In my experience and local building bylaws suggest;

5% for parking
8% for firetruck
12% for public Road
15-21% for driveway ( should be heated if you expect to drive on it in winter conditions)
Beyond that you'll need "the Hummer"

or better yet, a defender. and pushing 30 mpg, a much more environmentally friendly alternative.

besides, don judd had one.

i think they stopped importing defenders in 2003 because they weren't meeting emission standards. that's what my local land rover dealer told me.

actually, they were manufactured specifically for the u.s., not imported. the defender stopped sales in the u.s. in 1998 due to regulations requiring air bags for both passengers and stricter side impact requirements, which couldn't be implemented without a major design change. instead, we get the disco (which looks like a clown car) and the freelander, which looks worse than the p.t. cruiser. the emissions for the defender are slightly better than the hummer, unfortuantely.

luckily you can still buy them used, and since over 65% of manufactured defenders are still on the road, they've got some life to them. i've been trying to get my uncle to offload his for years now. he uses it on his farm, and it's only got 20k miles on it (it's a '93 110)

Sorry I'm not a fan of the Hummer it was the first thing that came to mind!

On another note a house I did for a friend insisted on building on the lowest part of his property, and required a 25% driveway down. I told him it was a joke and that he was crazy but neighbors had done the same and it seemed "okay" two years after its been built he's never driven his car down to the garage... why would you listen to your architect they just draw stuff... hahahahaha!

there's no way a defender will get 30mpg.
plus, as they've become a classic car (no longer available for sale - except used- in the states) people are asking astronomical prices for them. just seems counterintuitive as their beauty is in their basicness. really stomper truck like. similar reverse logic following the reissue of the beetle (no longer a car for the masses). i love the 90, but i could never afford one..

the latest defender gets 30 mpg.
the older ones are 15-20-ish.

most codes prohibit roads/driveways over 20%

the codes i refered to say not more than 1 in 8 but i was wondering if a 1 in 6 slope will work?

just because you 'could' build it, it would be awful and un-usable. if the code says 1 in 8, then you can't do 1 in 6.

time to figure out another solution to bring cars onto the site. maybe you need to hire an LA to do this for you...

No kidding Treekiller ...my "friends" driveway can't even be seen over the nose of my car.... how's someone suppose to drive and negotiate that????

teton pass is about 10% and it is simply frightening in places

California State Route 108 claims to have a maximum slope of 26%.  CA SR-4 has a claimed maximum slope of 24%.  A small economy car can negotiate either of these roads, as can a pickup truck with a 2400 pound slide-in camper on top and I have seen a large 5th wheel towed by a pickup truck navigate SR-108 at a substantial speed.  Full length 18 wheel tractor-trailer rigs have trouble and I have seen one slide backwards down the hill.  Both SR-108 and SR-4 get closed when there is any snow accumulation on the road.

Not being able to see forward is an issue with the vertical curvature of the road rather than the slope.

That said, in areas of California that get snow if you expect a fire truck to be able to arrive in winter and help out with whatever emergency the limit is 7% grade.

High slope driveways also have run off and erosion issues.

We do have an access road we have to build and landed on this forum; thanks for the advice.
By the way; the Defender's predecessors are the real deal ... these are my two best friends in Arequipa-Peru:
LR's Series 2a-109-SW, 1969 and 1970.