While driving my 2004 Dodge Intrepid on a weekly state-to-state commute, I got to thinking:
What would happen if while driving at highway speed (70 mph), or any other speed, I would suddenly grab the shifter, press the release, and move the lever from Drive to Park?
Is there an interlock to prevent this? what about on older cars? what would happen? Has anyone actually tried this?
All the automatics I’ve driven have an interlock to prevent shifting into Park or Reverse when the car is moving. There must be a minimum speed for the interlock to engage.
I don’t suggest testing it. However, on a few occasions someone has pushed the shift lever into Neutral on a car I’m driving – the interlock preventing the lever from going any further. It’s terrifying until you figure out what happened.
In every car I’ve driven after you turn the key into the on position and shift it into drive, you can’t shift it back into park until you turn the key back to the off position. As I’ve always owned manual car i’m constantly trying to shift automatics into park before i turn the car off. This is true for all cars I’ve driven, but the oldest car I’ve driven is an 81 riviera, so I’m not sure if it would be possible in older cars.
I had a 1965 Ford Ranchero that someone had shoehorned a 1972 302 and automatic tranny into. It would have been nice if they’d have upgraded the original brake master cylinder, but they didn’t. So one day on the way to work, I hit the brakes and nothing happened (if the car had a dual master cylinder, I’d have still had front brakes, since it was the rear brake line that went). Naturally, this had to happen to me in heavy traffic as I was headed downhill. In my various efforts to get the car stopped (the parking brake didn’t work, either) I jammed the transmission into park, figuring that it was better to dump the tranny than have my face slammed into the metal dash when I finally hit something. All I heard was a loud clicking sound and the car didn’t slow down a bit. I eventually got the car to stop by using the Fred Flintstone method: I put the car in neutral, shut the engine off, and stuck my foot out the door. Damn near crapped my pants, let me tell you. Almost as much fun as when the steering wheel in that car came off in my hands while I was driving it. :eek:
So, there ya have it, from someone who did it.
You stop very fast.
When I was 16, I was driving my 1984 Chevy S-10 along a deserted country road, at probably around 45 mph. Fooling around as 16-year-olds will do, I tried to shift it into neutral. Instead I went all the way into either reverse or park, I don’t remember which. The back wheels locked up, and the truck shuddered to a very quick stop, the back end kind of bouncing up and down. I was sure I had completely trashed the tranny or something, but surprisingly, there didn’t seem to be any noticeable damage at all. Drove teh truck for years after that with no problem.
Probably hit “Reverse.” I accidentally dumped the gear shift in my dad’s 1997 Suburban into reverse while driving as I was trying to adjust the steering column, it shuddered and stalled. I thought I’d killed it, but the engine fired right back up (once I got it out of reverse and into neutral) and I drove it home with no problems. Never told dad what I did, either. 
I’ve had the misfortune to do that with a 1975 Plymouth Scamp. It was a very damp drizzly day and the engine was constantly stalling. Luckily this was in a parking lot so I wasn’t travelling too fast. Anyway after about the 4th time the engine stalled, instead of putting it into neutral, I shoved it (in anger) all the way to PARK. Well, it made that hideous metal on metal sound that we must all be familiar with. I quickly put it into neutral and somehow everything was okay. I drove that car for years afterward without any transmission trouble.
Huh. I happened to be driving my car today with a big box in the passenger seat and it shifted position and pushed the gearshift into neutral. It took me a few seconds to figure out why my engine was revving so high.
I don’t know if a lockout mechanism prevented it from going further or I just got lucky.
I must be confused about what you mean. I have owned at least two dozen cars, various makes and models from 1969 to 2004. None of them have had an interlock where the car had to be off to shift into park. Almost all of them have had to be in PARK to turn the key to LOCK and take it out. The newer ones require you to put your foot on the brake to get it out of PARK, but no special tricks are required to put it in PARK.
Last summer, I drove my '75 Chrysler from Vancouver, Washington to New Orleans, Louisiana. At some point on the trip, I had the cruise set to 70 mph. I reached down to tune the (at the time, AM only) radio, which is located immediately to the right of the steering column. When I raised my right hand up to put it back on the steering wheel, I backhanded the shift and it went into neutral. I didn’t even realize what had happened until I heard the engine start to roar. You’d think that putting it into neutral would kill the cruise control. Thank God I didn’t knock it into reverse.
Anyway, when I was about 5, I remember my dad letting me sit on his lap to move the car into the garage ('75 Dodge Dart). When I thought we were far enough into the garage, I reached out and put the car in Park…his foot wasn’t even on the brake. The car stopped so hard I slid into the steering wheel and the horn honked!
Since then, I’ve never put a car into Park while it was moving. 
I remember an episode of “Lois and Clark” in which Jimmy discovers that the brakes have been cut on his little convertable. He cries out “Superman, help!” or something along those lines, and Superman flies in, lands in the passenger seat, greets Jimmy, puts the car into Park, then flies away.
I hope this is helpful.
Cool mental picture, Tuckerfan
I was helping a buddy with his old LandRover. His garage was such a mess that he could barely pull the car in so that we could work on it. There was a big step up from the steeeep driveway into the garage. Jeff was squeezing around the front of the vehicle to pop the hood at the same time I was walking around the back end of the truck. He pushed enough to roll the truck back and it started down the driveway. I stood behind it with my feet sliding down the asphalt and my arms extended. I said, “AAAAAAAAAAAA”, really loud. Jeff jumped in and hit the brakes. In immediate retrospect we couldn’t believe our stupidity. If one or both of us had died you woulda read about us in a Darwin Award email:(
This has nothing to do with the OP, but I’ve only owned standard transmission vehicles and I don’t know nothing 'bout no “Park”. 
As the son of an automatic transmission mechanic…
Park in an automatic is basically the same as neutral, except that a rod (I think it’s a rod) is pushed into a grooved section on a shaft. Usually what happens is that the rod bounces along the shaft, causing the clicking sound that Tuckerfan described. It probably won’t slow the car at all.
Ah yes, very informative, thank you all! I too have never owned an automatic vehicle before. The Dodge Intrepid is a sort-of exception, being my company vehicle
Curious though, about shifting into R while driving. I know that on manuals you can’t do it because of the planetary gear system. You’ll hear a lovely grinding sound until the car is almost stationary, at which point it’ll click in. I would think that there would be a similar arrangement with automatics? Or is there a delayed reaction?
I’ve done it twice in a Chevy Suburban.
When traveling at around 10mph on pavement, the truck kept rolling with a loud CLICK CLICK CLICK CLICK CLICK.
When traveling at around 5mph on dirt, the wheels locked and we skidded to a stop.
It might be cool to try it at 6, 7, 8, and 9 MPH, then graph the results. I mean, since you already have the 5 and 10 MPH results. Or not.
Manual transmissions don’t have planetary gears. Automatic transmissions do (with a few exceptions). In a manual transmission, trying to shift into reverse while moving forward will grind hell out of the edges of the reverse gears. If you can force it into position and then let the clutch out it’s likely to break gear teeth.
An incident like this is depicted in a later chapter of Tom Sharpe’s Ancestral Vices. An erstwhile but short-sighted constable is trying to prove a semi-retarded woman knows how to drive a car (if she can, it’ll lend evidence that she’s the masked assailant of local dwarves - it’s complicated). He puts her in the driver’s seat of a standard-transmission police car and she freezes up, panicking, with her feet on the clutch and gas pedals. The car begins to roll down a steep hill and in his desperation to get her feet off the pedals, he pushes the gear shift out of his way (into reverse). When her kicks her foot off the clutch, the forward speed of the car (~90 MPH) meets the reverse speed of the transmission (~9000 RPM) and hilarity ensues, along with shrapnel.
I’ve never had the brakes go out on me but I have practiced in case the situation arises (actualy as a teenager when I got my first manual transmission car I experimented with a lot of combinations of parking brake, gear shifting, clutch pressing et al to see exactly what I could and couldn’t do.)
Firstly I was under the impression that pulling the parking break while you’re travelling causes very rapid and dangerous stopping (locking wheels et cetera.) Well as I learned one day many years ago, it does not. I discovered if you find yourself moving down a hill in a manual and can’t stop yourself your best bet is to move it into neutral and apply the parking break over and over again.
The break won’t bring you to a stop the first time but as you apply it again and again I noticed it would bring the car to a stop.
The GM automatic overdrive I built had a drum / gear like part on the output shaft. A spring loaded knuckle fit into the teeth on that drum. A rod holds the knuckle off the drum except when the selector is in Park. The drum is not a typical gear with V like shapes for teeth. It is more like a drum with a 1" grove followed by about a 1" land. I believe the parking prawl system I described [tried to] is typical for many GM transmissions.
Also - Me too about the can put in park anytime, newer cars require brake to get out of park.