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Maximum common drive slope considering "icy" conditions ?Helpful Member!(5) 

RomanEngineer (Civil/Environmental) (OP)
13 Nov 08 9:04

Does anyone have information on the maximum slope a common drive can be?  I am located in north Georgia where snow and ice can be a concern from time to time.  There are existing drives that exceed 20% in the area; I just don't know how  passable they are in icy conditions.

The current regualtions for a subdivision streets set the maximum slope to 14%; since "common drives" are private the maximum street slope does not apply.  Interestingly the 14% street grade appears to conflict with the International Fire Code max of 10%

Any help is greatly appreciated

Thank you.
Helpful Member!  dgillette (Geotechnical)
13 Nov 08 18:26
Mine is 12-15%.  Sometimes takes a running start up a street with a slope of about 10%. Fortunately, it's short, straight, and faces south, with no real consequence to falling short and having to try again (no heavy traffic or cliffs).  On one side (the side with the 1-car garage, wherein my wife's car usually dwells), we have an electric heater that helps keep it clear, but it requires a little assist from a shovel if there's much snow.  On the other side, I shovel and spread pea gravel.  There are times I've had to put on chains or leave the car 50 or 100 m away if there was snow or freezing rain while I was at work.
frosty2
Regardless, I would shudder to think about a 20% driveway with ice on it, unless I had chains on all four for traction and braking.  Given my choice, I'd avoid anything steeper than 10%.  Fourteen is too steep, IMO.

I should note, however, that gravel is usually better than pavement in this regard.
Helpful Member!  cvg (Civil/Environmental)
13 Nov 08 18:35
10-12% is pretty common requirement for maximum in flatter regions of the country.  Grade difference between street and drive should be less to avoid ripping bumpers and tailpipes off.  In mountainous areas, steeper drives and public roads are more common since it is often infeasible to go flatter.  Four wheel drives plus lots of sand and salt are required in these areas.
Helpful Member!  oldestguy (Geotechnical)
17 Nov 08 9:54
I would not worry so much about those not being able to climb it.  However, what is at the bottom of the "slide"?  In our area (Wisconsin) most problems come from not being able to stop on the down hill run or running off to the side.

 Where is the sun with respect to this?  Sun shining on an asphalt pavement does wonders.  Even without the sun on a cloudy day day, that black pavement is warmer than concrete.
Helpful Member!  civilman72 (Civil/Environmental)
17 Nov 08 11:14
I deal with this all the time.  FWIW, here's the "order" of design I would recommend:

1. Hold design to 10% max.

2. If you need to go steeper than 10%, having pavement and south facing is most desireable.  Anything you can do to increase sun exposure is a plus (including removing vegetation).   

3. If you have to go steeper, but do not have good sun exposure, try to keep the steeper portion of the driveway away from the driveway/road intersection, to give vehicles coming down the driveway an area to stop, before sliding through the road and potentially hitting another vehicle.  For example, I would rather design a road with a 30'-40' flat area (<5%) at the road intersection, and then 14% for 200' up to the house, than having a 250' driveway with a straight slope from house to road at 12.5%.

4. If you lack sun exposure, gravel is actually better than asphalt (doesn't ice up as easy).

5. Finally, if it's economically feasible, you can heat the driveway.

Also, be very careful with liability!!!  If you design a driveway, and emergency vehicles are unable to access the house due to an improper design on your part (i.e. too steep, insufficient cl radii, etc.), you could be held liable.  So IF you go over a 10% design, limit your liability before you sign/stamp.  
Helpful Member!  rday (Structural)
25 Nov 08 18:09
I used to work for a county in the Colorado high country.

Our code stated maximum was 8% for drives and 6% for roadways. One of the local fire protection districts ran live tests using their equipment on one of the older roads in the county to help us develop the code. Emergency access drove the requirements.

I agree that 10% is likely the upper boundary. Any steeper and it gets really problematic for most vehicles.

civilman72 gives some good ideas to consider.

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