The car-sharing company Flexcar is planning to expand beyond its Northwest roots and enter the Washington, D.C., market this fall, where it will compete with Boston-based Zipcar. Both companies work to reduce the number of cars on the road by enticing customers to share vehicles and avoid the hassles of car ownership. Customers pay for the amount of time they use a car; the companies pay for gas, insurance, and maintenance. In Seattle, a Flexcar membership ranges from $35 for five hours of driving in a month to $525 for 100 hours. This spring, Flexcar added 29 cars and 560 members in Portland, Ore., by acquiring the company CarSharing. Further south, in the San Francisco Bay area, the car-sharing news is that 17 percent of the region’s 3.5 million vehicle commuters rely on carpools to get to work.