Tuesday, September 02, 2014

Rating SF drivers: Not the best, not the worst

Since there are so many variables involved in these rankings, as the author of the Slate piece (Which city has the worst drivers?) admits, he's indulging in more of a "parlor game" than a serious study: "If you don’t like my methodology, make your own." He includes a link to a spreadsheet with his rankings and the variables used.

There's a consensus that Miami has the worst drivers; both Slate and Travel and Leisure agree on that.

The Slate writer based his rankings on several sources, including starting out by inverting the Allstate ranking of the best drivers in the country.

On drunk driving, San Diego is the worst, and San Francisco comes in only 8th worst in the country. But the big variable on that list is enforcement, since Kicking Tires tells us that

San Diego has a sergeant and five specialized officers who spend 40 hours a week just stopping and arresting DUI offenders, according to Insurance.com. City officers are trained to watch traffic and search for even slight driver’s errors, such as stopping too long at an intersection.

Slate factors in miles driven into its rankings ("the number of miles that members of an average household travel by car in a year"). Since people in SF on average don't drive as far in our geographically compact city, we get a bad rating on frequency-of-accidents:

The Allstate rankings, for example, are based on the number of years between accidents. San Franciscans average 6.5 years between crashes, but they drive 74 percent as many miles as the average for cities in our survey, so we lower their years-between-accidents to 4.8 to account for how rarely they drive.

The sheer traffic density in San Francisco has to be factored in, since we have more than 10,000 registered motor vehicles per square mile (47 square miles, 477,314 registered vehicles).

But the biggest joker in the deck is how traffic accidents are counted, not only in San Francisco but everywhere else. That the city has a radically flawed method of counting accidents on our streets throws that important number in doubt.