Ever wonder just how the suggested speed through a curve is determined? Establishing the proper advisory speed for a curve is a critical step in ensuring the safety of our roadways. A curve speed study will establish the proper advisory speed that should be listed on the advisory speed plaque and posted with the advanced warning curve or turn sign.
The most common method to determine just what speed should be posted is to use a ball-bank indicator (BBI), a curved glass tube which is filled with a liquid. A weighted ball floats in the glass tube. The ball-bank indicator is mounted in a vehicle, and as the vehicle travels around a curve, the ball floats outward in the curved glass tube. The movement of the ball is measured in degrees of deflection, and this reading is indicative of the combined effect of superelevation, lateral (centripetal) acceleration, and vehicle body roll. Also, the ball-bank indicator test is normally a two-person operation, one person to drive and the other to record curve data and the ball-bank readings, especially if advisory speeds are being determined for a series of curves.
A series of test drives through the curve is made, starting with a relatively slow speed and proceeding at speeds five mph faster each subsequent time, measuring the degree of deflection shown on the BBI. As the speeds increase, so too does the degree of deflection. Using accepted values from the Manual on Uniform Traffic Control Devices (MUTCD), advisory speeds are determined by a combination of the degree of deflection for a particular speed range. The allowable degree of deflection is higher for slower speeds. Any speed 35 mph and above should not have a deflection greater than 12 degrees as shown on the BBI.
The curve advisory speed is set at the highest test speed that does not result in a ball-bank indicator reading greater than an acceptable level. For example, a series of test runs for a curve were started at 25 mph, with ball-bank indicator reading of about 6-degrees. This is well below the suggested criteria of 14 degrees for a speed of 25 mph. The speeds of the test runs were increased in five mile an hour increments until the speed of 35 mph gave readings of 10-12 degrees. These are the highest readings attained without exceeding the suggested criteria of 12-degrees for a speed of 35 mph or more, because the speed of 40 mph gave readings of 13-15 degrees. Therefore, this field measurement would result in posting an advisory speed of 35 mph for this curve.
Thus, when you are driving along our highways, especially on rural highways, and you see a curve sign with a supplemental plaque showing an advisory speed, you can feel confident it was scientifically determined and not just some person's estimate of a proper speed.
For more articles on Vehicle and Traffic Law and traffic safety, visit the Traffic Safety Board's website at: www.franklincony.org/ and click on the Traffic Safety Board from the pull-down menu under departments. "Like" us on Facebook as well. You may also call me, Dave Werner, at 483-1882 with your comments and questions or email me at: firstname.lastname@example.org