Suzanne DeChillo/The New York Times

The Bloomberg administration has aggressively expanded New York City’s bicycle infrastructure, creating hundreds of miles of bicycle lanes and encouraging residents to try out the cleaner, often speedier mode of transport.

On Thursday, the city said that the number of regular city bicycle riders increased by 8 percent this year, extending a decade-long trend that has nearly quadrupled the number of regular cyclists since 2001, according to the city’s Department of Transportation.

Clicker-carrying counters hired by the city ticked off the number of cyclists at six locations: the Manhattan side of the four East River bridges, the Hudson River Greenway at West 50th Street, and the Whitehall Terminal of the Staten Island Ferry in Lower Manhattan. The counts are taken 10 times a year, on weekdays between 7 a.m. and 7 p.m.

In the context of a city of eight million residents, the numbers are small: on average, the city recorded 18,846 cyclists a day this year at those six locations, up from 17,491 riders in 2010.

But in 2001, that figure was under 5,000. In 2007, it was about 9,300.

A few critics, including some bicycling advocates, have expressed concerns about the city’s figures, saying they are difficult to extrapolate for the city as a whole. John Pucher, a professor of planning at Rutgers University who is a fierce advocate of bicycle- and pedestrian-friendly cities, said he believed bicycling was on the rise, but that the city’s survey overstated the size of the increase.

“New York City D.O.T. is only picking those spots where bike commuting is increasing the most,” he said, and leaving out the Bronx and eastern parts of Brooklyn and Queens. “I happen to agree with their policies,” he added, “but their numbers are not representative of the city as a whole.”

The Transportation Department, in its official report on Thursday, took pains to say the counts represented trends in cycling habits, but were not an estimate of the total number of cyclists in New York.

A survey by the Department of Health and Mental Hygiene found that about 500,000 city residents ride a bicycle a few times a month, if not regularly.

Cycling appeared to be most popular in August, according to a month-by-month breakdown of the city’s data. (The full report is available as a PDF.) The bike path along the Hudson River attracted the most bicyclists of the six sites, and more riders were recorded at the Manhattan and Williamsburg Bridges than at the Brooklyn and Queensboro Bridges.

The expansion of bike lanes has coincided with a drop in pedestrian fatalities, and the city said bicycling-related injuries and deaths have not risen despite the growth in ridership.

Janette Sadik-Khan, the transportation commissioner, said in a statement that the city’s streets must “keep pace with new demands,” and she announced that her department would install 175 new hoop-shaped bike racks, made of ductile iron, across four boroughs, using the stumps of decommissioned, decapitated parking meters. The city has solicited vendors to manufacture an additional 6,000 racks, which could be on the way soon.