Each vehicle adds force to the collision,,, assume similar weight and the weight (M) is 1000 (units does not matter,,)
V1 is 50
V2 is 100
F=1/2 M(Vsquared)
F1 = 1/2 1000 ( 2500 ) = 1,250,000
f2 = 1/2 1000 (10,000) = 5,000,000
Force total is = 6,250,000
The truck adds 80% of the force,, the 50MPH car adds 20% of the force,,,
The best way to increase force is to double the velocity, rather than double the weight,,,
That squaring of the velocity really ups the ante of the truck,,,
Now, how that force is dissipated,, that is a different subject,,,
A truck hitting a bridge at 150 velocity would be
F3 = 1/2 1000 (22,500) = 11,250,000
That is almost double the energy of the two vehicles colliding,,,