A recent posting on rec.games.frp.gurps gave me the chance to blather on and on about how to handle collisions in a RPG. Regarding the huge damage that Vehicles would dish out for a 5 mph collision with a car:

John, you are correct to sense something funny here. The GURPS Vehicles rule does have a big flaw. Others have made helpful suggestions, but if I can wax pedantic here, allow me to dig to the heart of the problem and set things right.

You can quickly check the robustness of any RPG's collision rules with the "Earth test". Say the PC falls off his second-floor patio and plummets to the ground. How many dice of damage does he take?

Let's see, that's a collision with a 6.5 x 10^21 ton object... Whether the rules inflict damage on PC based on the colliding object's mass or on its HP (per Vehicles' inexplicably odd rule), we're talking... how many millions of dice of damage here?

Test failed.

Should anyone say, "but falling uses different rules", that's another problem. A fall is a collision; why the heck should the game have to offer two separate rules for the same situation? As someone else pointed out, GURPS' collision rules and falling rules can result in wildly different damages for a given impact speed, which is not good.

Here's where every set of RPG collision rules I've ever seen gets it wrong. RPGs always base the damage taken by an object in a collision on:

a) net collision speed; and

b) the other object's mass (or some stand-in such as HP).

This doesn't always work well, typically resulting in ridiculous damage when colliding with big objects at low speed (per your 5 mph example).

An RPG that wants to do collisions right needs to base the damage taken by an object in a collision on:

a) its change in speed due to the collision; and

b) its own mass.

(Yep, its own mass. That's the shocker, but it's correct.)

In GURPS, the above rule would suggest X dice damage per mph of speed change, and a damage multiplier based on the item's own mass.

This method always produces good results. Take the Earth test. Falling off a cliff and hitting the Earth at Y mph, any object will stop instantly; change in speed is the same as impact speed, and gives your base damage dice.

A human PC will then use some damage multiplier for human mass (not the Earth's mass!). A mouse falling off that cliff will use a tiny multiplier for mouse mass and may be harmed little, which is realistic. A truck will use a huge multiplier for its own great mass, and will be twisted beyond recognition. Which is what happens when trucks go off cliffs.

So why do humans come off so poorly in street collisions with runaway trucks? Because change in speed is key. In a 50 mph collision, the truck's huge mass experiences almost zero change in speed; the human's much smaller mass is rocketed to almost the full 50 mph. The truck takes almost zero dice damage for change in speed, times a big multiplier for its mass; the human takes many dice of damage for change in speed, times a small multiplier for his mass. If the multipliers are set properly, the truck will take little damage, and the human will be sauce.

Last example: 5 mph collision between truck and human. Again, no effect on truck. And little effect on human either – low damage dice for a mere 5 mph change in speed, times a small multiplier for human mass (not truck mass). That gives the results you're looking for.

(Of course, that ignores the human being run over by the truck. Crushing from huge weights is a separate matter...)

So, that's collisions done right. If you want to see a fully worked-up system for GURPS, head to Collisions. You might want to modify or simplify that for your own use, but as long as you get the basics right, as explained above, you'll get reasonable results.