Here in Philadelphia, we've seen steady growth in the number of bike lanes in recent years, and they're typically marked every couple of hundred yards with this image:
You can see from this flickr search that there's no national or international standard for how you might mark these lanes. But recently, there's been a subtle addition to the bike lane icons in Philadelphia:
What does it mean to add the helmet? The primary work of the bike lane icon (combined with the unbroken white line, in most installations) is in demarcating that portion of the road where bicyclists may safely travel. It points to that space and its purpose both for drivers who must observe laws about parking in it or crossing it and riders who may need encouragement to avoid sidewalks or a driving lane. The addition of the helmet, however, recruits the icon to another purpose. It operates as what semioticians would call an entailing indexical icon. It maps a relationship between a particular kind of user and an area of ground, but it extends the parallel between the icon and the actual user to one extra bit of behavior--the wearing of a helmet. It appears that both helmeted and non-helmeted icons appear in the same lane, and while it might be easy to go back and helmet the bare-headed icons, the persistent contrast actually makes the newer modified icons that much more evident. The distinction tells a story; it's the difference that makes a difference.
Are you seeing other examples of icons like this doing double duty?