The Montana Department of Transportation (MDT) deployed a State Truck Activities Reporting System (STARS) to automatically and unobtrusively collected information on the characteristics of commercial vehicles operating on Interstate and non-Interstate highways in Montana. The system used weigh-in-motion (WIM) sensors installed directly in travel lanes to collect vehicle weight and classification data at 28 sites. The system was evaluated over a two year period, and the data collected were used to support mobile weight enforcement activities and improve infrastructure pavement designs.
STARS demonstrated that WIM data could be used to allocate mobile weight enforcement resources to specific areas at certain times of the year and reduce the number of overweight vehicles on the road. During the first year of operations (May 2000 through April 2001), baseline data were collected from 16 STARS sites (WIM sites) to characterize overweight vehicle activity. A customized software program (MEARS) was used to generate monthly reports detailing weight, classification, and number of commercial vehicles at each site. The following year (May 2001 through April 2002) patrol officers, equipped with portable scales, were deployed to sites identified as having the highest number of overweight vehicles. With a fairly constant level of overall enforcement activity both years, WIM data were analyzed to determine the impacts of focused mobile enforcement.
As a secondary objective for the project, STARS data was collected at each site and compared to vehicle weight and classification data collected by the MDT (Data and Statistics Bureau) at each of 11 Weigh Stations. The STARS and Weigh Stations data were compared in terms of quality and quantity, and then calculations were made to determine the cost difference between designing a facility using WIM data (optimum pavement thickness) and designing a facility using Weigh Station data (overbuilt pavement thickness).
Since 90 percent of commercial vehicle traffic in Montana was estimated to occur on Interstates and non-Interstate NHS/Primary roadways, the study evaluated facilities in these areas. The precise location of each STARS site was dependent on siting requirements such as roadway grade and alignment criteria; however, to improve statewide coverage, most STARS sites were located on non-Interstate NHS/Primary routes since Weigh Stations already covered most Interstate routes. STARS data was estimated to be more comprehensive than Weigh Stations data since STARS sites collected and archived vehicle weight and classification data continuously, while Weigh Stations collected data only a few times each year. In addition, STARS data was expected to be more objective than Weigh Station data since inspectors at Weigh Stations tended to focus more on fully loaded vehicles. Avoidance activities were also of concern; therefore, at STARS sites portable classifiers were deployed to confirm avoidance activities were insignificant on potential bypass routes.
STARS data - Focused Enforcement
Throughout the extensive statewide network covered by STARS, the percentage of overweight vehicles in the traffic stream decreased 22 percent (8.8 percent of vehicles were overweight in the baseline year, and 6.9 percent of vehicles were overweight in the enforcement year). In addition, the average amount of overweight decreased nearly 16 percent (6,100 lbs overweight in the baseline year to 5,100 lbs overweight in the enforcement year).
The cost savings associated with the statewide reduction in pavement damage from overweight vehicles was $0.7 million. Costs were determined based on changes in pavement damage from overweight vehicles between the baseline and enforcement years. Dollar values for changes in pavement damage were gauged based on the total cost to provide highway service to the Montana highway system (1994-1996) and then adjusting for inflation to the baseline and enforcement years (2000-2001).
STARS data - Pavement Design
If pavements were designed based on fatigue demand calculations derived from archived WIM data instead of Weigh Stations data, Interstate and non-Interstate-NHS/Primary systems would save approximately $0.7 million and $3.4 million respectively in annual construction costs. These results were derived from calculations that show WIM fatigue estimates were 11 percent and 26 percent lower than Weigh Station estimates for Interstate and non-Interstate-NHS/Primary systems respectively.
The author emphasized that only one year of baseline data and one year of enforcement data were collected during this project; therefore, the results will have a considerable degree of uncertainty.
In addition, pavement fatigue estimates are only one factor transportation agencies consider when determining overall pavement design. Other factors include geometric constraints, construction practices, and resistance to environmental effects (i.e. thermal cracking, frost heaving, long term settlement, cracking at cold joints, etc.).