The hassle of booking and getting to the shared car may push some to use public transport instead
Is car-sharing green? Zipcar, a car-sharing scheme in the United States, claims that 90 per cent of its members drive 5,500 or fewer miles per year after joining the service -- PHOTO : BLOOMBERG

Washington - Getting around town used to be a pretty straightforward proposition. Those who could not (or chose not to) rely on leg power could take public transit, hail a cab or drive their own car. Things are slightly more complicated now.

Smartphone-based driver services such as Uber are challenging the traditional cab system. People are buzzing around cities on Segways.

Two popular alternatives are scooters and car-sharing services such as Zipcar and Car2Go. Price and convenience are obviously a factor in the increasing popularity of those options.

But some urban dwellers are moving away from car ownership for environmental reasons. If you are selling your car to green up your life, you want to pick the most environmentally friendly alternative.

So which is it: owning a scooter or sharing a car? A scooter's tailpipe emits less greenhouse gas than that of a car, but the difference is not as great as you might think. The average 50cc scooter weighs about 200 pounds (90kg), compared with more than 3,000 pounds for a car.

A scooter's fuel-efficiency advantage is not quite that dramatic, though, because cars have a lot of technology to manage gasoline efficiently. As a result, a typical scooter gets around 85 miles per gallon, compared with 28 for the average passenger car. A car emits 0.93 pounds of carbon dioxide per mile, compared with 0.31pounds for a scooter.

The greenhouse gases embedded in a vehicle - that is, the pollutants that result from producing, transporting and eventually disposing of it - can often be as significant as the tailpipe emissions. According to a 2010 study, manufacturing a typical gas-powered scooter produces approximately 650 pounds of greenhouse gas, and maintenance over its lifetime adds an additional 450.

Environmental analysts typically combine these embedded impacts with those of actual operation, then express the product's greenhouse gas emissions on a per-mile basis.

Assuming a scooter travels a little over 30,000 miles (4,828km) in its lifetime, it produces about 0.4 pounds of greenhouse gas per mile, 30 per cent more than tailpipe emissions alone.

Electric scooters are a better option. They generate 0.14 pounds of greenhouse gas per mile, according to that 2010 study.

Analysing car-sharing services is a far trickier proposition. Environmental researchers have conducted many analyses of passenger vehicles. One of the more commonly cited figures come from a 2008 PhD thesis; it found that passenger sedans produce approximately a pound of greenhouse gas per mile travelled.

But here is the problem with such analyses: They all assume that you drive the same number of miles, whether you have your own car or share one with, say, 19 other people.

By that reasoning, a group of 20 people who each drive an average of 5,000 miles a year would wear out a single car after about a year; over 20 years, the group would go through 20 cars. That is no different, the analysts figure, from 20 people who each own a car: They would also wear out 20 cars in two decades.

In either case, you would need to manufacture the same number of cars and you would cause the same environmental impact. In other words, a car produces 21/2 times as much greenhouse gas as a conventional scooter and seven times as much as an electric scooter, no matter how many people share it.

I suspect that the analysts' model of car usage is not quite how it works in the real world.

Zipcar notes that 90 per cent of its members drive 5,500 or fewer miles per year after joining the service, implying that this is a reduction from the average car owner's 13,476 miles. That is a shaky claim, but it stands to reason that people who own a car drive more than people who share one, just as TV owners watch more football than people who have to go to a bar to do so.

The book Poor Economics argues that one of the most important features of life in the developed world is a system that makes it more difficult to make bad choices. Milk is pasteurised, so the consumer does not need to decide whether to remove its harmful bacteria. Schools require students to be vaccinated, or at least make parents jump through hoops to avoid vaccinating their kids. These defaults are priceless.

A similar system - in which the default choice is a green one - may be good for the environment. Many new thermostats, for example, automatically lower the heat when we leave home, even if we forget to do it ourselves.

Car sharing is another example of this. When you own a car, the easiest way to get anywhere is often just getting behind the wheel. But if you do not own a car, the hassle of making a reservation and walking to the lot where the vehicle is parked may be enough to shift the advantage to walking, biking or public transit. Or, indeed, to a scooter.

Knowing your own lifestyle tendencies may be the key to finding the best solution. If you genuinely need on-demand transport, a scooter might free you from dependence on a car.

But if you are a car junkie who drives to the gym, the park and the bank just because you can, you might go for the car-sharing service as an exercise in tying yourself to the mast.

Washington Post