As part of an Urban Partnership Agreement project, the Minnesota Department of Transportation (MnDOT) added lanes and began operating a priced dynamic shoulder lane (PDSL) on parts of I-35W. Following the opening of these improvements, the frequency of rear-end crashes increased in certain sections, especially in the PDSL region. The object of this study was to determine if this increase was due to changes in traffic conditions or was a direct effect of the improvements. A preliminary analysis was done to determine the study region. I-35W, from its start to its junction with I- 94, was divided into 17 one-mile sections, and bi-directional (northbound and southbound) crash frequencies in Before-UPA (2006-2008) and After-UPA periods (2011-2013) were compiled for each one-mile section. The dominant crash type was rear-end crashes, but the changing trend of bi-direction rear-end crash frequencies from the Before to After period varied among the one-mile sections. Our interest lay in those regions where there was an outstanding increase in the rear-end crash frequency in the After period, which were approximately the I-35W HOT region (from TH-13 to I-494) and the I-35W PDSL region (from 37th Street to 26th Street). The I-35W HOT region and the PDSL region were divided into sections based on constant flow and geometry criteria as well as the availability of loop-detector data. Crash, loop detector, weather condition, and PDSL activation (only for sections in PDSL region) data for the Before and After periods were compiled for each section. Rear-end crash records were extracted using Minnesota Crash Mapping Analysis Tool (MNCMAT), and the original police reports were reviewed to confirm the crash times and locations. Loopdetector data were retrieved using MnDOT’s DataExtract tool. The source of weather condition data, namely rain and snow records, was MnDOT’s Road Weather Information System (RWIS). The PDSL activation status came from the MnDOT’s log of the Intelligent Lane Control Signal (ILCS) located at 37th Street. Logistic regression analyses estimating the change in rear-end crash risk following the UPA project, controlling for traffic conditions, weather conditions, and PDSL activations, were carried out for each section. For each hour during 2006-2008 and 2011-2013, the presence or absence of a rear-end crash became the dependent variable while independent variables consisted of traffic volume and lane occupancy (constructed using the loop detector data), the presence or absence of snowy or rainy conditions, before versus after the UPA project, and the presence or absence of PDSL operation. Results: 1) Most analysed sections in the I-35W HOT region showed no significant change in rear-end crash risk associated with the UPA project. Exceptions were Section N17 (northbound, just south of I-494) and Section S9 (southbound, just north of Minnesota River). Section N17 actually experienced fewer crashes after the UPA project, but the reduction was not as great as the change in lane occupancy would predict. The apparent increase in rear-end crash risk of Section S9 was possibly due to ambiguities in locating crashes during the Before period. 2) An “Inverted U” relationship between a proxy for traffic density, lane occupancy, and the probability of a rear-end crash occurring during an hour were seen in several sections. Crashes were most likely when lane occupancies were approximately 20%-30%, and crash likelihood tended to decrease for lane occupancies below and above this range. 3) The I-35W PDSL region experienced a substantial increase in congestion (defined as average lane occupancies exceeding 25%) following completion of the UPA improvements. This was due to removal of the bottleneck in the old I-35W and TH-62 commons, causing the bottleneck to move northward to the I-35W and I-94 junction. When controlling for the change in traffic conditions, there was no significant increase in rear-end crash risk attributable to the PDSL operation. This study demonstrated a methodology that could be used to evaluate the safety effects of freeway-related projects. To be more specific, this study worked out a way to estimate changes in hourly crash risk while controlling for variations in traffic conditions. MnDOT is interested in using PDSLs at other freeway locations. The impact of this research is to show that the current implementation of a PDSL did not have an adverse effect on safety. (Author/publisher)
Samenvatting