i seem to recall a warning light on Washington's olympic peninsula along Lake Crescent cyclists can trigger that alerted following motorists that bikes would be on a narrow, windy road for the next six miles!! i think it flashes of 45 mins to an hour after being triggered. there are additional warning signals at intervals.
ive also seen tunnel warning lights and think they are a very good idea. The ones in WA are push button, I think you can see the figure eight markings for an inductive sensor in the picture posted by Tom.