International roughness index
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.).
IRI is also used to evaluate new pavement construction, to determine penalties or bonus payments based on smoothness.
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.
[8] The IRI is used in managing pavement assets, as well as sometimes in evaluating new construction to determine bonus/penalty payments for contractors or for identifying specific locations where repairs or improvements (e.g., grinding or resurfacing) are recommended.
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.
The properties of the 'golden car' were selected in earlier research[12] 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).
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 of the suspension response at the golden-car simulation speed also determines the final IRI ride value.
Using World Bank terminology, these correspond to Information Quality Level (IQL) 1 and IQL-3 devices, representing the relative accuracy of the measurements.
These units can be used to certify inertial profilers as a reference device since they have a sampling interval less than 2.75-inches per AASHTO PP49.
The approach consisted of a sensor (initially ultrasonic but later laser) which measures the height of the vehicle relative to the road.
The quarter-car approach is considered more accurate in representing the motion felt by users and is most common.
This can be addressed with a 4-inch or 6-inch line laser mounted in the wheelpaths to mimic the tire patch width.
Two examples of this are the Pavemetrics system and the XenoTrack-True Solid State Lidarwhich have been adopted by many different OEM suppliers of network-level profilometer equipment around the world.
Since RTRRMS are generally affected by pavement texture and speed, it is common to have different calibration equations to correct the readings for these effects.
It is reported that the prediction of future IRI values may be easier than PCI as it includes less uncertainty.
