- Report Published -
|Higher Weight Limits for Trucks Hauling Gravel, Sand, or Crushed Stone in Coal Severance Counties|
|Department of Transportation|
|HB 2219 (Regular Session, 2001)|
HB 2209, enacted in the 1999 Session of the General Assembly, required that the Code of Virginia be amended and reenacted to extend the higher weight limits prescribed in subsection B of § 46.2-1143 to vehicles hauling sand, gravel, or crushed stone in the seven coal severance tax counties of Southwest Virginia. The bill required the Virginia Department of Transportation (VDOT) to "monitor the operation of vehicles under this subsection and the effects of such operation on the condition of the affected highways." HB 2219, which was enacted in the 2001 Session of the General Assembly, required these monitoring activities to continue with the higher weight limit provisions remaining in place for one year beyond the July 1, 2001 expiration date of HB 2209. This document serves to fulfill the requirement to report these results and forms the basis for a recommendation to the Governor and the 2002 Regular Session of the General Assembly as to whether the bill's provisions should be allowed to expire on July 1, 2002, or to continue, either in their present form or some modified form.
Purpose and Scope
The purpose of this study was to determine if vehicles operating under the higher allowable weight limit provisions cause pavements to deteriorate faster and, therefore, intensify maintenance and rehabilitation requirements than pavements bound by weight limits applicable elsewhere in the state. This was to be accomplished by conducting detailed field surveys at 18 in-service pavement sites representing the range of roadway and traffic conditions typically found on primary and secondary highways in Southwest Virginia. Ten of the sites selected were within the severance tax counties, and the remaining 8 sites were located elsewhere in Southwest Virginia. Thus, the 10 sites within the severance tax counties were to function as the experimental group, that is, the sites receiving the higher weight loads, and the remaining 8 sites were to function as the "control group," that is, sites that did not receive the higher weight loads.
The study had three limitations, which the reader should keep in mind when assessing the findings:
1. Because of varying interpretations of the provisions of HB 2209, it was impossible to locate a large number of representative control sites in Southwest Virginia. VDOT originally interpreted HB 2209 to allow higher-weight trucks to travel 50 miles in any direction from their point of origin, regardless of county destination. Because of this, one may assume that the higher-weight trucks traveled in 14 of the 18 sites monitored. Eight months into the study, however, in February 2000, the Attorney General's Office rendered an opinion regarding the fifty mile provision that prompted VDOT to prohibit trucks from hauling at the higher weight limits outside the severance tax counties altogether. This further brought into question the relevance of observations regarding rates of condition change between severance tax and non-severance tax sites. The reader is cautioned, therefore, that observations based on comparisons between severance tax and non-severance tax counties are not valid.
2. The twenty-six months available to monitor pavement performance was not enough time to allow the capture of data to determine whether there were significant differences in the rates of change among sites. Therefore, observations that involved comparisons between severance tax and non-severance tax (control) sites are inconclusive.
3. The tremendous number of variables that influence pavement performance and the vast resources and time required to answer questions related to truck-induced pavement damage would seem to speak against any extension of this study in an attempt to determine any peculiar effects higher-weight trucks may have in Southwest Virginia that they do not have elsewhere in the nation. The American Association of State Highway and Transportation Officials (AASHTO) has been leading a collaborative, comprehensive pavement research effort among all 50 states continuously for the past forty years to determine the effects of higher-weight trucks. This effort has included a study of the many variables (e.g., materials, construction techniques, local geology, roadway geometry, quality control effectiveness, weight limits and weight limit enforcement, axle configuration, tire pressure, climate, traffic volume and composition) that would influence the findings. The outcome of AASHTO's study is clear: the damage caused by heavy vehicles increases exponentially with corresponding incremental increases in weight. This outcome comprises the principle of pavement analysis used by all 50 state highway agencies for pavement design and research and is behind the pavement design procedure outlined in the AASHTO Guide for the Design of Pavement Structures, considered the "bible" of pavement engineers.
For these three reasons, the recommendations in this study are not based on conclusions about rates of deterioration observed among sites, but, instead, they are based on cost estimates derived from detailed structural analyses of the 18 sites conducted in accordance with procedures designed by AASHTO and the results of a literature review of other pertinent studies.
After the initial structural and functional conditions were documented at all study sites at the time HB 2209 went into effect (July 1999), the sites were monitored through documentation of the same condition indicators every three months throughout the original thirteen month study period. After the enactment of HB 2219, which mandated a one-year continuation of the study, a final condition survey of the study sites was conducted in September 2001. The intent here was (1) to establish initial baseline conditions from which subsequent measurements could be made, and (2) for comparative purposes, to permit the measurement of rates of change in condition within and between sites attributed to higher allowable weight limits over time. Specifically, visible surface distress, ride quality, wheel path rutting, and structural capacity were measured during three to four week periods in July and October 1999, January, April, and July 2000, and in September 2001. In addition, a detailed geotechnical (subsurface) investigation was conducted at each site in October 1999 to document pavement construction history and sub-grade support conditions. Sub-grade soil samples were extracted from each site during this phase of the work for moisture content determination, plasticity index computation, and grain size analysis in the laboratory. For the asphalt samples, resilient modulus testing was performed in the laboratory as well.
To develop site-specific information about traffic volume and composition, a survey consisting of vehicle counts, classifications, and approximate measurements of weights using weigh-in-motion technology was performed in April and May 2000. Data generated from the traffic survey augmented count and classification data compiled for each study site from pertinent average annual daily traffic (AADT) data published by VDOT's Traffic Engineering Division.
Results of the geotechnical investigation and traffic study were used in conjunction with pavement deflection test results to perform a detailed structural evaluation of all sites in accordance with the AASHTO pavement design and analysis procedure, which is widely used by U.S. state highway agencies. This analysis included (1) an assessment of the capacities of each site to support traffic at pre- and post-HB 2209/HB 2219 weight limits, (2) the thickness of pavement overlay that would be required to meet structural adequacy requirements under the weight limits in effect before and after HB 2209/HB 2219, (3) estimates of the cost to upgrade deficient pavements, and (4) an estimate of the cost of damage attributed only to the net increase in allowable weight limits over a twelve year period in the seven severance tax counties. The structural analysis also included an assessment of service life reduction associated with increased weight limits on under-designed pavements.
The study also included a review of the findings of several nationally significant studies conducted over the last fifteen years addressing the relationship between increased truck weights and pavement damage. This literature review was based on an Internet search of sites by the Transportation Research Board, AASHTO, the American Society of Civil Engineers, and the National Cooperative Highway Research Program.
The damaging effect of heavy trucks on pavements is not a new concept. Results of the most comprehensive study ever conducted on the relationship between heavy vehicles and pavement damage demonstrate that an incremental increase in vehicle weight results in an exponential increase in pavement damage. For example, a single pass of a three-axle single-unit truck with a gross weight of 54,000 pounds (the maximum allowed before HB 2209/HB 2219) causes approximately 8,000 times the damage of an ordinary passenger car. Likewise, one pass of the same three-axle truck with a gross weight of 60,000 pounds (maximum allowed by HB 2209/HB 2219) would cause approximately 50 percent more damage than the 54,000-pound truck. Stated differently, for this truck class, an 11 percent increase in weight results in a 50 percent increase in damage. Consider the effects of raising the allowable weight of a five-axle semi-trailer from 80,000 pounds to 90,000 pounds in accordance with the increase allowed by HB 2209. With an increase in gross weight of only 10,000 pounds, or 12.5 percent, a single pass of the 90,000-pound vehicle causes 58 percent more damage than the 80,000-pound vehicle. These examples apply to pavements that are structurally adequate to support the stated loads. The percentage increase in damage resulting from the additional weight would be drastically higher for structurally deficient pavements.
The exponential increase in relative damage corresponding to an incremental increase in weight is due to the severe reduction in the number of vehicle load repetitions that will cause fatigue failure, which is the result of increased stresses and strains within the pavement structure. An analogy would be the consequence of repetitively bending a thin piece of metal by hand. The metal could survive an indefinite number of repetitions if the degree of bending were quite small (i.e., lighter loads, lower deflections). However, if the degree of bending were increased only slightly, the metal would fail in fatigue after a finite number of repetitions. When the magnitude of truck volume over many years is considered, the cumulative effect of the net increase in damage resulting from a corresponding increase in weight becomes exorbitant.
Findings and Conclusions
• Although the period of time (twenty-six months) available to monitor pavement performance was sufficient to detect and document condition changes within sites, it was not sufficient to allow the capture of data to determine whether there were significant differences in the rates of change among sites. Therefore, observations that involved making comparisons between severance tax and non-severance tax (control) sites were inconclusive.
• For each of the 18 sites monitored as part of this study, visible load-induced distress increased, wheel path rutting increased, and ride quality decreased from July 1999 through September 2001.
• There were significant differences in the structural conditions of study sites.
• There was no consistent trend in structural deterioration for any site throughout the study period as determined by the pavement deflection analysis.
• Thirty-nine percent of the pavements investigated in this study were structurally inadequate to support traffic operating at weight limits allowed by HB 2219 for a sustained period of time.
• The cost of damage to primary and secondary roadway pavements within the seven severance tax counties caused by the net additional weight allowed by HB 2219 is estimated to be on the order of $28 million over a twelve year period. This estimate does not include costs associated with load-induced damage to bridges; motorist delays through work zones because of increased road and bridge repairs; safety and geometric roadway improvements; or loss of life and property resulting from the increased safety hazards of heavy trucks operating in mountainous terrain.
• The damaging effects on pavement performance of increasing vehicle weights are widely documented. The most comprehensive study of pavement performance under heavy vehicle loads, led by AASHTO, has been continuously underway since the 1950s. When applied to this study, pavement analysis principles derived from AASHTO's forty year national study demonstrate that the damage caused by heavy vehicles increases exponentially with corresponding incremental increases in weight for all classes of trucks affected by HB 2219.
In light of the structural evaluation and cost analysis performed for the 18 study sites and the literature review of pertinent studies, the provisions of HB 2219 pertaining to the authorization of additional weight limits for trucks hauling sand, gravel, or crushed stone should expire on July 1, 2002.
Further, in the opinions of the principal investigators of this research and the members of the Coal Severance Tax Study Steering Committee, the outcome of continued monitoring of the sites studied herein would serve only to support the findings of the widely accepted AASHTO research effort, which are based on more than forty years of continuous work conducted collaboratively by and for the 50 state highway agencies. It would seem that an attempt to replicate such a comprehensive and costly effort by continuing to monitor these Southwest Virginia sites would be redundant and is, therefore, not recommended.