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SUMMARY OF FINDINGS NCHRP Project 20-34 addressed the issue of determining what is actually accom- plished as the result of truck weight enforcement efforts. Traditionally applied measures, e.g., numbers of trucks weighed and citations issued, have provided indications of en- forcement effort. However, it is essential for valid measures of effectiveness (M.O.E.s) to provide measures of accomplishment in terms of actual enforcement objectives, i.e., deterring overweight truck operation and preventing pavement deterioration. This re- search project capitalized on WIM (Weigh-in-Motion) system capability and integrated a number of related scientific areas, i.e., pavement design principles and statistical applica- tions, to develop M.O.E.s which directly interpret WIM findings in terms of actual en- forcement objectives. Resulting M.O.E.s quantified the reduction in the number, propor- tion, and severity of illegally overweight trucks. This report documents of the research project, the objectives of which were as follows: I. To develop and validate truck weight enforcement measures of effectiveness (i.e., indicating what is accomplished as He result of enforcement activity). 2. To document findings in a user guide formatted to explain appropriate data collec lion methods, how to apply these methods, and how to interpret their results. Specifically discussed aspects of this research activity are as follows: (~) M.O.E. development and validation, (2) truck weigh-scale diversion, (3) permit-issued trucks, (4) enforcement effects on pavement life, and (5) user guidelines. Vl
M.O.E. Development and Valictabon Candidate M.O.E.s were developed via a series of analytical procedures. The most pro~sing candidate M.O.E.s were then validated In field studies via their application to ac- tual truck weight enforcement procedures. M.O.E. Development M.O.E.s were analytically developed in three steps. First, surveys of literature and state agency practice were conducted to establish existing enforcement evaluation procedures. Second, an expert pane! developed candidate M.O.E.s on the basis of truck weight enforcement objectives and the sensitivity of candidate measures to those objectives. Third, these candidate M.O.E.s were evaluated and raniced by application of the following objective cnter~a: Practicality of Application, Measurement Reliability, Sup- port of Statewide Random Sampling, Absence of Enforcement-Induced Bias, Data Collec- tion Melons Capability, Sensitivity to Infrastructure Damage, and Applicability to Data Collection Future Technology. The following M.O.E.s were thus established on the basis of their swtability to dem- onstrate truck weight enforcement effects: (~) Severity of Overweight Violations, (2) Pro- portion of Overweight Trucks, (3) Average ESALs, (4) Excess ESALs, and (5) Bridge For- mula Violations. These measures are sensitive to legal load-limit compliance objectives of truck weight enforcement procedures as well as the potential for overweight trucks to pro- duce pavement wear and tear. M.O.E. Field Validation Analytically-developed M.O.E.s were empirically validated in a comprehensive four-state field evaluation. Matched WIM data sets, collected under controlled baseline and enforcement conditions, were analyzed to determine the sensitiv- ity of candidate M.O.E.s to actual enforcement activity. Data collection conditions were controlled in order to avoid contamination from hour-of-day, day-of-week, and seasonal effects. . . vll
The field validation study design incorporated three essential measures-development methodological requirements: reliability, validity, and sensitivity. Reliability ensures that replicated applications will yield consistent results. In order to insure the reliability of recommended M.O.E.s, the field evaluation uniformly applied the M.O.E. sensitivity analysis to WIM truck weight data collected in four states representing northern, southern, eastern, and western regions of the United States. Validity of a measure refers to the degree to which it actually measures what it is designed to measure. The validity of tested measures In this study was established due to their relevance to truck weight enforcement objectives. Sensitivity requires that the applied measure produces a true Indication of the sought attribute or condition. The applied context of the field studies, i.e., controlled baseline versus enforcement conditions, ensured M.O.E. sensitivity. This four-state effort examined WIM data gathered In the presence of enforcement activities and compared it with data collected under non-enforcement affected flow conditions. Findings observed In each state are summarized as follows. California The California Department of Transportation provided output from a WIM scale located on I-5. An analysis of 3,678 truck combinations exhibited lower gross weights wad a smaller proportion of overweight axles during Me time when the weigh station was open. A su~sample of 2,370 tractor-sem~trailer combinations demonstrated lower rear-tandem weights with fewer instances of Excess ESALs when Me weigh station was open. Georgia Mobile truck-weight enforcement operations, utilizing an obtrusive portable roadside weigh scale, were conducted at a rural interstate location. An analysis of 483 combination trucks revealed a number of M.O.E. validation effects associated with observed axle and tandem weights. Under conditions of visible (and unexpected) mobile enforcement operations, the observed sample exhibited lower steering-axle weights, lower rear-axle weights, arid lower rear-tandem weights. Moreover, less severe Excess ESAL violations were observed during the enforcement period. During the surprise enforcement operation, a number of . . . vial
overweight trucks were observed to either park alongside the roadway or divert to alternate routes. Idaho A large volume of WIM data, i.e., gathered on approximately 29,000 commercial vehicles, was provided by Me Idaho Transportation Department. A comparison of baseline versus enforcement conditions during three different weekdays produced a number of significant findings. While no day-of-week effects were readily evident to indicate on which days enforcement effort would more likely be effective, all of Me tested operational measures were shown to be sensitive to enforcement activity. The M.O.E.s most consistently demonstrating sensitivity were: (~) We truck proportion exceeding 80,000 pounds, (2) the truck proportion web overweight tandems, (3) rear-tar~dem weight violation severity, and (4) Me truck proportion which exhibited Excess ESALs. While less frequently associated web enforcement activity, the following measures were also validated In the Idaho data: (~) higher average ESALs, (2) Bridge Formula violations, and (3) Me Duck proportion exhibiting Bridge Formula violations. Minnesota Data sets representing two weeks of continuous traffic monitoring were provided by the Minnesota Depar~nent of Transportation. Bending plate WIM data were gathered approximately five miles from a permanent tuck-weight enforcement scale during times when Me scale was open and closed. While generally weak M.O.E. validation findings were seen In these data, one data set exhibited a tendency to lower Bndge Formula violations and the other set produced a lower proportion of Overweight trucks and lower average ESALs. A number of ancillary issues affecting truck weight enforcement were also inves- tigated. First, a field study addressed the issue of overweight-truck weigh-scale diversion via usage of bypass routes. The applied methodology was to collect portable WIM scale data on bypass routes and compare these data with main line WIM data. Second, impli- cations for permitted overloads, with regard to their potential to confound M.O.E. appli- cation, were studied via a literature reviews and highway agency surveys to examine IX
state-of-art permit record systems. Third, pavement deterioration effects of overweight trucks were investigated. Pavement design principles were user to compare pavement life under specified axle loading conditions. Truck Weigh Scale Diversion This field study examined truck weight and travel trends on potential bypass routes which circumvented permanent weigh scales. WIM data were collected on both the main line and truck bypass routes in California and Florida. Similar truck overweight trends were observed in both states. Customary truck weight distributions, e.g., average gross and axle weights, did not statistically differ between bypass and main line routes. However, truck samples observed using bypass routes were consistently more likely to exhibit higher aver- age ESALs and Excess ESALs. Similar time-related travel trends were observed between the two states; despite differing hours of mainline permanent scale operation. In Flonda' In the presence of 24-hour and 7-day per week scale operation, the majority of overweight di- version activity occurred during the early morning hours, e.g., 6 a.m. to 9 a.m; In California, while time-of-day diversion travel was affected by permanent scale operation, a tendency was observed for early morning scale diversion, e.g., 3 a.m. to ~ a.m. Permit-Issued Trucks The presence of permitted overweight trucks in the traffic stream presents a for- midable threat to any WIM-based truck weight enforcement evaluation procedure. That permitted overweight trucks in the traffic stream can not be detected by WIM systems confounds the observed compliance with legal weight limits. However, the literature has indicated that traffic observations are insightful regarding violations in the absence of permitted truck flow data. Our survey of state highway agencies conducted during the course of this research established that existing record-keeping systems generally do not allow for adjusting x
collected WIM data to correct for permitted truck weight effects. Consequently, what is unknown at this time is the real effect of permitted overloads. However, our investigation of this problem did reveal that the Texas Department of Transportation does maintain sufficiently detailed and current records to determine the extent of overweight violations accruing from the presence of permitted trucks. Never- theless, due to the expense of associated hardware and operational costs, a solution that is sufficient to correct WIM-based truck weight surveillance for permitted truck presence is not generally available. Enforcement Effects on Pavement Life Appendix E of this report contains a comprehensive discussion of pavement de- sign factors and resulting deterioration effects associated with various truck overweight conditions. This discussion estimated effects of overweight trucks via the application of pavement design pnnciples that contrasted pavement life duration under specified axle loading conditions. Furthermore, the User Guide software includes an automated proce- cure to determine the pavement life effect of observed truck weight violations based on WIM-measured truck weights and existing pavement design characteristics. User Guidelines The primary product of this research consists of guidelines to instruct user high- way agencies regarding M.O.E. applications in their determination of truck weight en- forcement electiveness. The guideline consists of two parts. First, an M.O.E. Sampling Guide was included to assist highway agency person- ne! in determining data-collection site requirements. Data-collection site requirements depend upon the type of enforcement activity, i.e., regional enforcement program, corri- dor-specific enforcement campaign, or single-location enforcement activity. Site number requirements are given for specific traffic operational criteria, i.e., functional highway Xl
classification and proportion of trucks. Site-specific truck sample size requirements are also provided. Second, an automated User Guide is provided with this report to analyze and interpret WIM data. A user-friendly Windows software package, Truck Weight Enforcement Ef- fectiveness Too} (TWEET), allows users to directly determine the effectiveness of truck weight enforcement activities. The software calculates and statistically compares M.O.E.s between observed enforcement conditions. The user has the option of conduct- ing an automated pavement design life analysis, whereby the program determines the theoretical pavement-life effect resuming from the measured ESAL-Ioad~ng difference asso- ciated with the observed enforcement activity. .. .