Addendum: check out this post on Atlantic Cities by Paul Supawanich, which deals with another side of the equation.
Lyft's expansion into Hampton Roads is creating consternation at the State House. In the past few years, there have been numerous car-sharing innovations, from Zipcar, Lyft, and Uber to websites that allow users to more easily share rides in the same direction. These services are crucial to the future of urbanism in the US.
But some urbanists have been lukewarm. Fundamentally, they say, shared cars are still cars. These critics are missing the bigger picture. Car-sharing services allow vastly more people to give up car ownership. One recent study suggested that Zipcar and other car-sharing services replace 32 private cars with one shared car and have resulted in as many as 500,000 fewer new car sales since their appearance. Each of those private cars was taking up space in the city and resulting in more traffic.
Shared cars take all expenses of car ownership and boil them down to a trip cost. This is a critical change. Car ownership forces high fixed expenditures that become sunk costs for the owners. They've already shelled out for the car, so they might as well drive it. The car reorients its owner's brain towards living and shopping in places that were conveniently laid out for cars, rather than for people.
Think of it this way. Here's all the trips that a hypothetical family living in a relatively walkable urban area might make in 3 months. We'll say... central Winchester. The height of the bar represents the marginal value of having a car for each trip and the trips are organized from most to least valuable.
Once the accumulated value of the trips on the left is enough to justify owning a car, suddenly housing and shopping decisions are made based on the car. Since you're already driving to work every day, you might as well live in a place where you can only get to work via car. And your company might as well build an office that you can only get to via car. And your city might as well design its infrastructure around the fact that none of those employees are driving. And transit options disappear without customers. And now, suddenly, the marginal value of owning the car is extremely high for every commute, every errand, and even every optional trip. Indeed, you risk being unemployed and living in social isolation without it.
The secret of the shared car's ability to reduce vehicle trips lies in the marginal (meaning additional) value of the trips that people are currently taking. Every time you think about going somewhere, you have decisions to make. Do you drive to work or bike today (this is called "mode choice")? Do you run up to the grocery to get an extra box of pancake mix or do you wait until 3 when you'll be at the dentist already (this is called "trip-chaining")?
Some of those trips are extremely valuable - eg running your wife to the hospital when she goes into labor or (to use a less extreme example) being able to visit your cousins in a rural area - so much so that they force you to own a car. While there are lots of costs associated with driving, the biggest ones are fixed (or "capital") costs - the price of the car and its depreciation, the maintenance and upkeep of the vehicle, insurance and taxes, etc.
The costs people weigh when deciding what mode to use are variable costs - often just gasoline. Even these costs are usually underestimated. Maintenance from additional mileage doesn't factor in for most people. And buying gas is such a habit that few people actually sit down before each trip and calculate the cost of the trip.
All that to say, the real financial benefits of not driving only come when one can avoid even owning a car. And the really economically correct choices are made only when all auto-related costs are boiled down into a trip-by-trip cost that the driver can weigh against the value of the trip. Car sharing schemes allow people who are already close to being able to live without a car to finally give up car ownership.
