Optimal placement of point detectors on Virginia's freeways : case studies of Northern Virginia and Richmond.
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Optimal placement of point detectors on Virginia's freeways : case studies of Northern Virginia and Richmond.

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      Final contract report;August 2006January 2008.
    • Abstract:
      In Virginia, point detectors in the Northern Virginia and Hampton Roads regions are placed at approximately 1/2 mile spacing. This density is a product of early requirements for incident detection that have proven ineffective and perhaps unnecessary. There are other important uses of the data that likely have different requirements for detector placement than the original incident detection focus. For example, there is a desire to derive travel time estimates from the point detector data for the purpose of performance monitoring. To do this, the detectors are to be placed so as to effectively sample the conditions on a freeway. Unfortunately, little guidance exists on how to place detectors for effective sampling. The purpose of this research project was to develop a decision support methodology to identify the optimal locations of a finite set of point detectors on a freeway corridor in order to minimize the error in travel time estimation, within the constraints of available capital and maintenance funding. Case studies of freeway sections in three regions were conducted to demonstrate the utility of the newly developed tool. While there are potentially other important uses of the data collected by freeway point detectors, the recommendations in this report deal specifically with the issue of travel time estimation. The investigators found that the placement of detectors for the development of accurate travel time estimates will vary by location based on specific conditions. Arbitrary, evenly spaced detectors do not necessarily result in accurate travel time estimates. With carefully placed detectors that are well maintained, travel time estimates can be derived with an acceptable level of accuracy from point detection, under incident-free travel conditions. The methods developed in this research effort including the GPS data collection and the mathematical tool are effective in determining preferred detector locations when the objective is to minimize travel time estimate error. There is evidence that VDOT can reduce the number of detectors that are currently maintained by TMCs and can deploy far fewer than the 1/2 mile spacing guidelines, resulting in significant cost savings in both capital and operations and maintenance costs. Using the results of the Northern Virginia case study on I-66 EB where there are 20 detector stations currently deployed, the tool identified minimum travel time errors when data are assumed to be coming from 11 of those stations, a reduction of 45%. Data from VDOT's Traffic Monitoring System place the ongoing cost of a detector station at approximately $10,500. That results in an annual savings of $94,500 for this 11-mile roadway segment alone.
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