how does inertia affect collisions between cars?

Inertia is an object's resistance to change in motion. In other words the difficulty in moving an object or stopping it from moving. A heavier car has more inertia than a lighter car because it's harder to move. A faster car has more inertia than a slower one with equal mass because it's harder to get it to stop.

So the more inertia a car has, the more dangerous it is in a collision.