Inquiring minds also want to know:

1) What the heck is the difference between a "track hoe" and a back hoe? 2) Why the heck was the boom facing forward rather than the usual rearward.

Answer to speed question: Not necessarily all that fast. at all. With the boom facing forward it could have easily scissored (jackknifed) upward after initial contact with the first or second girder. Then inertia and leverage would pull the boom up poking it thru the bridge deck. Once the original cut is made, the answer to the extra credit problem is given. Namely it took only about half of the width of the bridge for a complete standing stop. You can't ignore friction on this one since both shearing and friction energy are expended by the boom while substantial friction energy is exerted by the flat bed trailer undercarriage in sliding and eroding the road bed beneath the bridge. Ditto for the drivers side tractor tread.

I also agree that the hydraulic cylinder or links were busted early on.

BTW, I think that the back hoe (track hoe) very nearly wound up wearing the bridge.

Inquiring minds further want to know what does a new overpass cost these days.

P.S.: The above analysis is the result of a rather long E-mail thread among a group of engineers that analyzed this thing to death a few weeks ago.

Best regards . . .

Jim