550,000 cows, is hoping to turn the manure from those cows into natural gas that not only can fire turbines to produce more electricity, but also fuel vehicles.

"We can put together the right package and right mechanism to help move it along," Idaho's energy czar, Paul Kjellander, said. "You've got to have somebody locally who is ready to take the risk and move this forward. But the state can provide the right type of incentives."

Similar efforts are already underway in Texas, Oregon and especially in California, which outpaces all other states when it comes to alternative energy and is home to more than 192 natural gas fueling stations.

The Golden State also isn't limiting itself to only animal waste as a source for fuel. An $8 million demonstration project between the University of California-Irvine and the Orange County Sanitation District is transforming methane from sewage into hydrogen fuel for cars.

If that project is successful, sewage treatment plants throughout the country could one day double as gas stations, giving the term "flushing money down the toilet" new meaning.

"The waste stream from society is being turned around and providing energy and transportation fuel for the society," said Scott Samuelsen, director of UC Irvine's National Fuel Cell Research Center, who helped develop the new fuel-cell device.

Natural Gas Has Limits

But you might want to wait a bit before running out to sell your gas-burning vehicle. The infrastructure to truly help natural gas take off as a vehicle fuel is coming along, but it's not quite there yet.

According to the organization Natural Gas Vehicle for America, there are now more than 120,000 natural gas vehicles and more than 1,100 fueling stations in the U.S.

Sounds great, but most of those vehicles in use by the government or in private fleets and only about half of those stations, which are dwarfed by the 200,000 regular gas stations in the U.S., are open for public use.

While one company, Toronto-based Fuel Maker, has developed an in-home natural gas pump, cutting down on the need for natural gas fueling stations, the equipment costs around $4,000 to have installed.

Perhaps the biggest obstacle to widespread use of natural gas to fuel vehicles is limits of the fuel itself.

Natural gas cars have some significant drawbacks. The lack of stations makes cross-country drives near impossible, they don't have as much driving range as gasoline-powered cars and their fuel tanks take up more space in the trunk of the cars.

There is also currently only one light-duty natural gas vehicle in production, the Honda Civic GX sedan, and that's only available for sale in California and New York.

Existing vehicles can be converted to run on natural gas, but the cost can range between $12,500 and $22,500, according to NGVA.

Benefits Fuel Natural Gas Efforts

So despite its limitations, what makes natural gas such an attractive alternative vehicle fuel?

For one, it produces significantly fewer harmful emissions than gasoline or diesel when used in natural gas vehicles, according to the U.S. Department of Energy.

So much so that federal tax credits can help cut down on the cost of converting a vehicle, and even the cost of that $4,000 in-home fuel pump.

The technology has also proven to be successful in certain areas. Roughly 22 percent of all new transit bus orders are for natural gas, and organizations as varied as delivery companies, airports and garbage haulers are already using natural gas vehicles in their fleets.

UPS is so impressed with natural gas vehicles it now has nearly 1,000 delivery trucks that run purely on compressed natural gas.

"[It is] a sustainable choice because natural gas is a cost effective, clean-burning and readily available fuel," said Robert Hall, director of vehicle engineering for UPS. "The company plans to continue to expand its 'green fleet' and to focus deployments in areas with air quality challenges."

And if T. Boone Pickens, who has bent the ear of President Barack Obama on alternative energy issues, has his way, even more of the vehicles will soon hit the streets.

The billionaire Texas oil man unveiled his "Pickens Plan" in the summer of 2008, calling for developing renewable energy sources and using natural gas as a transportation fuel to help wean the U.S. from its dependency on foreign oil.

"I like renewables because they're in America," Pickens told the Associated Press. "We have to get off of foreign oil. It's easy. The only resource we have in America that can go head-to-head to compete with diesel or gas ... is natural gas."

So perhaps before too long we won't be worrying about the price of oil as we fill up, but rather the rising cost of cow pies.