The International Roughness Index (IRI) is the statistic used to measure how smooth or rough a pavement surface is. The lower the calculated IRI, the smoother the pavement will ride. The higher the IRI, the rougher the pavement will ride. The units of IRI are usually in/mi, m/km, or mm/m. For more detailed information on IRI, refer to The Little Book of Profiling (Sept 1998), by Michael W. Sayers and Steven M. Karamihas. A relative appreciation for qualitative smoothness descriptions versus numerical values of IRI is given in Table 11-1.