If you're in Sioux Falls, Fort Collins, or Chattanooga, your chances of colliding with another vehicle are a lot lower than in most other U.S. cities.

The spread is substantial. The typical driver in Sioux Falls will go 13.5 years between collisions; that's more than two and a half times as long as the 5.1 years for Washington, D.C. drivers.

The results are part of Allstate's fifth annual "America's Most Improved Driving City" report, which ranks the 200 largest U.S. cities based on collision frequency

Among the most-improved cities were Alexandria, Virginia; Lexington-Fayette, Kentucky; and Arlington, Texas. Oddly, five out of the ten most improved cities versus last year were in Virginia (Alexandria, Hampton, Virginia Beach, Chesapeake, and Richmond).

Stereotypes do tend to hold at the top and bottom of Allstate's lists. Midwest and Middle American cities—many of them in regions with slippery winter-driving conditions—topped the list, while several New Jersey cities placed among the worst. The top-ten list held only one city, Eugene, Oregon, from a coastal state. The only surprise to this editor was Texas, which had a number of cities placing at the risky end. Massachusetts was omitted from the list.

Phoenix stands out as especially safe among big cities, with a collision likelihood that's only 8.8 percent above the national average. In addition to Washington, D.C., several other big cities—Baltimore, Philadelphia, and San Francisco—had especially high chances of collision.

To arrive at its lists, Allstate, which has about 11.3 percent of U.S. auto-insurance policies, analyzed its claim data for all collisions resulting in property damage claims. To help reduce the chances of influences like weather or construction, Allstate looked at a period of two years from January 2006 to December 2007. The figures were calculated toward "average years between collision" for drivers, and though they might represent an element of relative risk they're not being used to determine rates.

Here are the lists of safest and riskiest driving cities:

The Safest Driving Top Ten

1. Sioux Falls, SD
2. Fort Collins, CO
3. Chattanooga, TN
4. Cedar Rapids, IA
5. Knoxville, TN
6. Fort Wayne, IN
7. Lexington-Fayette, KY
8. Eugene, OR
9. Boise, ID
10. Colorado Springs, CO

Bottom of the List – Riskiest Driving Cities

1. Washington, D.C.
2. Baltimore, MD
3. Glendale, CA
4. Hartford, CT
5. Newark, NJ
6. Philadelphia, PA
7. Elizabeth, NJ
8. Providence, RI
9. San Francisco, CA
10. Los Angeles, CA

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