Superelevation Transition Question

[Joewsu]

Can anyone help me determine how to calculate the transition length from one superelevation to another superelevation? For examaple, going from a 4% super to a 5% super, what length would you transition over, for any generic speed?

 

[Maury]

You didn't say anything about a tangent section between the curves and I'm also assuming both curves are in the same direction. The AASHTO book on Geometric Design of Streets and Highways doen't seem to address this question directly. I would calculate the required length of runoff for the 5% s.e., then apply the tangent runoff portion to the end of the curve requiring the 4% s.e.

 

[Joewsu]

They are actually one curve. The super elevation changes within the entire curve length. It is along a ramp within an all direction interchange (N-S interstate to E-W interstate).

 

[ACtrafficengr]

The "Green Book" discusses runoff rate, as in feet of runoff per percentage of super. IIRC, it's something like 200' per percent at 55 mph. If I'm right, you would use 200' to go from 4% super to 5% super.

Be careful with compound curves like this. If it isn't obvious that the radius is changing, you may have to add some cues, like shorter chevron spacing or something.

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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