Determination of IRI Limits and Thresholds for Flexible Pavements

Abstract

The International Roughness Index (IRI) is an important pavement roughness measure that can be used to trigger appropriate maintenance treatments, approve new and rehabilitated roadways, and determine contractors’ performance incentives. This study developed a systematic approach to determine IRI limits and thresholds for flexible pavement. A total of 241 research participants were randomly recruited from 9 counties in 3 regions of North Carolina. Their perceptions of acceptable and unacceptable smoothness of preselected roadways were collected and analyzed. It was concluded that if the measured IRI value is less than or equal to 1.66 m/km (105 in./mi), most likely this section will be rated as acceptable; the rating will be unacceptable if the IRI value is greater than or equal to 2.37 m/km (150 in./mi). The acceptable and unacceptable IRI limits for urban and rural roadways in North Carolina were also determined. These findings can be used to derive IRI thresholds for construction approvals and contractors’ performance incentives. Furthermore, these IRI thresholds can be used to determine appropriate IRI trigger points for maintenance strategies. Although IRI limits and thresholds were determined using data provided by the North Carolina Department of Transportation (NCDOT), the method developed in this study can be easily adapted for use in other state DOTs.