much of nontraditional advertising infringes upon previously unmarketed space. in other words, we're intruding. so if we're going to interrupt people's environments, we'd better make it good.
this recent ad spray painted on the sidewalk for zip cars isn't good. or at least, isn't good enough to justify defacing a public sidewalk. yes, i see the copy at the bottom - it's washable. or "waskable." (it's kind of hard to read.) it's just not particularly engaging. i compare it to the sidewalk stencil done years ago for a lingerie store. it read, "from here, it looks like you could use some new underwear."
if a message can't elicit as strong of a reaction as that, don't paint it on anything - waskable or not.