Private road 6

[beameupscotty]

What would be the maximum grade break for a private roadway serving 4 residential units? I have an inverted crown section, so everything drains to the middle and this grade break would occur near a 90 degree turn. Client wants me to raise grade to more closely match existing ground, but I'm afraid to put more than a 2% change in slope (going from 2% to 4%)

Thanks

 

[goodwin7]

I hope this may help, I live in Tennessee and serve as a City Engineer for a small community. Our subdivision regulations that dictates our street design is developed with assistance from the State of Tennessee Planning Division. According to our subdivision regulations, a street to serve only 4 residential units would be classified as an access lane and the maximum grade is 12%, The regs give no algebraic difference in grade between forward or backward tangents, just a minimum vertical curve length of 150'. However, I think in your situation that technically you would want to check corner site distance since you are so close to a near 90 degree bend. This requirement is a point at 3.5'from the drivers eye to the pavement to a point 150' away at 3.5'above the pavement, I would raise the grade to that point and calculate what it would be. Regardless, if you are referring to vertical curve grade changes a 2% to a 4% is not unreasonable to do.

 

[Maury]

I checked an AASHTO publication, Geometric Design of Very Low-Volume Roads, for vertical curve requirements. The required length of a crest vertical curve is determined by the formula L=K*A, where L is the length of the curve, A is the algebraic difference in the percent of slope between the intersecting grades, and K is a coefficient that is dependent on the design speed. For a 20 mph design speed, K = 4. If the slopes are +4% and -4%, A would equal 8, and L = 4*8 or 32 feet. Because of the proximity to a 90% curve, this might qualify as a "higher risk" location and K(20mph) would increase to 5.