Mental Magpie wrote:
"Virg, the car will go forward and so will you when you're struck from behind. The seat is connected to the car and you're in the seat. Your body is thus not independent from the car. I don't know about the energy transfer up the leg because I think we're all imagining that part a little differently."
Your body is indeed independent from the car at the moment of impact. The separation of you and the car is what makes pressing the brake non-dangerous. When the car is struck from behind, it begins moving forward, but your body doesn't start moving with it until your standing intertia is absorbed by the seat cushion, which is compressed as it presses against you. During that time, the brake pedal is moving away from you, and then your body accelerates to match, but you're not firmly in contact with the pedal so there's no opportunity for the impact energy to transfer "up your leg" in any way. Unless you're hit from behind with a substantial amount of energy, the whole thing will be over before you meet up with the bottom range of the brake pedal anyway. The only way the energy of an impact could be delivered through a foot pedal to your hips is if you were bearing the pedal down as hard as possible with a locked knee and the car was struck from the front.
"On putting the car into neutral at a stop: I was taught that it was a bad idea because it makes it easier for you to be pushed into an intersection if you're rear-ended. Having the car in gear means that the engine's inertia will keep you from moving as far."
If your foot is on the brake, then the engine drag is extremely minimal because the brakes exert thousands of times more stop load than any transmission. Any impact capable of overwhelming the brakes will drive the car forward in a skid anyway since any car that can't exert more stopping power on its wheels than the tires exert on the road would be extremely unsafe to drive (antilock brakes exist specifically because brakes can easily stop the tires before the tires can stop the car). If you're at a dead stop and you've got your foot firmly on the brake, you're providing all the stopping power your car is going to offer no matter what you're doing with the engine, and a collision sufficient to make engine braking relevant will destroy the back end of the car and the transmission as well.