Pavement Roughness

Prior to 2000, PennDOT's pavement smoothness specifications had been based on the measurement of Profile Index (PI). Devices called California Type Profilographs were used to measure PI, and Pennsylvania Test Method (PTM) 424 defined which devices were acceptable and described how to perform the measurements.

In order to assure that pavement smoothness specifications resulted in a product that provides a smooth ride for the motoring public, PennDOT began to base pavement smoothness specifications on the International Roughness Index (IRI), rather than PI, in 2000. Whereas PI is a measure of the profile and the deviations from a smooth plane, IRI simulates a vehicle's response to the deviations. Therefore, IRI is an expression of the "rideability" of the roadway, as it is experienced by vehicle passengers. IRI was developed as a reference measure by The World Bank, and is based on a quarter-car simulation as described in NCHRP Report 228.

Rather than the profilographs used to measure PI, IRI is determined with devices called "profilers." Profilers are mounted with laser sensors and computers and may either be vans that travel at highway speeds, which PennDOT has been using for over a decade, or "Light Weight Profiling" (LWP) devices. An LWP is an off-road utility vehicle that can be easily used within the confines of construction projects, due to its limited size and weight, and added maneuverability.

PTM 428 defines the method in which to measure pavement profile and determine pavement ride quality for acceptance and payment using an LWP device. This PTM is provided in Appendix A of the Lightweight Profiler Certification Manual — Publication 589 (PDF). Unlike PTM 424, a list of acceptable models is not included. That is for two reasons: 1) the profiling technology is rapidly expanding and it is not in PennDOT's best interest to publish a PTM that may constantly need to be updated to include new LWP manufacturers and/or models; and 2) PennDOT wants to be assured that each individual LWP device is operated proficiently and provides accurate and precise results, and not make "blanket approvals" based on type. It is with this second reason in mind that a program has been developed to verify that all LWP devices are calibrated and operating properly.

The second aspect of the program is to certify that LWP operators are competent and capable of operating the device, as well as collecting, analyzing, and reporting accurate results. The certification program ensures control of the use of LWP devices; promotes competence, proficiency, and professionalism; and increases confidence in the test data. Operator certification is necessary because PennDOT offers no training for LWP operation. The vendors that supply LWP devices may provide training, but the extent and quality of training is beyond PennDOT's control and will vary depending on the vendor, device type, and software of each LWP device.

All LWP devices with acceptable test results will be designated with a decal to be applied by Roadway Inventory and Testing Unit (RITU), and all certified LWP operators will be provided a certification card.