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Measuring Curve Speed

MUTCD image showing curve signing

Ever driven a curve signed with a reduced speed? Curious how curve speed is determined?

The Manual on Uniform Traffic Control Devices (MUTCD) sets minimum signing standards and provides guidance to ensure uniform traffic control devices across the nation. New curve signing standards need to be implemented by 2019 and MDT is reviewing our current signing.

MDT interns, Calvin Reahard and Kevin Marsh, worked on a curve speed analysis project in Montana. Watch the intern video.

Using a ball bank indicator, which measures overturning force, Calvin and Kevin drove the curve in 5mph increments to determine the side friction. Their goal was to determine the speed that corresponds to a 12-degree ball bank indicator reading. Twelve degrees is the appropriate degree reading for a roadway with a driving speed of 35 mph and higher. Once a twelve degree reading (above and below) is obtained a bit of math determines the appropriate speed for the curve.

MDT uses the recommended speed to determine the correct signing for the curve. An example of curve signing is shown in the MUTCD image on the right.

You can read more about measuring curve speed at: