United States Department of Transportation
i
Evaluation of Geometric Design Needs of Freeway Systems Based on Traffic and Geometric Data
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2013-10-20
[PDF-3.73 MB]
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TRIS Online Accession Number:01651142
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Edition:Research Report
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Abstract:In Las Vegas, Nevada, the increased traffic competes for the limited spaces available in the freeway system and thus reduces safety performance. This study identified geometric design issues on freeway systems in Las Vegas, Nevada, based on available safety data for freeway ENEX and EX-EN segment types. For every segment, crash rate and severity models were developed. It was found that for EN-EX segment type, number of through lanes, curve radius, shoulder and median widths had a significant impact on average crash rate. The same geometric elements with an additional of AADT also indicated significant impact on severity crashes. Segments with large radius reduced average crash rate while it did not have an impact on severity crashes. Wide shoulders and medians reduced average crash rate and high severity crashes. Long segments reduced average crash rate while it did not indicate an impact on crash severity. The number of through lanes increased both average crash rate and high severity crashes. For EX-EN segments, curve with large radius, wide shoulders and medians reduced both average crash rate and high severity crashes. The number of through lanes increased average crash rate on I-15 while it reduced average crash rate on I-215. It did not have an impact on average crash rate on US 95. In addition, the number of through lanes increased the number of high severity crashes on I-15 and I-215 while it reduced high severity crashes on US95. High traffic volume increased average crash rate on I-215 while it did not indicate an impact on I-15 and US 95. As far as countermeasures to improve safety, it was identified that on an EN-EX segment, congestions usually occur in the following two situations. First, weaving would happen between traffic streams on to freeway and that off from freeway. Either of these two traffic stream become heavy, the freeway would become congested, which need separating them spatially. When the traffic from the on-ramp is heavy, ramp metering can be installed, which is a way to mitigate congestion and reduce crash on freeway. Second, if the traffic on the off-ramp could not be clearly quickly, the queue would back up to freeway and then make the freeway congested. In this case, signals at the end of the off-ramp need to be adjusted to make the off ramp traffic cleared on a timely manner. If this segment is long, warning signs would be recommended to warn motorists of the congestion downstream. An EX-EN segment is usually at the middle of an interchange, one side for traffic getting off freeway and the other side getting on freeway. The congestions usually occur in the follows two situations. First, the traffic on to freeway become heavy, making the merging on freeway congested, and this congestion would quickly spill back to this segment. In this case, it is suggested to install ramp metering. When the traffic is extremely heavy, separating the traffic on to the freeway spatially from that goes through the downstream segment is necessary. If this segment is long, warning signs are suggested to install on this segment, alerting the motorists of the congestion downstream. Second, if the traffic getting off the freeway become heavy, congestion on the off-ramp would back up to the freeway, which would create traffic turbulences for traffic even after passing the point of off ramp.
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