The safe assumption is "zero". If this is your objective, why assume otherwise?

Bear in mind ...

Wind.

Drainage.

Freeze-up. Nothing like a totally flat surface with no drainage to freeze into a big sheet of ice.

The floor of my garage is intentionally slightly sloped towards the garage door (drainage!) and my manual transmission car rolls out quite easily in neutral by gravity.

Getting an answer is not possible. You have to make each row of parking spots a slight swale so the cars would have to roll uphill in either direction to move. Even then, they could move until one axle finds the lowest spot.

Or intentional slope so that a vehicle in each parking spot is encouraged to roll further into the parking spot until it bumps against (preferably) a stop or kerb or (if push comes to shove) a wall. And arrange for drainage in the same way.

Intentional slope so that a vehicle operator immediately knows that they've forgotten something, as opposed to finding out three hours later that a gentle breeze has pushed their vehicle to the other end of the perfectly-level-and-frictionless parking lot, seems like a better approach ... and a lot more storm-water / winter freeze-up friendly if the drainage is done right.

What is the slope of your garage floor though? I checked online and there are 2 standards: 1/8" per foot and 1/4" per foot, which equates to 1.0417% and 2.0833%, respectively. I've always known that rubber-tired vehicles will roll at a 2% grade. I'm trying to figure out if a standard vehicle will roll on a 1% grade, or even a 0.8% grade in the absence of wind or seismic action.

Quote (BrianPetersen)

Intentional slope so that a vehicle operator immediately knows that they've forgotten something

This slope is too steep as it will damage the vehicle suspension over time. This may be suitable for hour-long parking at the restaurant but not suitable for long-term parking where vehicles are stored.

That's nonsense. Parking on a 1% grade won't damage any vehicle's suspension. Resting against a kerb won't, either. Gently rolling into a kerb at 1 km/h won't, either. The slight grade in my garage or driveway is plenty of a reminder if one forgets to apply the parking brake.

Parking on 5% grades won't hurt a car long term. There are many places where this a fact of life.
Though it is better to be facing up/down than across grade.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

Quote (EdStainless)

Parking on 5% grades won't hurt a car long term.

What about RVs though, where they require leveling blocks for parking on a slope? Will parking on a 5% grade for months at a time without leveling blocks harm the RV?

Also, I'm just wondering if a 1% grade (FAA maximum for airport aprons for aircraft parking, through in any direction) in the longitudinal direction on a smooth concrete surface where no wind is guaranteed (like in an indoor garage) will be enough to cause the typical road vehicle in one of the standard categories to start rolling. How about the FAA maximum allowable running grade of 0.8% in the first and last quarter of a runway, but inside a garage for road vehicles? How about a 0.5% grade (limit for most light rail and BART parking tracks), or even a 0.2%-0.25% longitudinal grade (limit for most railroads and metros for vehicle storage tracks), also indoors on smooth concrete?

RVs need levelling because most people find it uncomfortable to sleep on a significant slope, and the various water systems may need to be somewhere near level to work properly.

I would recommend doing an experiment with your own vehicle. Inflate the tires to max recommended and make sure the brakes aren't dragging. Measure how much force it takes to push or pull it. Try both directions.

There will be some vehicle that will roll if there is any slope where it is parked.

Trying to make a practical parking lot that is flat enough that no vehicle parked there will ever roll is impossible.

Here is yet another scenario - another car in this lot bumps into one of these unsecured parked cars and gives it a push.

BrianPetersons idea is likely your best hope for something workable. Slope the spots towards a curb. If you are doing back to back (front to front, front to back) parking then you need to figure out the maximum overhang of the 2 vehicles that might park in those 2 spots so you can space the 2 curbs for each spot far enough apart to keep those vehicles from hitting each other.

Of course, a simpler solution is to just let the owner of the vehicle who didn't secure it take responsibility for any damage when it rolls into something.

But the vehicle that is bumped will stop shortly afterwards because of friction. What I am trying to figure out is the typical net coefficient of friction for the entire part of the drivetrain after the transmission gearbox, including the differentials and tires, on a smooth concrete surface. Also, I neved mentioned that the parking lot needed to be practical, which is why I later added the scenario that it is indoors, where it is guaranteed to be free of wind. Here, I'm not leaving out the theoretical case in which there is plenty of money, so a roof like a train shed over the entire lasered-measured-during-construction parking lot (think ball game courts) can be built, and that the constraints are that the parking lot has to have a constant grade throughout and that only regular non-modified production vehicles (excludes cars with rubber tires replaced with solid metal with oil on top, for example) can park there.

I'm more interested in what use this information would be. Suppose I swore that 0.05 degrees was the number. Is that OK? Is that what you will use? If you are looking then for studies - who would ever do that study and what use would such a study be intended for?

That study would be used to determine whether it will be safe to park any regular vehicle with the transmission and parking brake both disengaged on a precisely-made sloped surface, such as a tennis court, under the condition that it is guaranteed there is no wind, no major earthquake, and no other vehicle or large animal would bump into it. This is mainly for curiousity purposes, as it is highly unlikely that a car will be parked on a tennis/basketball/etc. court, with the transmission and parking brake both totally disengaged, and on a totally-windless day.

That being an unrealistic scenario, the answer is NO.

And I spotted your similar thread in the Aircraft section, so now you are going to have to explain yourself.

No such study can be performed. How would you gather enough (all of) the current production vehicles to test that you could form a conclusion.

So you'd do the grand experiment, and then someone picks a new car up from the dealer, who has forgotten to reset the tire pressures down from the shipping pressures (40-50 psi). They jump out of their car and watch in horror as the car roll away over the cliff edge or whatever idiotic design feature is causing the problem.

Cheers

Greg Locock


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Actually, I accidentally did such an experiment. I have a mid-size station wagon from 2010 with all-wheel drive. I was getting stuff out of the cargo compartment and I leaned somewhere between a third and a half of half the vertical to get something from deep inside, which are 15° and 22.5°, respectively. The car was in Park but without the parking brake on and parked without wind on outdoor pedestrian-smoothness concrete in a spot which appeared to be totally level on. The car moved forward a few inches before it suddenly stopped from the parking pawl engaging. The car weighed 3508 lbf and I weighed 170 lbf.

The calculations are:
170sin(22.5°)÷3,508 ≈ 0.01854509
and
170sin(15°)÷3,508 ≈ 0.01254254

This means that the car in Neutral would have started moving on a concrete surface with the smoothness of a sidewalk without any wind or external forces besides gravity and friction somewhere between a grade of 1.25% and 1.85%. Given that this car isn't optimized mainly for efficiency, I would not be surprised if something that is primarily optimized for efficiency, such as a Prius Prime or Clarity Plug-In Hybrid, which both have especially-narrow tires with especially-high pressures, starts rolling at a 1.00% grade in Neutral, or even a 0.80% grade.

I managed to roll start a SAAB 900 on just the drainage grade in a parking lot in college. Worked on the 2nd try I think, had to push it back to the edge and point it at the storm grate.