Just a quick note - I happened upon a major 3-vehicle accident today on my way to a picnic. We rolled up moments after it happened.
My wife dropped me off, she continued to the picnic, I worked with the medics as we awaited extrication crews.

Vehicle 1: Minivan, 3x Passengers, 2 with major trauma, 1 serious.
Vehicle 2: Another Minivan, 3X passengers, all minor injuries
Vehicle 3: Pickup truck, 2x passengers, 1 major trauma, 1 serious injuries.

It was hectic, to say the least.

I worked vehicle 1, which was rolled onto it's side, into a large rock and on top of the other minivan. Both driver and passenger moderately entrapped. They missed some kind of a surface pipe by under 4", it looked like an inverted letter "J", and it was connected to a natural gas pipeline. I think that was good, I don't know what would have happened if they hit it, if it could have made a gas leak or not.
But the amount of crap in Minivan #1 was incredible, and it had all slammed up into the front (including a fire extinguisher that had been "attached" only to plastic, not metal, as I could see the plastic still attached to the mounting bracket.) Just getting all the crap out of the way to get to the patients took at least 2 full minutes, and we were working before the extrication crews got there. Not long for some patients, but an eternity when you're bleeding out. Pillows, tool boxes, coolers, blankets - lots and lots and lots of stuff. Secure your stuff, strap it down, do as much as you can to keep your stuff from being an impediment to rescue.

Just some perspective here from the front lines of the usual highway carnage.

A good warning that was brought up in an episode of mythbusters!

"...Just getting all the crap out of the way to get to the patients..."

Been there, done that. In my case, it took probably 10 minutes of frantic digging on my part to access the rear of a van (they were moving, had everything they owned in it) of the vehicle and the two critical kids I found. Blew out a disc in my lower back, two operations, not a day without back pain since April '83...

As one person put it: 'Traveling at 60mph if you slam on the brakes everything in the back not bolted down will be coming at you at most of 60mph'. Under those conditions a BOB is a potentially lethal bludgeon and a toolbox has the force of a battering ram.

And using just using sheet metal screw, or bolts alone, into the metal body won't do it. You need to have large fender washers under the bolts to spread the loads and prevent the bolts from pulling through when shock loaded.

If you regularly carry loads consider installing a strong barrier. These are required for many service and delivery vehicles.

Good point and excellently said.....I responded to one MVA of a van (work van) that had the driver and passenger seats only. In the back was the 'work gear'....and a few people...and nothing was secured. The van was on its side. Even more intersting was the fact that they had many containers of (powdered) paint pigment...that had ruptured. The guys were covered in the stuff. It took a few minutes to convince the ER staff that we didn't have a Haz Mat situation on our hands....LOL