32 ROUGHNESS OF RIDE The International Roughness Index (IRI) is a measure of how rough or smooth a road is, and how it affects the ride quality of a vehicle. The IRI is a mathematical calculation that simulates a vehicle’s response to the unevenness of a road’s surface. The lower the IRI, the smoother the road. The IRI is calculated using a profiler, a device that measures the distance between a vehicle and the road surface. The IRI is reported in units of inches per mile (in/mi), meters per kilometer (m/km) or millimeters per meter (mm/m). Roughness affects vehicle delay costs, fuel consumption and maintenance costs. Studies have shown that smoother roads can lead to better fuel economy. A rough ride on asphalt can be caused by a number of factors, including: • Deeper problems in the pavement, such as rutting, upheavals or depressions. • Traffic loading can cause pavement to deteriorate. • Weather conditions like rain, snow, frost and storms can contribute to pavement roughness. • The type of aggregate used in the asphalt mix can affect the surface texture. • Poor construction techniques can lead to pavement settling and depressions. • An unstable subgrade can cause pavement to settle and depressions to form. • Insufficient pavement thickness can lead to problems. • Improper curing and finishing of concrete can cause disintegration. The roughness of asphalt can be prevented by: • Considering pavement design. • Maintaining and rehabilitating pavement timely. • Paying attention to test sections.
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