In freshman physics, one of the first things you learn about is how collisions work. Coming from a physics background, I found the GURPS rules on collisions quite wonky, and leading to some rather odd results. For example, if you run into something bigger than you, why on earth should the damage you take depend on how much bigger than you it is? Inverting the problem (having the bigger thing run into you) is just a change of reference fram from the original. It doesn't matter if you are rammed by a VW Bug at 10 m/s or a battleship at 10 m/s, either case you end up having your velocity suddenly change by 10 m/s and the damage you take follows from that. As a consequence, my rules-tweaking instinct led me to come up with the following:
Damage: In a collision, both objects take crushing damage as if from a melee attack with a ST given by the Bulk of the smaller object times the relative velocity of the two in meters per second, divided by 5. Roll damage separately for both objects. The velocity is given by the amount the object would move this turn if it didn't run into anything, not the distance it did move.
Knockback: Damage to the larger object is multiplied by 3 for the purpose of calculating knockback. Damage to the smaller object is multiplied by 3 × the ratio of the larger object's bulk to the smaller object's bulk. Knockback can never be greater than the relative movement of the two objects. If a moving object is knocked back, the knockback is subtracted from its current Move.
Hard and Soft Objects: If one of the objects in a collision is soft (this includes most people and animals), damage is at -1 per die. Objects that are especially soft give extra DR to whatever collides with them. If the collision is with something very hard, such as a concrete slab or a boulder, damage is at +1 per die.
Sharp Objects: If one of the objects is sharp, it causes cutting or impaling damage, as appropriate, to the other object.
Hit Location: Roll hit location randomly. Damage to the skull or face also causes damage to the neck as a separate wound. Damage to the neck usually means the force was transmitted to the neck from another head blow, also apply damage to the brain as a separate wound. If a limb takes enough damage to cripple it, excess damage is applied to a separately rolled hit location.
Overrunns: If one of the objects runs over the top of the other, it can crush it, causing crushing damage with a ST given by half its Bulk.