Researchers with Allstate Insurance Co. analyzed two years of internal crash data to calculate the chance that drivers in 193 of the nation’s most populated cities would be involved in an accident.

Allstate, which claims a 12 percent market share of the nation’s auto insurance policies, found that Sioux Falls motorists average an accident once every 14.6 years — 31.6 percent better than the national rate of one every 10 years.

The city’s rate improved slightly from last year’s average of one accident every 13.7 years.

Officials in Sioux Falls again attributed the ranking to strong traffic engineering and driver education programs.

Following Sioux Falls were Fort Collins, Colo.; Chattanooga, Tenn.; Sterling Heights, Mich.; Warren, Mich.; Knoxville, Tenn.; Grand Rapids, Mich.; Cedar Rapids, Iowa; Lexington, Ky.; and Detroit.

Motorists in Washington, D.C., were most at risk, according to the study, averaging an accident once every 5.4 years.

Drivers in Detroit, ranked 11th in population, are likely to experience a crash once every 12.4 years, the best among cities with between 500,000 and 1 million people. Phoenix ranks the highest for safety among cities with more than 1 million people with a collision likely once every 9.8 years.

Massachusetts’ cities were not included as the company does not write policies in the state.

Allstate planned to give away free gas at a Sioux Falls filling station Tuesday to reward residents for their ranking.

Researchers studied about 2 million damage claims defined as any collision resulting in property damage filed between January 2005 and December 2006. A weighted average of the two-year numbers determined the annual percentages.

Allstate: http://www.allstate.com/