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Portland, OR's public bus system, called TriMet, is one of the best in the country. I ride it at least six times a week on my way to and from school. Before I can describe certain events revolving around the mechanism that controls the doors on the bus, I must first describe the mechanism itself.

A Tri-Met bus features two doors. One is in the front, and one is about two-thirds of the way back. Both doors open outwards, but the door in the front also slides inwards as it opens, so that it doesn't actually ever stick through the plane of the side of the bus. The door in the back simply hinges outwards.

The driver can open the doors in one of three ways. All three simply open the front door; the three methods differ in how the back door opens. The first causes the back door to not open, resulting in anyone trying to exit the bus to yell "back door!" up to the driver. The second causes a green light to turn on over the door and the door to unlock, requiring the person trying to exit to push the door open themselves. The third opens the door pneumatically and automatically.

An odd side effect of the third method is that the doors can't close until they have finished opening; the mechanism keeps trying to open the door until the door is fully open. In addition to this, the computer on the bus won't let the bus move if one of its doors are open. This brings us to today's events.

I mentioned earlier that I ride the bus to school. The particular bus I take goes from where I live to my school by way of downtown Portland. Consequently, my bus is usually full of people going downtown to work (an added bonus is this bus runs every 5-10 minutes or so).

So I get on the bus, we go downtown, and we stop at the first stop on the west side of the river. There happens to be a mailbox here. A plain metal box colored blue. The driver opens the doors...

And the back door gets stuck on the mailbox. It's only open half way, it can't open further, and attempts to pull it shut are hampered by the pneumatic mechanism trying to open it.

"Pull forward!" yells a passenger.

"I can't," says the driver. "The bus won't move if the doors aren't closed."

Some more tugging reveals that the door isn't closing any time soon. The driver shuts off the bus and restarts it, which has no effect. Most of the passengers, realising that this bus isn't moving for a little while yet, get off. They were just going downtown anyway, and we're already there. I, on the other hand, am going through downtown and beyond, so I stick around either to see if the bus gets unstuck from its small blue boxy prison or to wait for the next bus, which is 10 minutes away.

There is another passenger waiting with me, and he and the driver strike on an idea. This other passenger walks over to the back door from the outside and pushes it shut, then holds it against the strain of the mechanism. This, my guess is, tricks the computer into thinking the door is shut, and, indeed, allows the driver to pull forward from the mailbox and open the door properly. Door open, door close, back to my seat in the newly emptied bus, and on my way to school.

I guess the moral behind this parable is that every system has a single point of failure.


Update 10/14/02

As an aside, Necrospork tells me a little story about a woman who got her leg stuck between the door and a small box inside the bus, which required help from someone on the bus (Necrospork's father) to free. It would seem this simple flaw can be a danger to riders.

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