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17 agency has conducted reference measurements. Another exam- Most agencies also collect surface distress data at least ple is the certification process that has been proposed for pro- once every three years, with many collecting such data every filers (36). Verification of the consistency of the data is also year. Even agencies that still use windshield surveys reported important when changing service providers or when the ser- data collection frequencies of three years or less, resulting in vice providers (or the agency itself) use more than one pave- a high degree of temporal network coverage. ment data collection piece of equipment or technology. Friction data are generally collected once every two to three years, with a low percentage of agencies collecting data Location Referencing Consistency every year. For network-level friction data collection, a road- way is typically divided into segments, usually 0.5 to 1.0 mile The second key issue with the implementation of a new sys- in length, and a friction value is measured over the segment. For tem or data collection approach is the adoption of a common example, Indiana collects annual friction testing on Interstate location referencing method (37), or that appropriate and accu- highways and once every three years on other roadways (29). rate conversion procedures are provided. Non-standardized location reference methods can pose significant obstacles when Structural capacity data are collected with the lowest fre- new methods of collecting data or new methods of using data quency. Network-level structural condition data for an Inter- are introduced. A universal location referencing method based state highway can be assessed by taking as few as three FWD on the spatial and temporal characteristics of the data collected readings per mile, once every five years, resulting in 20% net- can reduce problems with year-to-year variations and time- work coverage per year. Studies in Indiana suggest that these history updates. Agency enforcement of data referencing can measurements, along with ground penetrating radar (GPR) prevent many time-history update problems (37). Generally evaluation, can provide reliable information with respect to speaking, spatial and temporal referencing of raw data is one the remaining structural capacity of a pavement system (40). of the most effective methods of ensuring historical continuity A significant number of respondents were unfamiliar with their and preventing the loss of historical data. agency's structural capacity data collection practices, suggest- ing that the collection of this pavement condition indicator is NETWORK COVERAGE AND SAMPLING conducted by an office other than the one in charge of pave- ment management. Another important issue that affects the quality of the pavement condition data is the network spatial and temporal coverage. Lanes Evaluated Network coverage and sample size are generally controlled by the type of data desired and their intended use. Pavement Another important issue related to data quality is the number condition data quantity expectations generally vary according of lanes evaluated. Most agencies (73%) reported collecting to: (1) the type of information required by the agency (and its data for one lane only along multi-lane roads, whereas only a intended use), (2) how often a particular piece of data is used, few reported collecting data along multiple lanes of the same (3) the expense and/or difficulty in obtaining that data, and roadway. Studies in Indiana have shown that in terms of pave- (4) changing federal requirements. The perceived rate at which ment smoothness, the difference between the driving lanes the pavement condition changes and the volume of data nec- and passing lanes is statistically insignificant (29). However, essary to provide useful information influences how often, if this type of agreement would not be expected in cases where at all, different types of pavement data are collected. Thus, separate lanes may receive different preservation treatments. all these factors influence the frequency of evaluations and When only one lane of a multi-lane road is being evaluated, sampling procedures. Automated condition data collection care is to be taken so that the same lanes are consistently eval- is generally considered ideal for collecting network-level data uated to be able to establish historical trends for developing because it allows for the efficient collection of large quantities performance models. Many agencies have recognized this and of data, and with the proper calibration and quality manage- have standardized which lanes are used for collecting data; for ment, data consistency can be assured (38). example, many agencies collect data on the primary direction on two-lane roads and on the outside lane in both directions Temporal Coverage on four-lane roads. According to the survey, most agencies collect smoothness data NEW DEMANDS IMPOSED BY CHANGING for their highways at least once every three years, with many BUSINESS PRACTICES collecting data every year. Given that the HPMS program managed by the FHWA formerly required the submission of Changes in business practices, such as the HPMS reassessment smoothness data on a sample of the network biennially (39), it and the adoption by AASHTO of the MEPDG, are expected to is not surprising that smoothness data are collected frequently. affect quality management practices. The type of data collected The reassessment of the HPMS now requires annual smooth- and their degree of detail will likely change, influencing the ness submission for the National Highway System. quality management practices used for their collection.