CAR-SHARING services, which let people use any car in a fleet, are relatively old hat. The earliest were founded in the 1990s. Most follow the same rules: members must return the vehicle to the place where they picked it up—inconvenient if you only need to go one way. Not so car2go. Unlike competitors such as ZipCar, car2go's automobiles may be left at any unrestricted street-parking space in a given city.

Daimler, the German carmaker which owns car2go (and produces the two-seat Smart hybrids in its fleet), launched the service in Ulm in March 2009, and began its roll-out in earnest in late 2011. Car2go recently set up shop in Seattle, Babbage's haunt. The firm enrolled 18,000 members in the city during its first 90 days, when it waived the £35 registration fee and offered some free service, too.

The firm has set its boundaries to cover all of Seatlle except some of its outermost reaches, as well as the University of Washington's campus (which has its own parking rules). A car may be parked temporarily anywhere during a trip, including far outside of the home area, so long as a member ultimately returns it to any point within the local zone.

In the United States, car2go has no minimum recurring charge (again, in contrast to ZipCar). Members who join in one city may drive cars elsewhere in the same country, though not yet across borders. The usage rate is $0.38 per minute up to a maximum of $13.99 per hour, or $72.99 per day. Cars may be driven any distance, but a fee of $0.45 per mile ($0.28 per kilometre) is added after 150 miles (240km). A 17% tax is levied on the fees.

A car may be reserved up to 30 minutes in advance at no cost, but you may also pick up any available car without prior notice, so long as no one else has booked it. Until a trip is formally ended, charges accrue whether the vehicle is parked, idle or in traffic. As with other car-sharing services, petrol, maintenance and insurance are included, making them a better deal for those with no car—and thus no separate insurance bill—than those with existing vehicles.

A car2go vehicle may remain at any two-hour or longer kerbside parking spot, whether it is a metered space or a free one. The company pays parking authorities to let its cars remain in on-street parking spots indefinitely, although the rules as to which spots qualify vary by town. In Seattle this amounts to $1,300 per car each year. Users still need to pay for parking in shorter-duration spots.

By deploying plenty of cars—322 in Seattle at the last count, with 250-500 in most other cities except Berlin, which boasts 1,200—and enabling one-way trips, the service allows taxi-like flexibility. People can, say, drive to a meeting point in separate vehicles but car-pool home, or use a car when hurrying to a meeting but take a bus back home. The firm's boss, Nick Cole, says it has considered joining forces with an airport parking lot, as it did in Austin, Texas, last month. This would undercut taxis, buses and park-and-fly drivers.

In principle, the service is predicated on the idea that users will shift cars about in such a way that they do not all end up in one place. This appears to be true in practice, too, although some businesses in several cities in which car2go operates have complained about its vehicles being legally parked for days at a time, displacing customers.

Your correspondent used the service for a recent trip to his doctor, where he would remain for an unknown period while having tests and waiting for results. Before departing, he checked car2go's real-time map, and spotted a vehicle a few blocks away. He also checked about the quantity of cars around his destination. A 10-mile trip mostly by highway set him back roughly $5 in one direction and $10 for the congested return journey. A cab ride would cost at least $29.50 in good traffic each way. Driving there yourself would cost in each direction $6 between petrol and a portion of depreciation and insurance, according to current mileage reimbursement rates set by the Internal Revenue Service.

Babbage released his car on arrival at the doctor's office. When his appointment was over, he walked a few blocks for lunch, and then checked the map again, this time through the firm's smartphone app. The same car happened to be free, as were four others within a short walk, and he used it to return home, parking it across the street from his house. Within an hour it was gone again, presumably picked up by another punter.

Some kinks need ironing out. The smartphone app, for instance, cannot be used to lock or unlock cars or enable a trip, even though all vehicles have a mobile broadband connection (a special card and a four-digit PIN are required instead; the key sits out in the open inside). Its satellite navigation technology has a distinct whiff of a five-year-old gadget. Nevertheless, car2go is a welcome addition to the "multi-modal" transit system that Seattle has stitched together in order to enable seamless travel by a combination of car-sharing, buses, commuter rail, light rail and streetcars. And another fine example of the inexorable rise of the sharing economy.