As the needle dips towards empty, Welling faces a familiar dilemma -- drive a few more kilometres and risk becoming stranded roadside, or find a place to fill up.

"If it's half full, I fill it up," says Welling, a water resources manager with the city of Santa Monica.

His abundant caution is well justified, since the city-owned car he's driving is a hydrogen-powered Toyota Prius, and the only local service station is tucked away down a dead-end side-street off Interstate 405.

"You only get 35 to 40 miles (56 to 64 kilometres) per tank with this car," says Welling, "so you've really got to stay close to the fuelling stations."

For all the hassle, Welling isn't complaining. If anything, he feels he's part of history in the making.

The Santa Monica hydrogen station is an early outpost on a vast "hydrogen highway" that Gov. Arnold Schwarzenegger hopes will eventually spread from southern California to the Canadian Rockies.

The idea is like something from one of Schwarzenegger's futuristic movies -- a string of refuelling stations where enviro-conscious motorists can fill their zero-emission hydrogen cars on coastline trips from Baja to Whistler.

"People are not going to buy fuel cell vehicles if there is not a place to fill up," says Dan Skopec, undersecretary of the California Environmental Protection Agency. "The governor is committed to help build a hydrogen highway in California and he wants to work with other states and provinces to do the same."

America's greenest governor will promote the ambitious plan next week during a three-day Canadian tour that will take him to Ottawa, Toronto and Vancouver.

Already, Schwarzenegger has found willing partners in British Columbia. Premier Gordon Campbell is forging ahead with plans to build an $89-million "hydrogen highway" that will stretch between Vancouver and Whistler in time for the 2010 Winter Olympics. Last month, he announced the province would use $45 million in federal funding to begin building 20 hydrogen-fuelled buses and the province's first fuelling stations.

Campbell and Schwarzenegger agreed in March to eventually link the two systems through the states of Washington and Oregon.

"I think what this hydrogen highway does is create a totally benign means of transportation in terms of emissions," Campbell said in an interview.

Hydrogen has long been touted as a clean alternative to gasoline because its only emissions are small amounts of water vapour.

And, indeed, most major car companies have prototype hydrogen fuel cell cars in development. Honda recently loaned its latest-generation fuel cell vehicle, the FCX, to a Hollywood actress for everyday use. General Motors will be releasing 100 of its Equinox fuel cell vehicles for market tests this spring.

Still, many experts doubt any of the vehicles - which still cost hundreds of thousands of dollars to produce - will hit sales lots for at least another ddecade.

For one thing, the technology has yet to catch up with the dream.

A key technical hurdle is fuel storage, because hydrogen-powered cars require extraordinarily large tanks to hold the same amount of energy as a regular gasoline-powered car. As a result, most hydrogen fuel cell prototypes have a maximum range of 160 to 250 kilometres on a fill-up, a major inconvenience when refuelling stations are scarce. The test cars used by cities like Santa Monica - which are powered by an internal-combustion engine retrofitted to run on hydrogen fuel - have a maximum range of about 70 kilometres.

The short range of today's hydrogen cars has forced Schwarzenegger to alter his original plan to fund hydrogen stations along California's major freeway. Instead, they are clustered around major urban centres for the convenience of municipal fleets.

"Although they work really well in these urban settings, they are not really ready to go between Sacramento and L.A.," says Gerhard Achtelik, manager of zero-emission infrastructure at California's Air Resources Board, a division of the state's environmental protection agency. "As the appropriate number of cars come out, you can expect the highway to expand."

In the entire state of California, however, only two dozen of a planned 250 hydrogen fuelling stations have so far been built. None, moreover, is open to the general public - which is just as well, since there are no hydrogen vehicles in private use anywhere in the state. Of the 170 hydrogen-powered cars and buses on the road, most are operated either by car companies or city fleets running demonstration projects.

"It's kind of premature to be deploying infrastructure for a hydrogen highway, in part because there aren't any practical cars," says Joseph Romm, a senior fellow at the Center for American Progress and a former assistant energy secretary in the Clinton administration. "And I don't expect there will be for quite some time. So driving (hydrogen-fuelled) cars up and down the West Coast is not something I expect a lot of people will be doing."

By contrast, Romm notes, there are 180,000 gas stations in the United States.

"One reason why people like their cars and drive so much is they don't have to worry they will have any trouble filling up."

Most experts believe the cost of hydrogen-powered cars will come down as technology improves and mass manufacturing capabilities are developed. Even so, opinion is divided on whether consumers will ever warm to hydrogen cars.

Welling, who uses his hydrogen Toyota Prius for work, says Santa Monica residents regularly ask him when they can buy one of the cars.

"This town is pretty green, so people are always coming over and give me the thumbs up," he says.

Others are wary. Rick Sikes, who manages Santa Monica's fleet of five hydrogen-powered cars, says one employee refused to drive the vehicle because he feared it was a "million-dollar bomb."

"I seriously doubt most Americans will want to drive a car where they are one or two feet from a 5,000 pound-per-square-inch canister of hydrogen," says Romm, author of Hell or High Water, which examines the viability of alternative fuels.

Schwarzenegger, however, remains determined to enforce much tougher greenhouse gas emission standards on all vehicles. The governor, who helped popularize Hummers by buying one of the gas-guzzling giants in the 1990s, had General Motors develop a hydrogen-powered H2 version of the vehicle in 2004. Within the next four years, California may require major car companies to produce at least 2,500 hydrogen-powered cars.

"There won't be millions of cars out tomorrow, but we expect thousands in the near future," says Achtelik.

Companies developing hydrogen technology are taking pains to make the vehicles and fuelling stations look as much like regular service stations as possible. The station in Santa Monica, for instance, has a pump with a digital display that shows volume, price per kilogram and total cost of a fill up - even though no city employee pays a dime when filling up.

The hydrogen nozzle fits in the same place as on a gas-fuelled car. The only difference is a second cable, which measures pressure in the storage tank and is attached to a receptacle on the bumper.

"You have to overcome peoples' reluctance to change by making things familiar," says Sikes. "The biggest problem that anybody has is plugging in that extra cable."

In British Columbia, the premier vows the province's fleet of hydrogen buses will be on the road in time for the Olympics and he's cautiously optimistic about completing a link of hydrogen stations to California by 2010 as well.

When Canada's first gasoline station opened in Vancouver 100 years ago, few people believed cars would replace horses and buggies as the dominant mode of travel, Campbell notes.

"Everyone who points out that we don't have a mass-produced automobile based on hydrogen is correct, but it is a technology which has huge potential."

Unofficial Schwarzenegger itinerary:

May 29: Toronto where he may meet Environment Minister John Baird

May 30: Speaks to Economic Club of Toronto at midday. Flies to Ottawa for possible meeting with Prime Minister Stephen Harper and reception at the American Embassy

May 31: keynote speaker of Pacific Economic Summit in Vancouver.