CORRELATION OF IRI TO PUBLIC PERCEPTION OF PAVEMENT ROUGHNESS. FINAL REPORT

Transforms relating PSI (Present Serviceability Index) to IRI (International Roughness Index) and also to RN (Roughness Number/Indiana, collected as Ride Score) were developed by this study. PSI for the purpose of this research is considered as an estimate of mean PSR for Indiana drivers. Supportive data were obtained by having 68 typical Indiana drivers ride and rate 66 one-half mile long road sections which were also tested with the FHWA ProRut (IRI) and a Model F Cox Roadmeter (RS later converted to RN). The sections were selected to approximate the roughness distribution of Indiana roads. Several transform functions were investigated to find appropriate equations to relate PSI to either IRI or RN. The following equations are recommended for calculating PSI (Indiana) from either IRI or RN: Bituminous Surfaced Pavements: PSI(RN) = 10.14 - 1.057 Ln(RN), R-squared = .86 and PSI(IRI) = 9.58 - 1.363 Ln(IRI), R-squared = .94; Concrete Surfaced Pavements: PSI(RN) = 12.57 - 1.416 Ln(RN), R-squared = .80 and PSI(IRI) = 14.25 - 2.328 Ln(IRI), R-squared = .85. The suggested transforms should be considered applicable in supporting appropriate programs. For conservative values the linear models denoted in the report are fairly good predictors of PSI from either IRI or RN. Appendix A shows transform comparisons among this study's results, a very recent limited INDOT study (100% State funds), and findings from a University of Illinois study (B. Al-Omari and M. Darter, Sept 1992). By applying a user statisfaction level of 85% to the recommended transforms, this study determined critical values of PSI, IRI and RN which define the point of separation between acceptable and unacceptable roughness. The numerical values corresponding to a user satisfaction level of 85% are as follows: Bituminous Surfaced Pavements: When PSI = 2.2, then RN = 1830 (expression of counts/mile) and IRI = 225 (in./mile); Concrete Surfaced Pavements: When PSI = 2.2, then RN = 1515 (expression of counts/mile) and IRI = 162 (in./mile). A profiler similar to the FHWA ProRut was completed as a result of this study. The Indiana profiler, or RIP (Rut-IRI-Profile) is capable of collecting profile and rut depth information using three to five laser/optocator sensors which are used in conjunction with accelerometers, operational hardware, C language software, and a SWAN 386-25/C PC. It is currently operating to collect average IRI and rut depths on each tenth mile of INDOT's road network. It also operates in a special test mode which stores elevation points as frequently as four times per ft so detailed longitudinal profiles can be generated and analyzed. By late Spring of 1993, INDOT hopes to add nine ultrasonic sensors to the RIP to improve rut measuring detail. IRI, however, will continue to be obtained with the laser/optocators.