For those that care
1000kg coming to a complete stop from 30mph exerts a force (approx) equivalent to 15000kg.
The numbers I asked my mate to work out we're based on 30mph because even if you were at 70mph, you wouldn't (probably) hit something and come to a complete stop.
Most likely you'd hit the car in front which would move, thereby reducing the force exerted.
Lets be honest, if you hit something immovable (say the concrete leg on a bridge), no matter what was in the back you'd be dead.
It took my mate (a maths teacher at quite a prestigious school) quite a while to work it out.
He says its not as easy as multiplying the weight by the speed. It's nM's etc etc.
Long and short...
A 15,000kg ratchet strap would hold at 30mph impact.
Would the vehicle? Who knows.
Any faster than that-you takes your chances.
A lot faster than that-you're toast either way