Though these services were invented for locals, car share programs like Zipcar, Hertz on Demand and Car2go can also be useful for renting a vehicle in an unfamiliar destination.

The newest generation of car share services, which allow drivers to pay as they go instead of paying by the day, is now a viable option for both travellers in the US and those in select countries around the world.

This month, the largest of these pay-as-you-go services, Zipcar, popped into the media spotlight when Avis, the global rental car chain, bought the company in a deal valued at $500 million. The acquisition will likely fuel the spread and marketing of Zipcar, and lead to expansion by its main rivals, Hertz On Demand and Car2go. In fact, Hertz plans to make tens of thousands of its 375,000 vehicles in the US available for reserving on an hourly basis by the middle of 2013, according to company spokesperson Paula Riveria.

And while these pay-as-you-go services were created with locals in mind, the industry’s increasing presence in major US and European cities makes them an appealing option for out-of-town visitors. Currently, Zipcar and Hertz On Demand members in one country can book a vehicle in a foreign country using their electronically coded membership cards. Car2go plans to add this functionality by June.  

Unfortunately, you can’t yet join on the spot while overseas. If you’re on holiday but not yet a member of a car-share program in your home country or if your home country doesn’t yet have the car share program, you would need a mailing address at your destination to sign up for an application, plus the patience to wait to receive the membership card in the mail.

For travellers, the main appeal of a car share service is being able to find vehicles scattered around a city at every hour of every day and night. It can also add spontaneity and flexibility to your trip; if you decide to make an impromptu day trip between, say, London and Stonehenge, you can just book a car and go without needing to figure out how to get there by train or bus. Similarly, if you want to quickly cross a city to see a part of town that isn’t easily accessible by public transportation, you can book the nearest available car and go.

The services allow travellers to reserve cars online or via an iPhone or Android app, and immediately pick up or return a vehicle without being limited by the office hours of a traditional car hire company, which may be closed for much of Sunday or have only a handful of locations around a city. In a nice touch, drivers age 21 to 24 don't pay additional fees to use Zipcar, On Demand or Car2go, while most traditional rental companies charge younger drivers a premium. Premiums also don’t rise if you are driving in a foreign country, though you should read the rules on policies carefully.

One drawback is that most car share services have very little availability at airports. In a rare exception, one-way rentals through Hertz On Demand are offered at all three major New York City airports, plus Baltimore Washington International Airport and Regan International Airport in Washington DC. Car2go has some vehicles at airports in Vienna and Hamburg.

Another catch is that car share programmes are not cut out for long journeys, because the services are priced to attract short-term users, not weeklong holiday-goers. After a certain threshold of about 24 hours of continuous use, the bill accumulated at by-the-hour or by-the-minute rate typically exceeds the cost of otherwise paying a flat daily rate on a traditional car rental.

The major services are all on par with each other on price.  All of the companies charge flat rate pricing — often as cheap as $8 an hour— which includes fuel, insurance, and roadside assistance. If a car runs low on fuel during a drive, a user can pay for petrol using a company credit card that is supplied with each car. Basic insurance coverage varies in specifics, but policies typically cover a damage deductible for common accidents of around $750 to $1,000.

Note: these for-profit car share services sometimes have government-run counterparts that can also be used for short jaunts around town, such as Paris’s Autolib', the city’s vehicle-sharing successor to its bicycle hire scheme Velib’. But these city-specific fleets have yet to spread much beyond Paris, making global programs like Zipcar, Hertz on Demand and Car2go better bets for globetrotting travellers.

Here’s a cheat sheet comparing the companies.

With 767,000 members, this is world’s largest car-sharing service. It has vehicles available around the world, with 10,000 parked across more than 500 locations, including 20 US cities, London and five other UK cities, and major destinations in Canada, Spain and Austria. Users pay a yearly membership fee that varies by location and a one-time application fee; rates include fuel and insurance.  

Hertz on Demand
Run by the global rental chain Hertz, this service has cars available in Berlin, London, Paris, Madrid, Sydney and the US (in Boston, Chicago, Denver, Miami, New York City, San Antonio, San Francisco and Washington DC). It has 1,600 vehicles scattered over more than 800 locations and serves 230,000 members, though about half of those members and vehicles are in and around New York City. Unlike Zipcar, Hertz on Demand doesn’t charge an annual membership fee; fuel costs are only complimentary for the first 20 miles driven per day; the company allows one-way trips; and most of its vehicles have been re-fitted with NeverLost SatNav devices that give turn-by-turn directions. Members can earn points in the Hertz Gold loyalty program.

Unlike Zipcar and On Demand, which rent by the hour, Car2go rents cars by the minute. Like On Demand but unlike Zipcar, Car2go permits one-way trips within the same metropolitan area. Uniquely, the 250,000-member scheme doesn’t require vehicles to be returned to designated parking spaces, and only uses a single model in its fleet of 6,000 vehicles worldwide, namely, the ultra-compact Smart fortwo, which some drivers find to be utterly adorable and others consider to be inadequately sized. (Zipcar and On Demand each have a broad range of vehicles.) The company, run by German automaker Daimler, rents 1,750 vehicles across six US cities (Austin, San Diego, Seattle, Washington DC, Miami and Portland, Oregon), six cities in Germany (Berlin, Düsseldorf, Hamburg, Cologne, Stuttgart and Ulm, three in Canada (Calgary, Toronto and Vancouver), plus Amsterdam, London and Vienna. Rates, which include fuel and insurance, vary by country, plus there is a one-time membership fee.

Sean O’Neill is the travel tech columnist for BBC Travel