

That Sunday, the city's yellow taxis sped through Manhattan's central business district at an average of 16.3 mph, according to the New York City Department of Transportation.

Graphic: Average Daily Taxi Speeds in Manhattan

The slowest date: Jan. 28, a Friday after the city had been crippled by a blizzard that dumped nearly two feet of snow over three days. The average groundspeed was 6.9 mph.

The city's slowest and fastest traffic days and a trove of other data were measured by the Department of Transportation using Global Positioning System data from the city's fleet of more than 13,000 taxi cabs. The data is included in the department's Sustainable Streets Index, an annual report to be issued Monday that describes in rare detail how people move through the nation's most densely packed city.

A line of yellow taxi cabs, cars and trucks clog Ave of the Americas as seen in this 2009 photo. Getty Images

"It's the most comprehensive set of traffic speed data of any city that I'm aware of," said Bruce Schaller, the deputy commissioner for traffic and planning, who compiles the index.

In a town where walkers often outpace crosstown buses, dealing with congested streets is a tie that binds the city. An average of about 778,000 vehicles converge each day in Manhattan's central business district, defined as all of the island south of 60th Street.

The report provides an uncommon amount of empirical data, from the percentage of commuters who walk or take public transportation to work (65%) to how many fewer pedestrians were injured along East 180th Street after street improvements (67%).

And it sheds light on what many motorists consider something of a dark art: discerning when they can best dodge traffic.

Some things that regular Manhattan drivers say they already sensed are clearly true, according to the report.

16.3 mph

Taxis' average speed in Manhattan's central business district on Aug. 28, 2011

• The fastest days for drivers are weekends and holidays.

• Mondays and Tuesdays are less congested than the rest of the work week.

• Spring and fall are faster than summer and winter, as are the days just after major holidays.

• Traffic slows down late in the year, as holidays and special city events clog the streets.

• Nothing slows things down quite like the United Nations.

The annual U.N. General Assembly brings hundreds of world leaders to Turtle Bay and results in a security clampdown, closing down streets and sending ripple effects throughout Manhattan.

"This city is a small city," said Anwar Sheikh, 56 years old, a livery cabdriver, who was parked on a recent weekday along Dag Hammarskjold Plaza, within sight of the U.N. building. "Like remember when President [Barack] Obama, [Bill] Clinton or [George] Bush come into the city? The city's terrible. You cannot drive. You stay here, you stay there, and then that's it."

The patterns are familiar, said Sam Schwartz, the former city traffic commissioner and engineer known as Gridlock Sam. He said the taxi-derived data shows similar trends as other measures of traffic speed.

It isn't surprising that traffic fluctuates with the seasons, spiking in the winter when there is bad weather and bottoming out in the summer when Manhattanites head for the beaches and hills, Mr. Schwartz said.

Still, drivers try to beat the averages, including Mr. Schwartz himself, a former cabby who said he is among the city's aggressive drivers.

"The other thing my wife hates about riding with me is if I see an opening, I'll see if it leads to a shortcut or not," he said. "And I'd say most of the time, it doesn't."

Professional drivers have their own gripes.

"Traffic managers," said Dennis Abrevaya and Robert Owens, in near unison, from the cab of their delivery truck, which was parked at the loading dock of Bloomingdale's on East 60th Street on a recent Friday afternoon.

The men both live in Queens and said they prefer attacking Manhattan in the early morning and after rush hour. But they are frequently at the mercy of traffic managers, the civilians in brightly colored vests who wave motorists through intersections during Midtown's jammed midweek afternoons.

Transportation Commissioner Janette Sadik-Khan and other officials have credited measures such as street lanes for bicycles and buses with improving the flow of traffic.

The stats show essentially no change in traffic volumes from 2010 to 2011. The previous year showed a roughly 1% increase in traffic citywide.

Drivers like Messrs. Owens and Abrevaya aren't convinced that the city's actions have reduced traffic.

"Nah, we don't buy that," said Mr. Owens, chuckling. "When is it less congested? When everybody's sleeping? At nighttime?"

A perennial battle is at work, said Mr. Schwartz, who praised the city's traffic management and its efforts to measure traffic patterns. That won't stop drivers—including Mr. Schwartz himself—from hoarding their own secrets for cracking the code of New York traffic.

"I can go to the Hamptons on a Friday night and not hit a traffic jam," Mr. Schwartz said, concluding an interview with a gentle boast. "But I'm not going to tell anybody how."

Write to Ted Mann at