The pain of gas prices and a desire to do good for themselves and the environment are pushing more people to ditch their personal wheels and rely on car-sharing.

The number of Denverites using car-share services has doubled in the past year — about 2,100 people were registered by last week — forcing coordinators to scramble to make enough vehicles available.

Car-sharing expanded by 16 percent nationwide in the past year, with 27 programs and more than 518,000 members registered to use 7,770 vehicles parked on city streets from New York to San Francisco.

Car-sharing companies cover insurance, gas and maintenance costs. Members pay a fee of about $10 a month and receive key fobs that contain radio chips that open and start cars that have been reserved by phone or online. Members also pay hourly fees — $4.50 to $6 in Denver.

Two car-share companies have positioned 46 vehicles around central Denver and Boulder. In Denver, they are haggling with parking authorities for a break on incessant ticketing.

Car-sharing coordinators are struggling to meet widening demand, and available cars aren’t always nearby when members need them.

“I need to take my mom to DIA,” Ann Scheerer, 49, puffed this week after pedaling a shared B-cycle from her Highlands home to a car-share Prius downtown. “Up until last month, I could always get a car. Now I’m having to scramble.”

Denver cartoonist Sean Heffernan, who recently bid adieu to his 1997 Ford Ranger, said he found romance thanks to his OccasionalCar subscription.

Chatting with a woman over coffee in Whole Foods, Heffernan, 52, “was feeling insecure that, because I don’t own a car, I’d look bad,” he said.

“Her eyes lit up” when he confided he’d turned to car-sharing, and she accepted his invitation to go out on a date in a shared Honda.

Two red eGO CarShare Prius hybrids parked in central Denver — one behind the Lower Downtown Tattered Cover and the other near the zoo — are the latest additions.

“We wish we had more cars,” said Karen Worminghaus, president of the nonprofit eGO. She increasingly fields calls from residents of outlying neighborhoods seeking service.

Susan Shaheen, co-director of the Transportation Sustainability Research Center at the University of California at Berkeley, said there is a delicate balance to car-sharing.

“When cars are spread out too thinly, you’re more likely to have people not able to get a vehicle as quickly as they want,” Shaheen said. “The risk is you might lose customers and not be able to cover costs.

“Is this a revolution?” Shaheen asked. She said it could be “a transitional phase to different mobility services,” such as bicycles, buses and light rail.

The Denver company OccasionalCar now has 925 members, up from 450 last year, with 18 vehicles in Capitol Hill and Lower Downtown, said president Russell Straub. He said he plans to add at least one car per month as membership grows by 30 to 40 drivers a month. “We’re definitely accelerating,” he said — and barely covering costs.

Until recently, Denver members could count on quickly finding cars, typically parked in a regular, rented spot. Rebekah Kik and her partner run the Two Tarts bakery from her Capitol Hill home and use an eGO Honda at 11th and Ogden for deliveries.

“It’s a lot easier to pay $10 a month and a few dollars an hour to do a delivery, rather than pay overhead of $350 a month,” Kik said.

But the Honda was gone one morning last winter, forcing her to ride a bus across town to a Prius at 16th and Market to deliver her pound cakes and swing by a grocery for flour.

Some of the converted say ditching their cars was more about lifestyle than money.

“I feel more a part of this place, the texture of it, the smell of it, the sounds,” said architect Marcus Bushong, 40, who took the 16th and Market Prius last Monday to go clothes-shopping and again Tuesday for a job interview.

“We’re getting more and more disconnected from the physical world,” living through computer screens, Bushong said. Extra walking, from his apartment to that Prius, helps ease his mediated-experience malaise.

State employee Carrie Grigg, 32, tried car-sharing simply for fun, “to see if I could go green,” and got hooked.

On lunch break last week, racing to return linens from her condo to Target, she reached her preferred Prius, half a block away, minutes after making a reservation.

“If they ever move it, I’ll be sad,” she said, waving her key fob near the door. “Until this reaches a threshold where I’m not able to get a car downtown, I’ll stay with it.”

Bruce Finley: 303-954-1700 or bfinley@denverpost.com