Maybe it's savvy civic engineering or just the courteous, slower rhythms of the Midwest, but a new study shows that motorists in this eastern Iowa (search) city are the safest in the nation.

Researchers with Allstate Insurance Co. (search) analyzed two years worth of internal crash and claim data to calculate the chances that drivers in 196 of the nation's biggest cities would be involved in an accident compared to the national average.

Allstate, which claims a 12 percent market share of the nation's auto insurance policies, found that Cedar Rapids (search) motorists average an accident once every 15 years, far better than the national rate of one every 10 years.

Motorists attributed the city's high grade to a variety of factors, including an efficient interstate that eases trips across town, the city's easy, grid-like layout and the slower pace and attitude of Iowans in general.

"I'd like to think it's because everybody is so polite," Connie Trautman, a 10-year Cedar Rapids resident, said Monday.

With a population of 122,500, Cedar Rapids is among the smallest of the cities surveyed by Allstate. But the report's findings also mirror results of an earlier study by GMAC Insurance Group that rated Iowa drivers high in knowledge about traffic rules and safety, putting the state third in the nation.

According to Allstate, the other safest cities behind Cedar Rapids were, respectively: Sioux Falls, S.D.; Chattanooga, Tenn.; Huntsville, Ala.; Knoxville, Tenn.; Des Moines, Iowa; Topeka, Kan.; Lakewood, Colo.; Fort Collins, Colo.; and Birmingham, Ala.

Allstate researchers say it's no surprise that some of the nation's biggest, densest urban centers are among the most likely places for accidents waiting to happen.

Washington, D.C., ranked 25th in the nation for population, is the least safe city for motorists with a chance of a fender bender once every 5.2 years.

But not all big cities rank poorly on the safety scale, according to the study.

Drivers in Milwaukee, ranked 19th in population, are likely to experience a crash once every 12.95 years, the best among cities with more than 500,000 people. Among cities with more than 1 million people, Phoenix ranks the highest for safety, with a collision likely once every 10 years.

Researchers studied 2.2 million damage claims, defined as any collision resulting in property damage, filed between January 2002 and December 2003, a broad enough period to limit the influence of external factors such as weather and road construction.