Calculated Speed

For curves and turns, the calculated speed is to be used as a guide for making the initial test run and as a check on the speed obtained by the use of the mechanical or electronic bank indicator. The calculated speed is not, however, to be used as the sole basis for selecting the posted speed. (See “Selecting Speed for Posting” at the end of this section for additional discussion.)
Calculate the design speed of the curve under consideration using the formula
(click in image to see full-size image)
where:
V
 = vehicle speed in miles per hour
R
 = radius of curve in feet
e
 = rate of roadway superelevation in feet per foot
f
 = side friction factor with value depending on speed as follows:
If the speed is:
then the sidefriction factor is:
  • 35 mph or greater
0.15
between 20 and 35 mph
0.18
20 mph or less
0.31