University of Virginia Library

DRIVER BEHAVIOR:

Imagine yourself approaching a circular curve (without transition)
at 40 MPH; you slow down to stay within your lane or, if the
road is clear ahead, you encroach on the adjoining lane to maintain
speed. In either case you have selected a transition course to
travel around the curve. If you encroached on the adjoining lane,
you created a traffic hazard to yourself and to others.

Suppose, too, that the curve was superelevated and you drove
comfortably around the curve without skidding and increased your
speed where the road straightened out again. Had the curve not been
superelevated, you would have slowed down around the curve to avoid
skidding. You experienced the conditions Barnett intended to improve
to make roads safer.

In order to persuade engineers to design for transitions with
superelevation, he developed tables to simplify calculations and

reduce engineering costs. The tables were published in Barnett's
book, Transition Curves for Highways.

Because Barnett's tables resulted in minus superelevation for
curves flatter than 22° at 25 MPH, and 4.5° at 55 MPH, AASHO chose
to use the equation found in Seelye's:

You will find that Virginia Highways uses standard normal crown
and 0.0 superelevation for curves flatter than 22° at 25 MPH, and
flatter than 4° at 55 MPH.