I'm here to tell you: Don't be afraid. After years as a driving instructor, and thanks to rental car companies that apparently lost their minds and let me loose with their vehicles, I can assure you: Your engine won't let you destroy it (at least not like this). And shifting into neutral at speed could save you from yourself.
Why Neutral Is Your Friend
Here's why: Government and private studies have shown that pretty much all sudden-acceleration crashes happen when the driver unintentionally presses the accelerator in the mistaken, but unshakable, belief they are pushing the brake. If you hit the brake, you'd know it: I have done hundreds of experiments - some that started above 100 mph - in which I pushed both the accelerator and brake to the floor. The vehicles came to a full, complete stop with the engine screaming wildly. The brake will easily overpower the engine in every reasonably well-maintained vehicle.
That said, throttles can stick open; I've experienced several. If the throttle is stuck and the driver fails to press hard enough on the brake, or pumps the brakes until the vacuum assist is depleted, the car may not come to a full stop.
The point: Regardless of whether your throttle is actually stuck or you're just mistakenly slamming on the gas, the answer is to put the car in neutral. If the throttle is truly stuck, shifting into neutral will prevent the car from accelerating. If you're like most sufferers of unintended acceleration - you're just aiming for the brake and missing - then the same is true. You won't be able to accelerate no matter how hard you hit the gas if the car is in neutral.
In my experience, neutral also comes in handy when you're in the passenger seat. If Grandpa mistakes the gas for the brake, moving the gear selector into neutral may prevent a crash. The ability to slow a learner's-permit-holding teen with a quick slap into neutral quells many parents' fears. (Slap the gear selector, not the teen. I know they deserve it. But don't.)
Why You Shouldn't Be Afraid of Shifting
The person seeking my advice typically ignores my question and, panic rising, asks, "But what if it goes into reverse or park?" It won't. First, depending on your car, the shift lever won't move to the reverse or park position unless you press the physical button on the shifter. New cars with electronically controlled transmissions might not have such physical barriers, but they'll ignore your inappropriate request to park or go in reverse while you're moving. A few go into park at about one-half mile per hour.
Neither a U.S. Department of Transportation spokesman nor I could find any law that requires automakers to idiot-proof the transmission. However, no vehicle I've tested in the past 15-plus years will go into reverse with the car moving forward.
So there you go. Your engine won't blow if you shift it into neutral while in motion. And the answer to "What happens if you accidentally select reverse or park?" is either "Nothing" or "You can't."
Source : http://autos.yahoo.com