Roughness Index and Thresholds for Urban and Low-Speed Pavement Management

State highway agencies monitor pavement roughness to assess road network condition and construction quality assurance. States are also required to report the International Roughness Index (IRI) on the National Highway System (NHS) under the National Highway Performance Program. The IRI often overstates roughness experienced by vehicles and occupants of vehicles traveling on low-speed and urban roadways. This is because the IRI simulates vehicle response for a travel speed of 50 mph. Vehicles travel much slower on urban and low-speed networks, often between 0 and 35 mph. At lower speeds, the same roughness is experienced over a longer period of time and feels less intense.* *This is important because more than two thirds of the vehicle miles travelled on the U.S. road system occur on urban and low-speed roadways.

Additional research is required to establish an improved metric and threshold values for roughness on urban and low-speed roadways. This is necessary for effective use in the applications of road roughness measurement, such as construction acceptance and triggers for intervention based on functional performance.

The research would improve the relevance of data already being collected and submitted to the Federal Highway Administration (FHWA) via the Highway Performance Monitoring System (HPMS) and improve transportation asset management and performance management of our nation’s highways.

The objective of this research is to identify an urban and low-speed roughness index and threshold values for management of urban and low-speed pavements. This includes construction acceptance criteria, intervention thresholds, and fairly categorizing the quality of pavement segments (e.g., good, fair, poor).

The research would improve the relevance of data already being collected and submitted to the Federal Highway Administration (FHWA) via the Highway Performance Monitoring System (HPMS) and improve transportation asset management and performance management of our nation’s highways.

Without this improvement to existing practice, the functional quality of high-speed and low-speed pavements cannot be judged on an equitable scale. The mischaracterization of roughness on urban or low-speed highway segments can skew the timing of maintenance and rehabilitation treatments, thus reducing life-cycle optimization. It is anticipated that the refined index and thresholds will facilitate better management of the functional quality of urban and low-speed road networks, and better characterization of road network status.

This research is of interest to pavement management, construction, and maintenance staff. Asset management philosophy has moved to the forefront and state agencies need quality methods to track performance of pavements for development of maintenance and rehabilitation programs. In addition to long-term performance monitoring, ride quality indices are used for construction acceptance. The previous NCHRP study developed the scale for the index; however, threshold values must be established for use in project acceptance. Finally, the threshold values would also be used by maintenance staff to determine when preservation activities, such as grinding, should be performed to minimize roughness that can accelerate deterioration of pavements.

To facilitate immediate implementation of the results, sample source code for the revised roughness index is required. The research will also produce a draft (or revised) AASHTO specification for using the index and thresholds in construction quality assurance and in pavement management.