max street slopes

[johnhan76]

We are designing a subdivision road (speed limit 25 mph). The local standards allow up to 15% max grade but I am uncomfortable going above 12. no reason to be nervous but i have always used 12 as a rule of thumb. any insight would be appreciated.

 

[RJLDesign]

I agree 12% usually is not what you want to design above. How ever going to 15% can be done but should only be used as a last resort method.

However having a heavy grade on the road can save you having to need a "slow point" in the road. Because people are not going to fly down the grade and they are not flying up it either.

Be careful when you flatten off the grades to. most local stds. have a maximum VC radius and you may have a problem with sight distance at the crest.

Do you have to bench you pads along side the road. Seems you have some steep terrain there. If you have to cut into the hill to have benched pads, maybe your road can be pushed down to 12%.

Then all you have to worry about is balancing your earthworks ! and I know how fun that is

Hope this helps

Rj

 

[KRSServices]

In residential subdivisions and in a perfect world, I would not design above 11%. If at the top or bottom of the gradient there are stop or yield signs, and a few driveways, I would not go more than about 7 to 9%. If there is a possibility of freezing, then I definitely would not recommend anything higher than 8 or 9%.

I know trucks are not supposed to frequent a residential neighbourhood, but for disabled people and people whom require special deliveries via trucks (oxygen bottles, propane, etc) the steeper slopes increase the probability of a serious accident.

KRS Services

http://www.krs-services.com

 

[johnhan76]

Thanks for the help - I am accustomed to flat ground. I have flattened the final grade to 12%. I performed a cross sectional anaylsis using autodesk LDD. At one section there is an 8' centerline cut and a 13' cut up to the lot on the high side. THis seems unacceptable - my references show a max grade of 7% for driveways. 13'/.07 is over 100' which is way unacceptable. I will regrade the road for small cut - what should i shoot for as a max cut and is my reference right about the 7% driveway?

 

[CarlB]

In hilly areas around town here, many driveways are in the 15% to 20% range. But as for the driveway at the house where the car will park, 7% is not a bad maximum to hold (although again here some are around 10% -need good brakes). As long as you have a relatively flat landing where the driveway meets the street, I'd say up to 15% driveway grade is acceptable.

 

[johnhan76]

Do you recommend some type of vertical curve in the driveway where it meets the street. I was sort of concerned about the car scrubbing the bottom. I have a 10' landing (shoulder with utilities) adjacent to my street that is graded at 2% towards the street.

 

[CarlB]

Yes you should have a short vertical curve - say 20'. It would be pretty hard to construct it without a vertical curve using normal heavy construction equipment. A 15% grade break without a vertical curve would cause some vehicles to drag bottom or tail end.

 

[gjeppesen]

That brings up a good point. Could somebody tell me how to design (and when I need to design) a vertical curve for a driveway?

I don't have a particular situation at the moment.

Thanks.

 

[Maury]

Architectural Graphic Standards has vehicle dimensions that suggest that any change in grade of 8 degrees or more should have a vertical curve. This is a change in grade of about 14%. I use a vertical curve any time the change in grade is more than 8%. A curve length of 20' should be fine for any drive, and a 10' length will work for grade changes of 8-12%.

 

[johnhan76]

Keep in mind that this 8% change in grade only applies to driveways or other roads where the traffic is very slow. We design subdivision roads to have curves when there is a grade difference of 1% or more.