The Immortal Pedestrians powerup, known in Carmageddon as Pedestrians From Hades, Things From Hades or, on the GOG release, You Killed a Kitten, has quite a simple effect: it makes pedestrians unable to be be killed, at all. Even drowning them is useless. They can still be pushed around, and blood drips also squirt off, so one can still have fun with them, but they can't be damaged.