Hi.

Planning to extend coverage of the back yard with ~40mm high gravel for an area automobiles park.  Already did a section using about 25 mm sized gravel;  the quarry has finer sizes, like 12 and 8 mm.

Which should provide better traction?   There is a mild slope, like 5 degrees.  Gravel goes on top of polyethilene film placed over the soil (to prevent weeds from poking trough).

Here is a suggestion.

Quote

Crushed stone works well for driveways and paths because the angular pieces tend to hold together as the gravel settles, creating a stable, solid surface that doesn't shift or migrate under the weight of vehicles and foot traffic.

In general, driveway gravel should be relatively large, between about 3/8 and 3/4 inch in diameter, as compared to the gravel used for footpaths, which is typically 1/4 to 3/8 inch in diameter.

https://www.hunker.com/13424503/types-of-gravel-for-driveway

17 hours ago, zapatos said:

Here is a suggestion.

https://www.hunker.com/13424503/types-of-gravel-for-driveway

Good suggestion +1

I'll add the following to this.

The stones should not be all of the same size, whatever size you choose.

The gravel should be 'graded'.
This means it should include a range of sizes from the biggest to quite fine.

The point is that you are trying to get the smaller sizes to fill the gaps between the larger sizes.
And then the even smaller sizes to fill the smaller gaps
and so on.

This produces a dense compact 'pavement' to run vehicles on.
Original roads were made like this by McAdam.
These were further compacted by the action of traffic wheels and weights grinding them together.

Single sized aggregate will always be unstable and slip about under the action of wheeld vehicles.

 

Zapatos comment about angular stones being better than rounded oned quite correct. These achieve better particle interlock.