Click for next page ( 53


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 52
52 Is the total number of vehicles counted within expected Creating easy-to-use data summaries, ranges? (Note that the range used should be fairly large Flagging questionable data, and because truck volumes in particular can vary significantly Removing poor data after their quality has been con- from day to day.) firmed to be poor by qualified staff review. Are there any unusual time-of-day traffic patterns that would indicate the potential for some type of counter Staff review of these summaries and the performance of failure or inappropriate counter setting? (For example, independent reviews of irregular data collection results should does the volume at 1:00 a.m. exceed the volume present still be done for all collected data. As noted earlier, traffic can at 1:00 p.m.?) vary considerably from location to location, and knowledge of Are hours of data missing from the dataset? site-specific traffic patterns and independent review of ques- Have the scale's diagnostics reported any problems? tionable data are keys to successful quality assurance programs. Most of these checks assume that a trusted dataset exists against which new data can be compared in order to deter- 5.6 PURCHASE EQUIPMENT mine the presence of unusual data. For permanent data col- WITH A WARRANTY lection sites, the best place to get these trusted datasets is immediately after the site is first installed and calibrated. The When purchasing equipment, it is good practice to obtain initial calibration effort should ensure that the site is working a warranty on the life of that equipment. The warranty should correctly, that the vehicles crossing the sensors are being cor- specify the expected life of the sensor given specific uses of rectly counted and classified, and that the weights are accu- that sensor. For example, a 5-year warranty on bending-plate rate. Data collected immediately after calibration should then weigh pads might be specified given a lane volume of less serve as the initial trusted dataset. than 5,000 trucks per day. If traffic patterns change over time (and the validity of Warranties provide agencies insurance against poor manu- these changes is independently confirmed), additional trusted facturing quality control and also provide incentives to ven- data patterns can be developed, stored, and used as part of the dors and manufacturers to improve the quality of their equip- quality assurance process. ment. Warranties are not free, but limit the cost of equipment For short-duration counts, it is important that data collec- replacement. For a vendor, the added revenue obtained in tion crews that set equipment confirm the equipment's proper return for a warranty becomes profit, so long as the equipment operation at the outset before leaving the site and then recon- performs as specified. However, if equipment fails prema- firm the equipment's proper operation prior to picking up the turely, a significant loss to the vendor occurs. This approach sensors at the end of the count. (That is, the crews should per- provides a significant incentive to correctly predict the life form a consistent, routine check to ensure that the counter is span of sensors and other equipment. correctly counting and classifying vehicles each time it is Warranties of overall system performance have been suc- placed.) Office-based reviews can then compare the col- cessfully used by some states. These warranties extend pro- lected data against the short-duration counts made to confirm tection beyond the sensor and accompanying electronics to the equipment operation, as well as against earlier (historical) quality of the data produced by the data collection systems. counts made at that site or at nearby sites on that roadway. These warranties are only effective when the highway agency In addition to these basic data checks, a number of addi- can supply appropriate site conditions (e.g., smooth enough tional routines are often provided by equipment vendors or pavement) to make the warranty valid and when a mechanism developed by individual agencies. These should be reviewed, to monitor compliance with the warranty is in place. tested, and used whenever they offer cost-effective improve- For example, with WIM equipment, it is likely that the ments to available procedures. equipment vendor subject to this type of warranty requirement The key to any quality assurance program is that the rou- will require the site to meet the site specifications defined in tines available are tested and used. This requires resources ASTM E 1318 specifications. While these conditions may and effort, but results in substantial improvement in the qual- be met immediately after sensor installation, it may be nearly ity of data collected and supplied to users. Over time, qual- impossible to meet these conditions after two additional ity assurance practices will help identify poor equipment and years of pavement deterioration. Such difficulties can make poor data collection practices, which can then be discarded performance warranties unenforceable. or modified as appropriate. These practices also will help This example points out that if a highway agency chooses agencies improve their knowledge of traffic patterns in the to require data quality warranties from outside vendors, it state, which is a major benefit to an agency. is necessary to set up a quality assurance program that can Wherever possible, quality assurance tests should be auto- be used both to detect equipment that needs repair and to mated. However, the automated tests should primarily be determine when the site conditions no longer meet warranty limited to specifications. The specifications developed for LTPP SPS