Since I work for a car sharing organization, people often ask me what makes a city or neighborhood ideal for car sharing. While certain factors are easily measurable or obvious (density, walkability, and mixed use development), others are a just as important but not as apparent. I’ve come up with three such “soft factors” (soft because they are not hard measurements which can be gleaned, say, from census data). These seem to be quite indicative of whether car sharing will thrive, and seem to be good for creating livable cities as well—as long as livability is not intertwined with car ownership.

They are the availability and cost of parking; the frequency, reliability and speed of a transit network; and the prevalence of urban congestion.

1. The cost and availability of parking. Owning a car is expensive. However, once you start paying for parking, you’re throwing money at little more than a 100-square-foot plot of ground for your car not to drive. Once this cost gets over about $100 a month, it contributes significantly to lower car ownership. Enmeshed with this factor is the availability of parking. It’s almost always possible to find street parking if you look hard enough. But if you have to circle a block six times, jockey your car in to a tiny spot, and/or move it every third day to the alternate side of the street, it makes car ownership more of a burden than a freedom.

Cities where car sharing thrives are not cities where it is easy to find a parking space. One of the major reasons car sharing took off in cities like Boston, Philadelphia and San Francisco is that they were able to advertise that their cars always had “reserved parking,” a godsend for residents who had to deal with expensive private lots or arduous on-street spaces. All of the sudden, they could take a two hour car trip, get home, and not have to worry about how many blocks away the nearest spot would be. Or, if they gave up their private spot, they might find a couple grand in their pocket at the end of the year.

2. The frequency, reliability and speed of a public transit network. The three adjectives here generally go hand-in-hand-in-hand, with the exception of a minor explanation regarding speed. Speed is relative. Sure, antiquated subways in Boston, New York and Chicago may creep along through ancient tunnels or els, but compared with the gridlock above (or below)? Well, private right-of-ways do have their advantages. And are they reliable? Well, about as reliable as highways which, at any time, may devolve in to a traffic jam.

The most important piece of the transit puzzle seems to be frequency. Or to put it differently, “can you walk to the nearest bus line and get on a bus without knowing a schedule.” This generally means that most lines should have midday headways of 15 minutes or less. And while grade-separated, rail transit carries a large fraction of riders in many of these cities, reliability and frequency seem to be more important to car sharing than the exact mode. Seattle, for example, was until a few months ago a bus-only transit system (We’ll ignore the monorail and one-mile streetcar.) and the new light rail line doesn’t serve many high-car sharing neighborhoods. Still, most lines run every ten or fifteen minutes all day and in to the evening, and while they’re not particularly fast, they come pretty often.

Do a lot of car sharing users walk or bike? Yes. But if it’s raining, or cold, or they just want to make use of transit, the ability to walk to the corner and not have to wait 25 minutes reduces the need and desire to own a car. (Especially when it might take that long to find a parking space; see factor 1 above.)

3. The prevalence of urban congestion. This is probably the most confusing of the three factors, since I don’t mean congestion on freeways leading in to the city in the morning and out in the evening. What it refers to is the prevalence of random traffic jams and tie-ups. In other words, how often during non-peak periods (middays, evenings and weekends) is there horrible traffic for no apparent reason? How often do you get in your car and, because a lane has been blocked off or a light has malfunction or an inch of snow has fallen, a trip that should take ten minutes takes half an hour? How often do you sit and watch a light a quarter mile ahead and realize that there are 40 cars ahead of you and only two are making it through each cycle? And how often is there some event—a parade or a race or a visiting dignitary—which so screws up the traffic system that no one in their right mind would drive downtown?

In cities which support car sharing, everyone’s had the experience of sitting in traffic on a Saturday afternoon for, well, no apparent reason. Urban congestion is not just that there are too many cars on the road, but that they are dynamic urban environments which sometimes don’t mesh with the automobile. If one small protest or minor accident closes off a main street corner, it can cascade across the street network, creating gridlock at a time it’s not expected. Of course, as anyone driving in any of these cities knows, there’s no time when there’s never been traffic.

Are these the only three factors which contribute to a dynamic car sharing market (or, in other words, make owning a car so unpalatable that many people do without)? Of course not. Also important are population and employment density, walkability (which has to do with these factors) and, to a small extent, the availability of bicycle facilities, the cost of gas, planning ordinances, physical geography and the like. But, from what I’ve seen, these are some of the most important factors, and they not only create a city with good car sharing prospects, but one in which people actually want to live.