Max Deflection without Horizontal Curve

[MichSt]

This does not seem to be covered in AASHTO, but I’ve seen two equations used to determine the max deflection that can be used without a horizontal curve:

1. arctan(1/design speed)
2. arctan(60/design speed^2)

What is the appropriate criteria & reference to use for this situation?

My application is a 4-lane local road with 35mph design speed.

Thanks.

 

[dficker]

In Ohio, ODOT uses equation #1 for high speed (50 Mph and greater) and equation #2 for low speed; rounded to the nearest 15" (ODOT location and Design Manual, Vol 1 Fig 202-1E). For 35 MPH, max deflection will be 2^ 45'

 

[MichSt]

Thanks dficker…do you have a hard reference for those equations (e.g. AASHTO, or something like that)??? My state DOT does not have these equations in there road design manual and I was looking for a reference to back-up the use of these equations.

 

[ACtrafficengr]

You could use the MUTCD taper equations

V > 45 mph L = WS²/60
V >= 45 mph L = WS

and plug 1 in for W to give you a rate.



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"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail."

Wm. Phelps Eno, ca. 1928

 

[MichSt]

ACtrafficengr...I see what your saying. That must be how the equations I have shown were developed. So, is that saying that a taper is basically a horizontal deflection w/o a curve? I think I would agree with that, but it seems like round-a-bout reasoning. I’m really looking for a more definitive reference.

Thanks