Smoking Only at the Curb

Last year while visiting San Francisco, my husband and I stopped to get a sandwich at an Italian deli. We sat at a small table outside, and I noticed this sign which I'd never seen in San Diego.

Although it has the symbol of no smoking with a red circle around the silhouette of a burning cigarette, the wording of this law seems to be focused on the rights of the smoker.  It says where smoking can be done: at the curb or - if no curb - at least 15 feet from any opening into or out of the store.

So, what's a curb?  It's the raised cement edge - usually created by the sidewalk - next to which cars park or pull up to load or unload passengers.  Below is a photo of a storm drain (where the water goes when it rains) in the street. Above the storm drain in the top third of the photo is the raised cement curb.