Roughness and IRI
Pavement roughness is recognized around the world as a key performance measure both in terms of pavement life as well as cost to the user and quality of experience.
When it comes to pavement condition, rough roads typically have greater extents and severities of key pavement distresses including, but not limited to, cracking, potholes and rutting.
When it comes to user experience and cost, rough roads impact the user in terms of their riding comfort, safety, fuel economy and vehicle maintenance expenses.
Roughness of pavements is evaluated by measuring the accumulated difference in the height of a road surface (the longitudinal profile) over a prescribed length such as the number of inches of elevation change per mile of the number of meters of elevation change per kilometer.
Industry standards require pavement elevation measurements to be made using an inertially-corrected measurement platform so that measurements are made independent of the response of the measurement vehicle’s chassis to road conditions.
Filters are applied to longitudinal profiles in order to focus on the specific wavelengths which contribute the most to user discomfort.
While different roughness metrics exist, the International Roughness Index (IRI) is without question the recognized standard around the world.
While traditional inertial profilers are limited to the use of a total of two point lasers, one in each wheelpath, Pavemetrics® Laser Crack Measurement System (LCMS®-2) is able to extend roughness measurement across the entire travel lane using 4,000 3D points of measurement. As well, the LCMS’ built-in Inertial Measurement Units (IMU) ensure measurements that are independent of vehicle response to conditions.
LCMS longitudinal profile is measured according to ASTM E950: Standard Test Method for
Measuring the Longitudinal Profile of Traveled Surfaces with an Accelerometer Established Inertial Profiling Reference) and measurements are made to a precision and bias of a Class 1 profiler.
IRI is calculated according to ASTM E1926: Standard Practice for Computing International Roughness Index of Roads from Longitudinal Profile Measurements.
LCMS roughness measurement methodology provides a number of advantages over the traditional method including:
- 4,000 points of measurement across the entire lane as opposed to only 2 measurements
- An optional lane-tracking algorithm which allows the user to correct for driver wander which would otherwise lead to less repeatable and less relevant reporting
- An optional pothole filter which allows the user to exclude a single instance of a pothole or similar localized roughness feature which would erroneously skew the condition of a road section which is otherwise in excellent condition
- An optional tire-bridging algorithm which is designed to optimize roughness measurement on tined or grooved surfaces
Output from the LCMS roughness module includes raw longitudinal profiles, computed indices and waveband analysis.
Raw profile is reported using the PPF and ERD formats for direct import into ProVal which allows the user to verify precision and bias, investigate localized roughness and to calculate IRI and other indices.
Computed indices include both IRI and HRI (Half-car simulation) and are output in XML format.
Waveband analysis filters longitudinal profiles using three band pass filters in order in order to report three separate profiles which respectively highlight short wavelength, medium wavelength and long wavelength pavement distortions.
E950; Can collect longitudinal profile and compute IRI with the precision and bias of a Class 1 Profiler
The Australian 3D Roughness Experience
Authors: Richard Wix and Simon Barlow (ARRB Group)
Advantages of Laser Scanning for 3D Road Profiling