ARE THE CRITERIA FOR SETTING ADVISORY SPEEDS ON CURVES STILL RELEVANT

The purpose of the study presented in this paper was to evaluate the validity of current criteria for determining advisory speeds on horizontal curves. The study also explored whether the actual posted advisory speeds followed these criteria and how motorists complied with posted advisory speeds on curves. Curve geometry, spot speeds, and ball-bank readings were measured at 28 locations on two-lane highways: 16 in Virginia, seven in Maryland, and five in West Virginia. Each site consisted of either a single curve or a series of curves where only the first curve was studied. Results revealed that the criteria for determining advisory speeds are not valid for modern vehicles. At most curves, posted advisory speeds were not only well below the prevailing traffic speed, but also below the recommended values as suggested by the ball-bank indicator and the Traffic Control Devices Handbook nomograph. This resulted in little compliance among drivers. The study also found noticeable variation in the application of the existing ball-bank criteria from curve to curve and state to state. Alternatives for determining realistic advisory speeds on curves are discussed.