Advantage I-75 CVO Weigh Station Simulation Model
Weigh Station Simulation Model
One of the outcomes of conducting the evaluation of the Advantage I75 Mainline Automated Clearance Project (MACS) was the development of a weigh station simulation model. This simulation has already proven to be a valuable decision-making tool for state transportation officials in their design and planning activities in the area of commercial vehicle operations.
Recent studies have shown that commercial vehicle traffic is increasing annually at double digit rates, creating more challenges for state and federal transportation officials in such areas as infrastructure maintenance and traffic enforcement. This increase in truck and bus traffic can present an even greater strain on a state's already limited transportation budget.
As state transportation officials are confronted with increasingly difficult investment decisions to improve commercial vehicle operations and weigh station performance capabilities to accommodate the increasing number of trucks and buses, officials need all means available to accurately make investment decisions and to work more intelligently, not harder.
The video clip shown here introduces a simulation model that embodies the major concepts and methodologies for weigh station operational test evaluation and the use of simulation models for decision making with regard to commercial vehicle operations and evaluation.
The purpose of this research project was to evaluate the effect that electronic clearance has on weigh station queue length and the number of "queue-based" bypasses resulting from weigh station overcrowding. Data collected during the study indicated that long weigh station queues and trucks bypassing stations due to overcrowding are common at certain weigh stations.
The small number of transponder-equipped trucks during the evaluation period demonstrated that it was not possible to measure significant changes in queue length and queue-based bypasses during the evaluation. The decrease in queue length and bypasses due to queue proved to be quite small when only one or two percent of the total commercial vehicle population are equipped with transponders for electronic clearance. In addition, there is extensive variability due to such factors as local traffic.
Therefore, the Center for Transportation Research and Education developed a simulation model to estimate the potential effect of electronic clearance on queue length and queue-based bypasses. Simulation is also used to assess the expected improvement under various assumptions about the proportion of transponder-equipped vehicles.
BACKGROUND and TEST METHOD
The Center for Transportation Research and Education at Iowa State University built its model by applying earlier weigh station simulation modeling developed by the Applied Physics Laboratory at the Johns Hopkins University using the Arena simulation software. The software is quite sophisticated and can provide continuous animated displays of a number of parameters such as traffic flow, queue behavior, and processing time at the weigh stations.
The flexibility of this simulation program, combined with the rich information base supplied by the weigh station data collection, provided the evaluation team with enough data to illustrate the expected effect of many electronic clearance scenarios (e.g., varying the population of transponder-equipped vehicles) on the primary variables of interest (bypasses and queue length).
In addition, other variables of interest, such as current and expected travel time savings attributable to electronic clearance, are incorporated in the simulation model that can graphically illustrate and assist key decision makers in evaluating their investment in electronic clearance technology.
These models were validated using the data collected from operational weigh stations. An enhanced evaluation of electronic clearance was completed by investigating current and expected effects of electronic clearance at six weigh stations along the Interstate 75 corridor. The capability to assess these factors using simulation is currently being developed.
C. Evaluation Approach Used
The simulation model was used to evaluate the electronic clearance effect on reducing trucks' travel time, unauthorized bypasses, and queue length at weigh stations. The model is extremely flexible. This model, while initially developed for a limited number of sites, is easily modified to simulate other weigh stations along any corridor. The simulation model is coded in Arena, which provides an integrating environment for graphically building a model using SIMAN simulation language. The animation capability of Arena enables users to view the changes in performance of the weigh stations resulting from implementation of electronic clearance of commercial vehicles.
D. Statistical Methods Used to Analyze Data
The output of the simulation model includes data that describe weigh station performance. These data, produced in summary form, include the proportion of trucks bypassing the weigh station due to queue, queue length, and processing time at the weigh station.
These summaries also provide a measure of the precision of the estimates. The data collected and analyzed as part of the field test are used in the simulation models. These data are analyzed using standard statistical inferences including measuring the mean, standard error of the mean, standard deviation, and distribution of a number of variables, such as truck interarrival times. These statistical summaries are used as input data for the simulation model and to verify that the simulation model can reproduce real conditions.
The advantage of a validated simulation program is the capability to simulate various scenarios at selected weigh stations that would otherwise be impossible to perform. For example, it is estimated that by the year 2004 the number of trucks on U.S. highways will increase by 13.4 percent*. By the same date the number of transponder-equipped trucks will increase. The simulation model is able to simulate these projected traffic conditions.
This ability to simulate diverse traffic conditions will aid state transportation officials in their decision-making processes concerning whether to design new weigh station facilities or re-engineer existing facilities.
*U.S. Freight Transportation Forecast to 2004. Prepared by DRI/McGraw-Hill. Submitted to the American Trucking Associations Foundation. Second Annual Report. February 1996.
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The video clips provided here illustrate three scenarios: