Most people have heard the term "dead man's switch" and know that it has to do with train locomotives, and maybe even know that there might be a pedal or a handle that the engineer had to push or manipulate or else the train will automatically stop. Anecdotally, if it's a pedal to be depressed, a brick would do, and you could go about your business :)
For my HCI class I show a brief clip from a History Channel documentary about locomotives. I recently found it on YouTube. You can see a modern "alerter" (instead of a dead man's pedal) and the engineer's explanation from 6:50 to 7:35 in the video.
The March/April 2011 Technology Review showed a "drowsiness detector" that works on the same principle but for automobiles.