Based on figures from an annual count of bicyclists conducted since 1984, the New York City Department of Transportation announced on Thursday that commuter cycling rose a remarkable 35 percent between 2007 and 2008.

The data [pdf] are suggestive, not definitive, but officials said they considered the trend to be genuine.

The annual count, known as a screenline count, was first conducted in 1980 and has been done annually since 1984. This year’s count identified 12,583, up from 9,327 in the previous year.

The counts were were historically taken once a year, during the middle of the week for a 12-hour period, 7 a.m. to 7 p.m. Last year, the Department of Transportation expanded the time window to 18 hours and added two additional counting dates to add more robustness to the sample. (The count was adjusted to allow for a year-to-year comparison.)

The Commuter Cycling Indicator, as the annual tally is known, was conducted at the Brooklyn, Manhattan, Queensboro and Williamsburg Bridges; at the Whitehall Terminal of the Staten Island Ferry; and along the Hudson River Greenway, at West 50th Street.

According to the department, the 18-hour count showed that over a quarter of the cyclists counted used city streets earlier in the morning and later in the day than previously believed. Certain bike routes have become particularly popular: The number of cyclists crossing the Williamsburg Bridge has quadrupled from 2000 to 2008, to 4,000 on a typical day.

“This growth is real,” said Wiley Norvell, a spokesman for Transportation Alternatives, the leading advocacy group for cyclists and pedestrians in New York City. “It’s born out by what I see on the streets every day, and we’re reaping the rewards from the city’s investment in bicycling over the last several years. More and better-designed bike lanes are producing more cyclists and more first-time cyclists.”

Since 2007, 140 miles of new bicycle routes have been added to the on-street bicycle network.

From 5 to 6:30 p.m. Thursday, on the Manhattan side of the entry to the Williamsburg and Brooklyn Bridges, the department distributed more than 1,500 bicycle lights over the next few days, pegged to the end of Daylight Saving Time this weekend. Cyclists are required under state traffic law to use white headlights and red taillights from dusk until dawn on all New York streets.

“This unprecedented increase shows we are well on the way toward our goal of doubling the number of bike commuters,” Janette Sadik-Khan, the transportation commissioner, said in a statement. “As these numbers rise, cyclists should take all safety precautions, while drivers must be vigilant when sharing our streets with this growing population.”