Courtesy of Grant Gordon

The chalk car parked on Haight Street in August 2009

If the residents of the Haight, the Castro, the Mission and other San Francisco neighborhoods were given a giant public chalkboard, what would they write?

Grant Gordon knows — thanks to his 2001 Toyota Echo.

Sixteen months ago, Gordon, a Daly City native, covered the car — its metal surfaces, at least — in black chalkboard paint, converting it into a mobile, erasable canvas. Whenever he parks, he leaves a box of multicolored sidewalk chalk hanging from the window for passers-by.

“I’ve given the outside surface of my car away to everyone,” said Gordon, 31.

The car has become a rolling billboard for San Francisco neighborhood free speech. Depending on where he parks, local passions — and quirks — are revealed.

As soon as the paint on the car was dry, Gordon pulled into his first space, on Castro Street, right in front of the Castro Theatre. (“I have parking karma,” he said, knocking on wood.)

Soon there were drawings of men’s and women’s genitalia all over the car, he said, but also “stuff from all over the world, different languages.”

Next stop: Haight Street, in front of Amoeba Records (again with the parking karma!). The car was quickly adorned with peace signs, flowers and sayings like “J’adore hippies.”

Over time, Gordon found that each neighborhood tended to produce predictable scribbles: sexual doodles in the Castro, mushrooms and marijuana in the Haight, Spanish-language messages in the Mission. But there were also plenty of renegades.

Two teenage boys at 9th and Irving coated the whole car in solid colors. Someone at 16th and Market expertly sketched a San Francisco skyline across the roof. One day in the Mission, the scribblings ranged from “Love your neighbors, travel the world” to “I (heart) cheese.”

Some themes, of course, appeared citywide, like baseball messages as the Giants closed in on their World Series championship.

Gordon takes frequent photos of his car and posts them on his Web site, chalkcar.com. When he feels like it — every couple of days or weeks — he takes a hose to the car and starts fresh.

“I really want to take it to the Financial District,” Gordon said. “But I can’t afford parking down there.”

The smiley, red-bearded former preschool teacher, now studying film at San Francisco State University, got the idea for the art car before going to Burning Man in 2009. It took him and a few friends three or fours hours and five or six cans of Rust-Oleum Specialty Chalk Board spray paint to transform his once-green car. The total cost was about $80, including pizza.

Gordon has only once censored what was written or drawn on the car, when a group of over-eager South San Francisco teenagers used it to promote their personal phone numbers and Facebook addresses.

Given the chance to send a message, he said, “people are overwhelmingly positive.”

This article also appears in the Bay Area edition of The New York Times.