Lost in the line of cars crawling toward the Bay Bridge every weekday afternoon are a few wayward souls: motorists looking for parking.
Exactly how many is difficult to determine. But if you ask the San Francisco Municipal Transportation Agency, it's more than a few.
The SFpark pilot program features meters that produce data telling users if spaces are available and where, and allows users to fill meters via smartphone or computer.
To promote the high-tech meters the agency began installing in eight neighborhoods in 2011, The City's transportation planners are presenting facts that make some transportation advocates balk.
Among them is the statistic that 30 percent of all congestion in The City is caused by frustrated drivers circling the block for that elusive parking space.
That figure has been repeated by local think-tank SPUR, transit agency Director Tom Nolan and current transit agency Transportation Director Ed Reiskin — who at a Board of Supervisors meeting on May 2 proffered "20 to 30 percent" as a more accurate reflection of congestion caused by parking seekers.
It's based on "the most comprehensive study to date that is used by the industry," said transit agency spokesman Paul Rose.
Yet the data to back up the number is a bit thin. For instance, the figure is an average based on a total of 10 studies conducted in eight cities over a period of 80 years.
That's too long a time frame — comparing 1920s Detroit to 2013 San Francisco is useless — and the studies included have too wide a range, from 8 percent to 74 percent, for the 30 percent "average" to be meaningful, says Tony Kelly, a neighborhood activist and staunch opponent of SFpark.
However, the transit expert who created the data doesn't think the transit agency is being devious.
"It's a harmless, very shorthand way to get across an idea that they plucked from a book," said Donald Shoup, the professor at UCLA who wrote the book — "The High Cost of Free Parking" — from which the 30 percent figure was gleaned.
The transit agency is currently conducting its own study, using bicycles to mimic cars in select neighborhoods during the day. But in the meantime, the real number is unknown.
In the book, Shoup offers "34 percent" as the average figure for traffic caused by parking seekers. In reality, it could be more or less.
"I don't know whether it's 30 percent," Shoup said. "It may be more. What's important is that cruising causes some level of traffic, a lot of pollution, and is expensive."
The transit agency is offering 30 to 33 percent as an average, but even that "seems too narrow," said Dr. Robert Saltzman, a professor of decision sciences at San Francisco State University who conducted a parking study in West Portal that did not offer a percentage on parking seekers.
The real number, Saltzman says, may be impossible to figure out. "You have to be able to infer people's intentions as they drive in their cars," he said.
Meanwhile, the 33 percent figure is being used to sell the public on parking meters — and the possibility of meters in residential neighborhoods.
A plan to install up to 5,000 meters in the northeast Mission district was halted after neighborhood outcry, but a long-debated plan to activate meters on Sundays and past 6 p.m. daily went into effect Jan. 1.
Parking meters produce about $50 million annually for the transit agency, half of which is from citations, Reiskin said in May.
In the streets
• 30% San Francisco congestion caused by parking seekers, according to SFMTA
• 10 Studies figure is based on
• 1927 Year of first study from which data is used (Detroit)
• 8% Least congestion caused by parking seekers (New York City, 1993)
• 74% Most congestion caused by parking seekers (Freiburg, Germany, 1977)
• 8-45% Range in congestion caused by cruising in three N.Y. studies
• 0 Studies conducted in San Francisco