Evaluations of Ride Equation

The Texas Department of Transportation (TxDOT) annually collects profile data over the state-maintained highway network. TxDOT uses the profile data to determine ride quality based on the present serviceability index (PSI). The indices determined are stored in the Pavement Management Information System (PMIS) database, and are published in the PMIS reports prepared annually by the Materials and Pavements Section of the Construction Division. The existing equation for determining PSI from measured profile is based on ride measurements obtained from a rating session conducted in the late 1960s. Since its original development over 30 years ago, a number of changes have taken place that requires a re-evaluation of the current equation. These include: changes in vehicle design, such as improvements in vehicle suspension and handling characteristics, and the switch from the predominantly large automobiles used in the late 1960s to the smaller and more fuel efficient mid-sized and compact cars of today; migration from the response-type roughness measuring devices, i.e., the Mays ride meters used by TxDOT in the 1970s and early 80s, to the inertial profilers that are now standard within the department; and change in the interval used to report PSI, from 0.2- to 0.1-mile. In view of the above changes, TxDOT funded a research project with the University of Texas at Arlington and the Texas Transportation Institute to investigate the adequacy of the current ride equation to estimate ride quality under present-day conditions. To accomplish this objective, the researchers and project monitoring committee conducted two ride surveys on asphalt and portland cement concrete pavements for the purpose of collecting user opinions of ride quality with which to evaluate the existing equation. It was decided in the initial project meeting held during the fall of 1998 between researchers and TxDOT project members, to limit the length of each survey section to 0.1 mile intervals to maintain consistency with the standard PSI reporting interval. The present report documents the research efforts conducted in this project. Before discussing the research effort, a short history describes the evolution of the current ride equation or PSI.