Capital Bikeshare is growing and is opening more stations. This one is at Fifth and F streets NW in Washington. (Sarah L. Voisin/The Washington Post)

Washington’s Capital Bikeshare system has been in business for nearly five years. Barcelona’s bike-share program opened in 2007. And New York City’s relatively nascent Citi Bikes launched in 2013.

There were just 13 bike-share systems in 2004. Jump to 2015, and there are more than 855 systems worldwide, everywhere from Finland to Australia to China, which, at 237 systems, has more bike-share programs than any other country.

Now that many of these systems have grown out of their infancies, there is more extensive and reliable data about who uses them and whether they are an effective alternative to cars and traditional modes of public transit.

In a report for the London-based journal Transport Reviews, researcher Elliot Fishman, director of the Institute for Sensible Transport, examined data and studies on large bike-sharing systems worldwide and found that no matter where the programs are, convenience is the driving factor in whether people use them.

D.C.’s Capital Bikeshare surveyed more than 11,100 members and found that 69 percent of respondents said getting where they need to go fast was “very important” in encouraging them to use the program.

The distance between a rider’s home and the nearest docking station also is key to motivating people to pay for a membership. Montreal residents living within three-tenths of a mile of a docking station, for instance, were about three times more likely to have used that city’s bike-share program. This sort of data, according to Fishman’s research, explains why a system’s success is partly contingent upon the density of the network.

Fishman also looked at who is using bike-share programs. In general, people who use bike-share are more affluent and better educated than the general population, although typically they earn less than cyclists who don’t use the service, according to Fishman.

In London, early bike-share users were overwhelmingly affluent, but between 2010 and 2013, the percentage of low-income residents using the system doubled from 6 to 12 percent, Fishman said.

In the District, the income difference between Capital Bikeshare users and the general population is striking: Fifty percent of members had an annual household income of more than $100,000, according to Capital Bikeshare data. By comparison, census data shows that the region’s median household income in 2010 was $84,523.

Bike-share users also tend to be white. In the District, for example, only 3 percent of users identify as African American, despite the fact that African Americans make up nearly 50 percent of the population, according to ridership data from Capital Bikeshare.

So why should cities strive to address socioeconomic and racial gaps among users?

For one, there are the obvious health benefits: In London, bike-share has resulted in an additional 74 million minutes of physical activity for residents.

And Fishman writes that most people use bike-share as an alternative to other forms of public transportation, potentially reducing crowds in transit systems.

But not as many people are using bike-share to replace cars, which has the potential to be the programs’ greatest benefit. Capital Bikeshare says 55 percent of respondents in a 2014 survey said they used a car less frequently since joining bike-share, although it’s unclear how often they opt for a bike over a car.

Fewer cars and more bikes on the roads mean less congestion, reduced air pollution and a more physically active population. Fishman says researchers need to spend more time analyzing why some people don’t participate in bike-share and identifying potential barriers for non-users. This research, he says, can help fix underutilized systems and help guide the building of new ones.

“Bike-share’s performance in replacing motor vehicle trips has been less than most expected, and future efforts to transfer trips previously done by car to bike-share will help underpin the potential benefits of bikeshare,” Fishman wrote.

Clarification: This story originally compared the income of Capital Bikeshare users to D.C.’s median household income. Because Capital Bikeshare is present in the Maryland and Virginia suburbs, it would be more accurate to compare bikeshare users’ incomes to that the median income in the entire region.