By most measures, San Francisco is a great place to walk and bike, with its compact street grid, mixed-use neighborhoods and relatively mild weather. But a new study  conducted by UC Berkeley professor Michael Jerrett  suggests the city may need to focus on taming traffic before kids will get the full health benefits of that dense development.
Streetsblog New York's Noah Kazis reports on the study , which links traffic volumes to youth obesity:
Jerrett shows that not only does the built environment matter, but traffic volumes matter too. His team's long-term study tracked children from across Southern California, starting from ages 9-10 and continuing through high school. Controlling for a wide variety of factors, they compared the children's body mass indexes (BMI) to the density of traffic near their homes.
Children living within 150 meters of high-traffic areas were found to have, on average, BMIs five percent higher than those living near low-traffic areas. Only the immediate surroundings seem to matter: Traffic levels within 300 or 500 meters didn't affect BMI.
The researchers put forward two reasons for why traffic volumes contribute to obesity. High asthma rates could be part of the equation, making kids less likely to engage in physical activity. Kids - and their parents - also seem to be especially sensitive to the real or perceived danger from cars, much more so than adults.
To put the findings in context, a regular San Francisco block is about 600 feet, or about 180 meters. If kids live on a street with a lot of traffic, or if the next cross street is overrun with cars, there's a real chance they'll be less likely to bike or walk.
Ben Caldwell, who runs the Presidio Community YMCA's Bicycle Program, said the city needs to do a lot more than just install bike lanes and sharrows to make kids feel safe traveling the city's streets by bike or foot. "Few parents think that bike lanes and sharrows are enough to keep kids safe," said Caldwell. "Frankly, I think they're right in most cases."
Even with a long-standing injunction  on any improvements to the city's bicycle infrastructure, cycling has grown at an impressive rate  over the past three years. Still, Caldwell said it will take a lot more to get kids out of their parents' cars and riding bikes and walking to school, the library, or the park.
"Rather than a Bike Plan , we need a trails and bikeways master plan that looks at the entire network of trails and bikeways and paths and staircases and plazas, the entire non-motorized transportation network," said Caldwell.
But where to start?
"There's infinite number of examples of where we could do better out there," said Caldwell. Park Presidio, for example. "It's a nasty street but actually has a lovely path along the ridge that is effectively useless because it's so nasty to cross all the streets."
He suggests looking at the Buchanan Street mall in the Western Addition, which could be fixed up to provide a mixed-use pathway from the Rosa Parks school to the Buchanan YMCA and even Hayes Valley.
Not that San Francisco lacked for reasons to bring more livable streets to its residents, but Jerrett's study is a hard reminder that our tolerance for traffic has measurable consequences.