"A 'sharrow' -- the word is an amalgamation of "arrow" and "share the road" -- is a larger-than-life thermoplastic symbol of a bicycle topped by two chevrons pointing the way forward.
Sharrows have been increasing in popularity nationwide, and got a boost in 2009 when they were officially entered into the federal transportation engineering canon.
Like many experts on transportation bicycling, Tom Fucoloro, proprietor of the Seattle Bike Blog, wasn't enthusiastic about them. Sharrows are spread so indiscriminately on Seattle streets, he said, that "they mean nothing now." He has noticed that there seems to be "slightly less aggression" from drivers when they're in place. "But does that mean all the streets without sharrows are worse?"
A 2004 study prepared by Alta Planning + Design for the San Francisco Department of Parking & Traffic "says sharrows slow car traffic slightly, and make bicyclists a little safer. But they are even better at keeping drivers at a distance from parked cars -- once again, bike infrastructure benefits more than just people on bikes."
From Seattle Bike Blog: Elly Blue's take on sharrows after a recent trip to Seattle: Tom Fucoloro describes his visit with "Elly Blue, a prolific bicycle writer and zine editor from Portland. Our conversation made its way to sharrows, which are the topic of her most recent story for Grist. Seattle's sharrows are a rather confused road marking, but legally, they don't do anything."