THE ACCESS LINKS FOR THIS RECORD WERE REPORTED AS BROKEN. THE RECORD MAY HAVE BEEN RETIRED BY THE HOST. International Roughness Index is a statistic of a pavement surface s longitudinal profile that quantifies ride quality as the estimated accumulation of a passenger vehicle s vertical movement (in inches) as it drives over the subject road for a distance of one mile. The reported IRI units are, therefore, inches-per-mile, with a lower IRI value depicting a smoother ride. This Transportation Performance Measurement reports the average IRI calculation determined from laser-measured elevation changes in each wheel path.