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From 2005 through 2009, there were 2,246 bike crashes in San Francisco, a city where cyclists and cars jostle for position on crowded streets.

During the same time period, there were 1,416 crashes in Contra Costa County, where long roads run through quiet suburbs and countryside.

Which is more dangerous for bikers? The answer, surprisingly, is Contra Costa County. 

The reason is that Contra Costa County has far fewer cyclists than San Francisco. An average of 11 percent of Contra Costa bike commuters were in crashes each year from 2005 through 2009. During the same period, just 4 percent of San Francisco bike commuters were in crashes, the lowest rate in the Bay Area

The crash numbers come from the brand-new Bay Citizen Bike Accident Tracker 2.0, which maps every bike crash in the Bay Area from 2004 through 2009 that was reported to the California Highway Patrol. Every city is required to report to the CHP every collision that its police officers write up.

The crash numbers alone don't really measure danger, because they don't take into account how many people are bicycling on the roads. We decided to compare the accidents with how many bikers are on the roads in each county.

The number of cyclists is difficult to calculate. The only uniform number available across all Bay Area counties is from the U.S. Census Bureau, which asks people if they commute to work by car, public transit, foot or bike. The number does not give a full picture of all the cyclists because it doesn't include people who bike on the weekends for fun or who use a bike to commute only a few days a week. But it does give a relative indication of how popular biking is in each county.

In Santa Clara County, which had the highest crash total, with 3,509, 6 percent of bike commuters were in crashes. Alameda County, which had second-highest total, with 3,406, 7 percent of bike commuters went down. Marin County had a higher rate, with 9 percent of bike commuters getting into accidents, but Marin also sees a flood of tourists who often get into accidents on their trips over the Golden Gate Bridge, probably inflating the rate.

Here's a ranking of accident rates per bike commuter. Click on column headers to sort:

Overall, the numbers show that urban areas, surprisingly, can actually be safer for bikers than more rural and suburban areas. San Francisco, for example, had far fewer biking fatalities than other counties. Santa Clara and Contra Costa counties had the highest number of fatalities, followed by San Mateo and Alameda counties.

Who's to blame for all the bike accidents? Out of the more than 14,000 bike crashes in the Bay Area over the five-year period, bikers were to blame for 52 percent of them, according the police who wrote up the accident reports and sent them to the California Highway Patrol. Just 34 percent were the fault of car drivers.

The raw numbers don't tell the whole story. Solo bike crashes are included in the data, and in most of those cases, bikers are found to be at fault. In bike-versus-car and bike-versus-pedestrian accidents, bikers were at fault 48 percent of the time, while cars were at fault 37 percent of the time. 

And if you had any doubt, the vast majority of accidents were indeed caused by men, according to the data. In all nine counties, men caused more bike accidents than women, whether they were the cyclist or the motorist in the situation. Bear in mind that gender was not noted in about 40 percent of the data; those accidents aren’t included in the chart below.

But more men ride bikes than women do. "Typically there are between two and four times more male riders out today," said Robert Schneider, who has a doctorate from the Department of City and Regional Planning at the University of California, Berkeley.

We took the top three overall violations and showed how many times they were cited in each county. In Alameda, most accidents were caused by a “wrong side of road” violation. In San Francisco, “right of way” violations were most frequent.

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