Development of IDOT’s Proposed Smoothness Specification Based on the International Roughness Index

Project Details









Hyung S Lee, William Vavrik


Illinois Department of Transportation


Benefit cost analysis, Comfort, Life cycle analysis, Service life, Smoothness, Specifications

Project description

The existing smoothness specification implemented by the Illinois Department of Transportation (IDOT) was based on profile-index measurement, which was also widely used by other agencies. There is a national and international trend, however, toward using the international roughness index (IRI) as a standard smoothness index for pavement management as well as construction acceptance. Therefore, the primary objective of this study was to develop an IRI-based smoothness specification for IDOT. It was desired that IRI thresholds be developed objectively by quantifying the benefit of pavement smoothness. The benefits quantified within the framework of life cycle cost analysis and benefit-cost analysis generally revealed that smoother pavements are anticipated to exhibit increased service life, reduced life cycle cost (including agency and user costs), and improved safety. IRI is also a crucial component of user comfort, especially for high-speed facilities. IDOT’s new IRI-based specification was developed while considering these benefits along with the increased cost quantified as the incentive payments to be made for smoother pavements. In addition, several aspects of IRI thresholds and the smoothness assessment schedule were investigated for achievability and risk. Although the risk of moving from a profile-index-based specification to an IRI-based specification was found to be relatively low (i.e., 4.4% and 6.6% risk assessed for high-speed and low-speed facilities, respectively), it indicated that IDOT and IDOT’s contractors may need to work together to adapt to the new specification and continue to refine the IRI-based specification as more experience is gained.