International Roughness Index
Encyclopedia
The International Roughness Index (IRI) is the roughness index most commonly obtained from measured longitudinal road profiles. It is calculated using a quarter-car vehicle math model, whose response is accumulated to yield a roughness index with units of slope (in/mi, m/km, etc.). Since its introduction in 1986, IRI has become the road roughness index most commonly used worldwide for evaluating and managing road systems.
The measurement of IRI is required for data provided to the United States Federal Highway Administration, and is covered in several standards from ASTM International
: ASTM E1926 - 08, ASTM E1364 - 95(2005), and others. IRI is also used to evaluate new pavement construction, to determine penalties or bonus payments based on smoothness.
The United States National Cooperative Highway Research Program (NCHRP) initiated a research project to help state agencies improve their use of roughness measuring equipment. The work was continued by The World Bank to determine how to compare or convert data obtained from different countries (mostly developing countries) involved in World Bank projects. Findings from the World Bank testing showed that most equipment in use could produce useful roughness measures on a single scale if methods were standardized. The roughness scale that was defined and tested was eventually named the International Roughness Index.
The quarter-car math model replicates roughness measurements that were in use by highway agencies in the 1970s and 1980s. The IRI is statistically equivalent to the methods that were in use, in the sense that correlation of IRI with a typical instrumented vehicle (called a "response type road roughness measuring system", RTRRMS) was as good as the correlation between the measures from any two RTRRMS's. As a profile-based statistic, the IRI had the advantage of being repeatable, reproducible, and stable with time. In the math model, the simulated vehicle speed is 80 km/h (49.7 mi/h). The properties of the simulated vehicle were selected in earlier research to provide high correlation with the ride response of a wide range of automobiles that might be instrumented to measure a slope statistic (m/km). The damping in the IRI is higher than most vehicles, to prevent the math model from "tuning in" to specific wavelengths and producing a sensitivity not shared by the vehicle population at large.
The slope statistic of the IRI was chosen for backward compatibility with roughness measures in use. It is the average absolute (rectified) relative velocity of the suspension, divided by vehicle speed to convert from rate (e.g. m/s) to slope (m/km). The frequency content of the suspension movement rate is similar to the frequency content of chassis vertical acceleration and also tire/road vertical loading. Thus, IRI is highly correlated to the overall ride vibration level and to the overall pavement loading vibration level. Although it is not optimized to match any particular vehicle with full fidelity, it is so strongly correlated with ride quality and road loading that most research projects that have tested alternate statistics have not found significant improvements in correlation.
IRI was originally developed for summarizing lengths of road to determine if and when repairs or maintenance are needed. It can also be calculated as a continuous property of a profile by using a moving average to provide a "roughness profile" IRI profiles are sometimes used to evaluate new construction to determine bonus/penalty payments for contractors, and to identify specific locations where repairs or improvements (e.g., grinding) are recommended.
The measurement of IRI is required for data provided to the United States Federal Highway Administration, and is covered in several standards from ASTM International
ASTM International
ASTM International, known until 2001 as the American Society for Testing and Materials , is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, and services...
: ASTM E1926 - 08, ASTM E1364 - 95(2005), and others. IRI is also used to evaluate new pavement construction, to determine penalties or bonus payments based on smoothness.
History
In the early 1980s the highway engineering community identified road roughness as the primary indicator of the utility of a highway network to road users. However, existing methods used to characterize roughness were not reproducible by different agencies using different measuring equipment and methods. Even with a given agency, the methods were not necessarily repeatable. Nor were they stable with time.The United States National Cooperative Highway Research Program (NCHRP) initiated a research project to help state agencies improve their use of roughness measuring equipment. The work was continued by The World Bank to determine how to compare or convert data obtained from different countries (mostly developing countries) involved in World Bank projects. Findings from the World Bank testing showed that most equipment in use could produce useful roughness measures on a single scale if methods were standardized. The roughness scale that was defined and tested was eventually named the International Roughness Index.
Definition
The IRI was defined as a mathematical property of a two-dimensional road profile (a longitudinal slice of the road showing elevation as it varies with longitudinal distance along a travelled track on the road). As such, it can be calculated from profiles obtained with any valid measurement method, ranging from static rod and level surveying equipment to high-speed inertial profiling systems.The quarter-car math model replicates roughness measurements that were in use by highway agencies in the 1970s and 1980s. The IRI is statistically equivalent to the methods that were in use, in the sense that correlation of IRI with a typical instrumented vehicle (called a "response type road roughness measuring system", RTRRMS) was as good as the correlation between the measures from any two RTRRMS's. As a profile-based statistic, the IRI had the advantage of being repeatable, reproducible, and stable with time. In the math model, the simulated vehicle speed is 80 km/h (49.7 mi/h). The properties of the simulated vehicle were selected in earlier research to provide high correlation with the ride response of a wide range of automobiles that might be instrumented to measure a slope statistic (m/km). The damping in the IRI is higher than most vehicles, to prevent the math model from "tuning in" to specific wavelengths and producing a sensitivity not shared by the vehicle population at large.
The slope statistic of the IRI was chosen for backward compatibility with roughness measures in use. It is the average absolute (rectified) relative velocity of the suspension, divided by vehicle speed to convert from rate (e.g. m/s) to slope (m/km). The frequency content of the suspension movement rate is similar to the frequency content of chassis vertical acceleration and also tire/road vertical loading. Thus, IRI is highly correlated to the overall ride vibration level and to the overall pavement loading vibration level. Although it is not optimized to match any particular vehicle with full fidelity, it is so strongly correlated with ride quality and road loading that most research projects that have tested alternate statistics have not found significant improvements in correlation.
IRI was originally developed for summarizing lengths of road to determine if and when repairs or maintenance are needed. It can also be calculated as a continuous property of a profile by using a moving average to provide a "roughness profile" IRI profiles are sometimes used to evaluate new construction to determine bonus/penalty payments for contractors, and to identify specific locations where repairs or improvements (e.g., grinding) are recommended.
Further reading
- "Relating Road Roughness and Vehicle Speeds to Human Whole Body Vibration and Exposure Limits" by Ahlin & Granlund in International Journal of Pavement Engineering, Volume 3, Issue 4 December 2002 , pages 207 - 216.
External links
- "International Roughness Index" at The University of Michigan Transportation Research Institute (UMTRI)