Relationships Between Ball Bank Indicator Readings, Lateral Acceleration Rates, and Vehicular Body-Roll Rates

The objective of the current AASHTO criteria for horizontal curve design is to select the radius and superelevation so that the lateral acceleration perceived by the occupants of vehicles traversing horizontal curves is kept within comfortable limits. It is considered good design practice to provide roadways on which these comfortable lateral acceleration limits are not violated within an appropriate range of speed values. However, because of geometric constraints, some horizontal curves require significant speed reductions to maintain motorists’ comfort levels. The ball bank indicator has been used since the 1930s to measure the speed at which occupants become uncomfortable. Reported in degrees, the reading of the ball bank indicator is the sum of a vehicle’s lateral acceleration and the body-roll minus the roadway superelevation. Ball bank indicator readings and lateral acceleration levels are both indicators of safe speed based on vehicle occupants’ comfort levels. Both have been used for highway design since first being introduced, although they are related only theoretically. The criteria for using ball bank indicators have been modified only slightly in the last 50 years, while vehicle technology and societal changes have experienced significant changes. Ball bank indicator readings were correlated with lateral acceleration rates. Additionally, the influence of vehicular body-roll on ball bank indicator readings was investigated. Ball bank indicator readings and lateral acceleration values were found to be highly correlated. The influence of body-roll on ball bank indicator readings appears to be negligible when using typical passenger cars to determine safe speed on horizontal curves.