Dockless bike-share services attracted a lot of attention when they rolled out in U.S. cities last year, but they still have a long way to go before they overtake traditional, station-based systems in ridership.

Dockless bikes made up 44 percent of all shared bikes on the street last year, but accounted for a mere 4 percent of the trips. In fact, the bikes in station-based systems are used five times as often as those in dockless systems.

That’s according to a new report from the National Association of City Transportation Officials (NACTO), which noted that dockless bike-share companies are now operating in at least 25 U.S. cities. “The large influx of dockless bike shares across the U.S. has not yet translated into substantial mobility gains,” the researchers wrote.

The low ridership numbers could justify the relatively cautious approach many cities have taken with dockless bike shares. Indeed, the first operator of dockless bikes in the country, BlueGoGo, has already filed for bankruptcy. "The extreme degree of venture capital funding, coupled with generally low ridership," researchers wrote, "brings questions as to the overall sustainability and volatility of the dockless bike-share market."

NACTO further noted: “Cities are proceeding cautiously, watching the results of pilot efforts, and encouraging dockless companies to share more data so that cities can better evaluate and understand how dockless bike share can further city goals of safety, equity and sustainable mobility."

There is good news for bike-share services overall, though. Ridership throughout the country grew by 25 percent between 2016 and 2017, with 35 million trips taken. “In just a few years, bike share has proven successful in an ever-widening collection of cities,” said Linda Bailey, NACTO’s executive director, in a statement. “Cities continue to build streets that provide safe places to ride, and people are responding by biking by the millions. Bike share is increasingly an integral part of the day-to-day transportation mix, and we see no end in sight to this flourishing new mobility option.”

The introduction of dockless bike-share services more than doubled the number of shared bikes on U.S. streets, from 42,000 in 2016 to 100,000 in 2017.

The key to increasing dockless bike-share ridership may be in how the services are used. The NACTO report found some evidence that the riders of dockless and station-based bike-share services use them differently. In Seattle, for example, nearly half of all trips from station-based services are made during rush hours, and three-quarters happened on weekdays. “In contrast,” NACTO researchers wrote, “dockless bike share in Seattle has an evening peak but no morning rush hour peak and trips are spread out over the day with highest use seen on weekends, suggesting more recreational use.”

The NACTO report also found:

• Several major cities introduced dock-based bike shares last year. Those include Honolulu, Detroit, New Orleans and Charleston, S.C. Honolulu's service is now the eighth most popular in the country.

• Station-based systems in Boston, Chicago, New York City and Washington, D.C., made up three-quarters of all trips taken nationally, the same share as they had the year before.

• A third of all bike-share systems offer discounted passes for low-income riders.