Maximum Driveway Slope

[msquared48]

I have commonly limited driveways to 6 to 10% maximum slope in past projects, but a project I am preparing a proposal for is currently shown at 30% by the architect.

I am considering walking away if the slope cannot be lowered as I do not want the liability for that design. It is a very constrained site sitting on a steep slope site anyway.

My portion is to design a vehicular access bridge with the same slope...

I am going to email the Architect, but before I do that, would like some input as to whether or not I am all wet...

Thanks in advance for any input.

Mike McCann, PE, SE (WA)

 

[cvg]

you're dry
I would limit to 20% maximum and require a transition section on both ends to prevent high center/bumper dragging. Under no circumstance would I have the cars actually parked on a 30% slope.

 

[msquared48]

cvg:

This is for vehicular access, not parking. Transition sections are present...

Still the same thought?

Mike McCann, PE, SE (WA)

 

[miningman]

I have operated underground equipment on 25% ramps. Heavy duty 4 wheel drive systems were necessary and even then it was tough on the equipment. These days we restrict ourselves to 15%. For normal street vehicles , I would say the architect is insane to even consider this value.

 

[fel3]

https://www.onlyinyourstate.com/northern-california/san-francisco/steepest-streets-san-francisco/

San Francisco's steepest streets are in the 30%-35% range.* I have driven some of the streets listed in the article and they are disconcertingly steep. They're also really fun when you have a manual transmission, like I did in my first car. Drive roads this steep every day and, it seems, replace your brakes every month (and clutch, if you have one). Seriously, though, grades this steep are so much more dangerous than flatter grades, especially when wet, icy, and/or when visibility is compromised and grades this steep are much harder on vehicles. But, surely you know this.

Waipio Valley Road on the Big Island of Hawaii has a 25% grade (per Wikipedia) and most car rental companies say you can't drive their cars down it. When we visited in 2005, we rented a 4WD with low range from Harpers specifically for this road and to drive up to the top of Mauna Kea.** Expensive, but worth it, and the low range was invaluable. Even the steepest climb I have done in my stock Grand Cherokee--climbing bare granite to reach the top of Bald Mountain near Shaver Lake in the central Sierra Nevada--is only about 25% max.

Back to the question at hand--If you plan to go ahead with the project, I suggest finding out what the local jurisdiction has to say about maximum driveway grades. They may not allow 30%, or even 20%. A few other questions come to mind: does the homeowner plan to tow a trailer up or down the driveway, is the site prone to snow, ice, and/or heavy rains, is the driveway long enough that fire department access must be provided,*** etc?

So, while quite a few streets (and probably driveways, as well) exceed 30% grade, I would not want to touch this design myself. It's not that I couldn't design it, it's that the potential liability will likely far exceed the value of the project to you. If you get sued because of an accident, the plaintiff's attorney might legitimately argue that 30% (or whatever slightly flatter grade you might come up with) is outside of normal standards and thus was a negligent design. A Google search for //maximum driveway grade// and similar turned up lots of recommendations (some professional, some not) for much flatter maximums than 30%. I think the plaintiff's attorney would start there, then bring in an expert witness who would likely state that something like 15% or 20% max is the "standard of practice." That doesn't mean he's correct, but it would be an uphill (pun intended) battle to sway the judge or jury otherwise.

* BTW, 35% is "only" 19.3°. Only.
** I didn't find the max grade for the Mauna Kea Road, but my recollection from driving it is about 15% for the unpaved section.
*** A typical maximum grade for fire apparatus is 15%. In fact, this requirement deep-sixed one of the few development projects I have ever worked on: the site was so steep that we couldn't make the developer's layout or any of our alternatives work without getting the roads to 20+%. Nearly 20 years later, the site is still undeveloped.

-- Fred

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[msquared48]

Thanks guys. I guess i’m Not all wet then... yet.

Mike McCann, PE, SE (WA)