SAN FRANCISCO – Zipcar and Flexcar, the two largest and only nationwide car sharing companies, are merging. The new company will operate under the Zipcar brand and use its technology.

Car sharing businesses charge membership fees for round-the-clock access to fleets of vehicles spread across metropolitan areas. Members pay hourly, and sometimes per-mile, rates when they drive the cars. Gasoline and insurance are covered in the price.

While the effects of the merger are uncertain, company officials, industry experts, and members said the move could help expand the practice of car sharing.

City CarShare, a nonprofit group, brought car sharing to the Bay Area in 2001, and the two for-profit companies followed four years later. City CarShare has 15,000 Bay Area members who share 230 vehicles; Zipcar has 12,000 members using a fleet of 430, according to company officials. Flexcar membership figures were not available, but the company has a fleet of 200 vehicles in the Bay Area.

Flexcar and Zipcar members said their concerns over shrinking competition would be outweighed by the increased access to cars — both in the Bay Area and in other cities served by the merged company.