Everyone who buys auto insurance in Philadelphia knows it. So does everybody frightened by the Schuylkill Expressway's merge-or-die ramps, Lincoln Drive's snakelike twists, or Roosevelt Boulevard's multilane madness. Philadelphia is a risky place to drive.

Now Allstate Corp. has stepped forward to confirm that conventional wisdom. The auto insurer says that among residents of 193 cities it examined, Philadelphians had the sixth-highest frequency of accidents in 2008 -- and the most among residents of the nation's 10 largest cities.

On average, Allstate's Philadelphia policyholders have collisions once every 6.4 years, the insurer said. By comparison, New Yorkers wait eight years between accidents, Clevelanders 10.7 years, and residents of Sioux Falls, S.D., 13.5 years.

Five smaller cities' residents did worse in the comparison, which examined claims data from the 193 largest cities where Allstate sells auto coverage -- a set that excludes Boston and other Massachusetts cities.

Bringing up the rear among cities beyond the 10 largest was Washington, D.C., whose residents averaged accident claims every 5.1 years. Also worse than Philadelphians were drivers in Baltimore, Hartford, Conn., and Newark, N.J.

Allstate announced the study yesterday with a news release headlined, "Philly Drivers Rank Worst of Largest Cities in Fifth Annual 'Best Drivers' Study." The Northbrook, Ill., company said it had analyzed collision frequency "to identify which cities have the safest drivers."

That conclusion was greeted skeptically by state officials, as well as by outside insurance and traffic-safety experts.

Jenny Robinson, a spokeswoman for Pennsylvania's Department of Transportation, said Philadelphia had its best year last year in a decade, based on PennDot's count of "reportable crashes" -- accidents that result in death or injury, or require a vehicle to be towed away.

"We see highway safety improving in Philadelphia, and it's really hard to see why the Allstate data conflict with that," Robinson said.

Other experts similarly questioned Allstate's use of property claims as a measure of safety.

"There's a lot of metal contacting other metal, but that's about all you can say," said Martin T. Pietrucha, director of the Thomas D. Larson Transportation Institute at Pennsylvania State University. "But is the number of fender-benders in your town a measure of safety?"

A civil engineer who studies the "human factors" in car crashes, Pietrucha said Allstate's claims data could be skewed if Philadelphians were more likely to report smaller accidents to their insurers -- a possibility that other studies have suggested.

Another factor that may limit the study's significance is that it compared cities rather than metropolitan areas.

Some large cities, such as Houston and Phoenix, have incorporated hundreds of square miles that would typically be suburbs in the Northeast. On average, those cities' drivers face much less congestion -- "a very important factor" in accident frequency, said Donna Lee Williams, director of government and industry affairs for AAA Mid-Atlantic and a former Delaware insurance commissioner.

"I don't want to say that Allstate's findings aren't valid," Williams said. "But it's really difficult to claim that Philadelphia drivers are the worst in the country."

Williams, Pietrucha, and other experts said Philadelphia drivers faced a tough mix of challenges, including not just congestion but aging infrastructure, narrow streets crowded with parked cars, and roads that did not meet the most recent standards.

For example, Pietrucha said drivers in rowhouse neighborhoods suffered visibility problems because homes were built to the sidewalk and cars and SUVs filled every available parking space.

"When you've got parking that goes pretty much to the street corner, and you've got to look around a car to see if someone's coming, those are the kinds of things that lead to a lot of crashes," Pietrucha said.

Capt. Mike Murphy, commander of the city police Accident Investigation District, said studies of Roosevelt Boulevard's high accident rates have led to better enforcement and education efforts, which he said have helped reduce fatalities.

"We have to change people's behaviors and the psychology of how they drive," Murphy said. "Any type of distractions can cause accidents -- people eating in their vehicles, talking on the cell phone, dealing with children -- any distraction."

Insurance rates themselves lend support to Allstate's characterization of Philadelphia.

According to the state Department of Insurance, a sample insured Philadelphia driver would have paid a premium of $5,372 to $6,169 for full coverage in 2007 through the state's assigned-risk pool.

That same sample driver would have paid substantially less in all of Philadelphia's suburban counties, and less than half that price, or $2,121 to $2,828, for the same coverage in Allegheny County, where Pittsburgh is located.

Contact staff writer Jeff Gelles at 215-854-2776 or jgelles@phillynews.com.