Safety Theater

September 27, 2009

Back in June, I posted a note following the crash on the Washington DC Metro, which sadly caused the deaths of nine people, injuries to a number of others, and considerable inconvenience to thousands of commuters and other travelers.  One train ran into another train that was stopped, probably because of a failure in a safety system that is supposed to keep trains apart.  In my original post, I noted that some of the initial reports of the accident suggested a physically improbable chain of events.

Today’s Washington Post has a follow-up article on one of the short-term “fixes” that was put in place by Metro shortly after the accident.  Some of the cars involved in the crash were of an older type, which is known to have structural deficiencies that might prove dangerous in a collision (as indeed they were).  This was originally discovered some time ago; because funds were (and are) tight, the decision was made to replace the old cars with new ones as they became available, rather than to retro-fit safety improvements to the old cars

Shortly after the accident, Metro announced that it was reconfiguring its trains, so that the older cars would not be used at the ends of the train, but only in the middle, surrounded by newer (and stronger) cars.  The Post story says that this change was made in an attempt to improve public confidence, not as the result of any specific analysis:

One of the first moves Metro officials made after a subway crash killed nine people this summer was to sandwich older rail cars, similar to one crushed in the accident, between newer, sturdier cars. While repeatedly portraying the move as one that might improve safety, interviews and newly obtained documents show Metro conducted no engineering analysis before launching the initiative.

It was an example of what Bruce Schneier  calls “security theater”: something that has little or no effect on actual security, but is designed to make people feel better.  Perhaps we should call this “safety theater”.

I am, in a strange way, somewhat relieved to know that the car “sandwiching” decision was not taken on the basis of any engineering analysis, because, if it had been, I would have grave doubts about that analysis.  The suggestion that was made when the decision was announced was that the stronger, stiffer cars on the ends of the trains would protect the less-robust ones in the middle from damage.

Now admittedly it is an over-simplification of the conditions, but if we assume those stronger cars to be perfectly rigid, interposing them between the colliding object and the weaker cars would make essentially no difference.  The kinetic energy of the colliding object has to go somewhere; a perfectly rigid body would transfer essentially all of that energy to the weaker car(s) in the middle of the train. (If, on the other hand, the newer cars have energy-absorbing “crumple zones”, like modern automobiles, then the change would help.)

Imagine putting an egg on the counter, and hitting it with a hammer; obviously, it will break.  Now suppose that you put the egg on the counter, and suspend over it a 0.25 inch thick steel plate, on springs so it is just touching the egg.  If you then hit the plate with a hammer, do you think the egg will be protected?  Neither do I.

I do understand that the dynamics of an actual collision are much more complicated.  What I find a bit unsettling is that the original “improvement” was accepted pretty much uncritically by the media and the public, except for a few other old curmudgeons I know who apparently remember something of Physics 101.

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