As with other cab companies, Yellow’s drivers are independent contractors, and company cabbies who own the vehicles they drive will decide whether to convert them to compressed natural gas, Bewley said. “It will be up to them,” he said.

Yellow is paying for the conversions to compressed natural gas in-house, Bewley said. As many as 500 taxis could be converted at a cost of $10,000 to $15,000 per cab, he added.

Not all company-owned vehicles in the Yellow fleet will go through the conversion process, Bewley said.

There are some 30 vans that carry patrons in wheelchairs that will continue to run on gasoline. The reason: At the moment, nobody does conversions of vans of this type to natural gas, according to Bewley.

Yellow has between 30 and 40 hybrid vehicles that won’t be converted, he added.

As of this week, there were 19 natural gas-powered taxis with permits to operate at DFW, according to David Magaña, the airport’s manager of public affairs.

By comparison, there are about 2,000 cabs with permits to serve DFW, Magaña said. The program giving front-of-the-line privileges to natural-gas-powered taxis—at least temporarily on hold—is still in its infancy, he said.

And to be sure, natural gas isn’t yet suitable for all vehicles.

The tanks that hold compressed or liquefied natural gas take up nearly twice as much space in vehicles than do tanks that hold regular fuel, according to Jimmy Ramsey, business development manager at Clean Energy Fuels, a California company that supplies natural gas used in vehicles by Yellow Cab and Cowboy Cab, among others. Clean Energy Fuels was founded by North Texas energy investor T. Boone Pickens, who remains the largest shareholder of the business.

In addition to taking up more room for fuel tanks, natural gas is harder to find at the pump. There are only nine fueling stations in the Dallas-Fort Worth area for natural-gas vehicles that are open to the public, Ramsey said. One of those fueling stations is located at the front of the line at DFW, and another is near Dallas Love Field.

As a fuel for vehicles, natural gas is “not ready for the everyday driver because you can’t go across America on it,” he said. But as driver fleets switch over to it, other users will follow, he said.

Clean Energy is looking to add another three to five natural-gas fueling stations in North Texas in the next year or so, Ramsey said.