United States Department of Transportation
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An Analysis of Expected Delay Reduction by Replacing a Congested At-Grade Intersection with an Interchange
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1991-07-01
[PDF-8.88 MB]
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Edition:Interim: November 1990 - July 1991
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Abstract:This research has documented calibration of an off-line computer traffic simulation mode, TRANSYT-7F, in terms of its suitability to model traffic flow through signalized streets, especially for saturated conditions. The latter half of this research is devoted to evaluating the potential effectiveness of grade separation at congested intersections of two arterial streets. The calibration of the model indicated that the model generally overestimates delays as compared to observed delays. The differences between observed and simulated were less for unsaturated conditions and more pronounced for saturated conditions. It appeared that the model does not have the capability to realistically estimate delays for saturated conditions. Evaluation of grade separation of congested at-grade intersections showed that the reduction in delay was directly related to the flow rate at the intersection. The higher the flow rate, the greater the reduction in delay. The cut-off flow rate entering the critical intersection, at which grade separation would be an effective option, was found to be 5000 vph. The analysis indicated the beyond entering flow rate of 5000 vph, the delays at the at-grade intersection increase exponentially. In order to truly benefit from grade separation, intersections with an entering flow rate of more than 5000 vph would result in enough delay savings to justify the high cost of installation.
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