Development of Speed-Related Ride Quality Thresholds Using International Roughness Index

Abstract

The International Roughness Index (IRI) has been used as a road roughness index on highways for nearly two decades. Research shows that subjective ratings such as present serviceability rating or mean panel rating could be reasonably predicted from IRI. Acceptable IRI values for highways have been developed and published by FHWA. Such IRI standards, however, are not applicable to local streets. Because vehicle speeds are lower on local streets, a comfortable ride can still be achieved at higher IRI values. There is a need to determine the acceptable IRI threshold values for local streets with various speed limits. Such IRI threshold values allow local pavement management officials to compare objectively the ride quality of streets with different speed limits. It was found that human ratings depend linearly on the logarithm of the rate of change of the vertical acceleration, namely, the jolt or jerk experienced by the raters. By analyzing the IRI records in the Long-Term Pavement Performance database and the corresponding calculated jolts, the authors found that jolt is linearly proportional to IRI at a given speed and approximately linearly proportional to the travel speed for a given IRI. By assuming further that the same jolt will lead to the same ride quality and that Interstate highways are operated at 120 km/h (75 mph), the authors use the jolt corresponding to the IRI thresholds set by FHWA for highways to develop speed-related ride quality thresholds at different travel speeds. The variation of jolt for roads with the same IRI value is also discussed.