In Washington, DC, motorists are required to stop for pedestrians in the crosswalk. In places where cyclists are allowed to use the sidewalk, they have the same rights and duties as pedestrians, and motorists are required to stop for them.
I would say that zero percent of motorists understand this -- if it weren't for an encounter I had today. I was waiting to turn left from a left turn lane, and I had crept forward into the crosswalk. An opposing motorist saw me stopped in the crosswalk, thought I was crossing, and stopped to let me go. Predictably, the cars behind him let loose their horns in a fussilade of anger.
So there is at least one motorist who's aware of the law. It would be taking your life into your hands to count on there being more than one, though.