Car2go, the car-sharing service offering one-way trips in cities, needed five years to sign up its first million members. The second million required only 21 months.

Car2go, a Daimler subsidiary that launched in 2009, announced Tuesday that it has reached the two million member milestone. Every 1.5 seconds a member takes a trip in one of its 14,000 vehicles in 30 cities across nine countries.

The company doesn't say if it's profitable, but the rapid uptick in membership points to a broad enthusiasm for the sharing economy, and a shift in how the world accesses transportation.

Related: How car-sharing is already helping cities with their transit issues

With Car2go, customers paying a one-time $35 membership fee can rent a vehicle parked on a nearby public street and leave it elsewhere in their city. There's no need to reserve a vehicle ahead of time, or return it to a designated parking lot. Car2go cuts deals with local governments so that its vehicles can be parked in public spots.

The company began service in its first Asian city, Chongqing, in April. The interest in China has been unprecedented -- Car2go has signed up 113,000 customers in less than six months. The membership base in Chongqing has nearly surpassed Car2go's fifth most popular city, Vancouver, which launched in April 2011.

The company's most popular cities are Berlin and Hamburg, both of which have more than 130,000 members.

Car2go's fleet of vehicles are mostly two-seater Smart cars. They leave little room for cargo, but are easy to park in cities. The company has branched out some, adding four-door Mercedes-Benz sedans and SUVs in its European fleet.