At truck stops spread across the nation’s highways, they’re popping up like all-night diners and caffeine pill displays.

Brand-new natural gas pumps sit mothballed alongside their diesel counterparts, waiting for enough trucking companies to sign on that it makes sense to flip on the switch and begin delivering fuel.

But Jim Harger, marketing director for Clean Energy Fuels, the U.S. leader in natural-gas filling stations, insists executives are not worried about the 70 pumping sites his company installed last year.

“The question was always: Is it the trucks first or the stations?” he said. “If you look at the infrastructure we placed, it helps to solve the so-called chicken-and-egg problem. We are maybe a year out ahead, but it won’t take long to get enough critical mass to open the stations.”

Already the fuel of choice for new power plants, natural gas is considered a potential replacement to diesel fuel within the transportation industry.

Garbage trucks and city bus fleets are already shifting over in significant numbers.

And in the past six months, trucking executives have been jumping over each other with announcements. Peterbilt began full production on a natural gas model at its plant in Denton, Texas, last month, and companies including Procter & Gamble are pledging to shift their trucking budgets away from diesel.

But with firm numbers on orders hard to come by, questions remain about how deeply invested the industry actually is.

There have been exceptions. In June, UPS, the largest trucking company in the country, announced that next year it will only buy natural-gas trucks for its U.S. small-package business — the bulk of its operations.

That, along with deals with other trucking companies, will allow Clean Energy to begin operations at truck stops in San Antonio, Mesquite and Amarillo within the next three months — in addition to operations already under way in Dallas and Houston, Harger said.

The drilling boom in fields like the Barnett Shale and Marcellus Shale brought natural gas prices down to historic lows last year. They have rebounded since but still remain a fraction of what they were in 2008.

T. Boone Pickens, the billionaire energy investor and Clean Energy’s largest shareholder, said that with the large shipping companies signing on, it would only be a matter of time until small companies followed suit.

“Natural gas is going to be much cheaper than diesel for a long time,” he said. “If I’m a competitor of yours and my fuel costs are half yours, I’m going to win. It’s so simple you ask yourself why wouldn’t you do it.”

But so far, the myriad small firms that make up the trucking industry at large have been slow to join in.

The cost of buying a natural-gas truck can run up to 40 percent more than diesel. And companies are worried about factors beyond fuel costs. These include additional weight that could force them to cut their loads and the resale value on a $160,000 natural-gas truck, should the industry not expand as advertised, said Glen Kedzie, energy and environmental counsel for the American Trucking Associations.

“For the most part, you’re not seeing huge orders coming in. People are dabbling with three or four trucks here and there,” he said.

“This is really the first alternative to diesel we’ve seen. There’s been discussion over the years, but it never went anywhere. … That’s not to say (natural gas) is not going to happen in the future, but it’s cautious optimism.”

In April, truck engine manufacturer Cummins began production on a natural gas engine believed to be a breakthrough for the industry. But the company has not released data on how many engines have been ordered, and a spokesman said the company was only projecting 5 percent to 10 percent growth in the next five to 10 years.

“Natural gas continues to be a small part of our business,” Jon Mills, the spokesman, said.

Dillon Transport, an Illinois-based trucking firm with a hub in Dallas, began running natural-gas trucks early last year. The company now counts almost 10 percent of its 420-truck fleet on natural gas, with more on order, but has been careful not to overcommit, said marketing director Phil Croft.

So far the company has limited use of the trucks to shorter routes, such as one moving limestone and sand from Dallas to an asphalt plant in Waco, Texas, a trip that does not require refueling.

“Our (long-distance trucks) are still diesel. We’re worried they could get too far from a station, and we don’t want to convert everything all at once. But we’re sold on the concept,” Croft said.