Analysis of Work Zone Crash Characteristics and Countermeasures (2018) / Chapter Skim
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NCHRP Project 17-61 6 CHAPTER 2 Causal Assessments of Work Zone Crashes Overview Much of the work zone safety literature is based around methodologies that develop statistical associations between work zone crashes and work zone features. Knowledge on crash causation, including the road environment as one of many factors in the causal chain, is limited.
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NCHRP Project 17-61 7 Methodology Work zone crashes (defined as those occurring within a defined work zone area) were extracted from the database for calendar years 2011 and 2012.
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NCHRP Project 17-61 8 Even so, crashes determined to be directly related were not necessarily caused by the work zone and may have been only slightly influenced by the work zone's presence. Conversely, crashes not determined to be directly related could still have been influenced by the work zone, but the officer failed to capture this influence in the crash report.
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NCHRP Project 17-61 9 Table 1. Summary of Crash Causes Directly Related to Work Zone Presence Crash Type # of Directly Related Crashes % of Coded Crash Typea 1st Most Common Cause 2nd Most Common Cause 3rd Most Common Cause Rear-end 679 18.1% Stopping/ slowing due to work zone presence (64.7%)
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NCHRP Project 17-61 10 peak periods, were not included. The majority of these crashes (73.0%)
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NCHRP Project 17-61 11 crash reports, it appears that failure to yield on the part of the merging vehicle was a common cause of crashes. Common Cause #2: Confusion due to work zone traffic control A total of 34 crashes (21.3% of the directly-related angle crashes)
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NCHRP Project 17-61 12 Recovery from intrusions into the work zone activity area This casual category included seven crashes (4.1% of directly-related sideswipe crashes) involving non-work vehicles unintentionally entering a work zone activity area, and then sideswiping vehicles in the adjacent travel lane as they attempted to recover back to the travel lane.
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NCHRP Project 17-61 13 Crash protection/shadow vehicles at 1) lane closures or 2)
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NCHRP Project 17-61 14 Overall, 24.1% of the directly related crashes occurred during work zone congestion. Of these, 73.3% (261 crashes)
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NCHRP Project 17-61 15 NMVCCS Analysis Methodology NMVCCS records contain more than 600 variables that capture details related to the driver, vehicle, roadway, and the environment, including crash narratives, photographs, and diagrams collected at the crash scene. The NMVCCS database includes variables indicating whether there was a work zone present at the crash site and whether traffic barrier was present.
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F igure 2. Initial classification of NMVCCS crashes in or near wo rk zones on freeway/interstate roadways.
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NCHRP Project 17-61 17 Roadway departure Late/forced lane change Rear-end due to congestion "Other" situations As expected, the classification with the highest frequency was the rear-end crashes due to congestion (25 crashes, or 45.5% of the 55 crashes)
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F igure 3. NMVCCS crashes in or near work zones on freeway/in terstate roadways based on direct relation to the work zone.
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NCHRP Project 17-61 19 Thus, a re-classification of the 12 crashes in this group, this time based on cases where work zones could have had an influence, results in six crashes being work zone related and another six grouped as having no clear work zone influence. Out of the six cases classified as work zone related, in four occasions the right lane leading to the off-ramp was shifted and had temporary concrete barriers (in one case the right lane was closed with cones)
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NCHRP Project 17-61 20 An initial classification of the crashes is shown in Figure 4. Similar to freeway crashes, those not related to vehicle-roadway interactions (e.g., roadway design, surface conditions)
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F igure 4. Initial classification of NMVCCS crashes in or near wo rk zones on non-freeway/interstate roadways.
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NCHRP Project 17-61 22 The second sub-group included roadway departure crashes (eight cases) , where driver performance errors including inattention, health issues (two cases)
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F igure 5. NMVCCS crashes in or near work zones on non-freew ay roadways based on potential influence of the work zone.
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NCHRP Project 17-61 24 Results of Analysis Freeway/Interstate Crashes The re-examination of the 55 crashes on urban and rural freeways resulted in the groupings by type of crash shown in Figure 6. Only one crash was not related to vehicle-roadway interactions (e.g., roadway design, surface conditions)
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F igure 6. Initial classification of LTCCS crashes in or near work zones on freeway/interstate roadways.
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NCHRP Project 17-61 26 In a separate category there were seven sideswipe crashes involving at least one truck. In five cases, the lane configuration was modified by construction, and for this reason were considered related to the work zone presence.
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Figure 7. LTCCS crashes in or near work zones on freeways/interstates based on direct relation to the work zone.
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NCHRP Project 17-61 28 Non-Freeway/Interstate Crashes A total of 21 work zone crashes from the LTCCS records were identified and analyzed at facilities without full-access control. Following a similar format used for the freeway/interstate crashes, nonfreeway/interstate crashes were first classified into vehicle-roadway or vehicle-vehicle interaction categories, as shown in Figure 8.
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F igure 8. Initial classification of LTCCS crashes in or near work zones on non-freeway/interstate roadways.
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Figure 9. LTCCS crashes in or near work zones on non-freeway/interstate roadways based on potential influence of the work zone.
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NCHRP Project 17-61 31 Summary of Findings from the NMVCCS and LTCCS Analyses A breakdown of the freeway and non-freeway crashes grouped by main categories and subcategories for each of the two databases is shown in Table 4. Overall, back of queue rear-end crashes related to speed drop/congestion were by far the most common type and represented about 48% of all work zone related crashes (64 of the 133 crashes)
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Table 4. Summary of NMVCCS and LTCC Crashes where the Work Zone Influence was Likely Category Sub-category NMVCCS LTCCS Total Freeway Non-freeway Freeway Nonfreeway Speed drop/ congestion At back of queue (driver distraction / inattentive / failed to recognize stopping traffic)
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NCHRP Project 17-61 33 New Insights Gained Through the Analyses The combined results of the analysis of the three different databases illustrate the significance of work zone-induced congestion upon crash potential. Certainly, work zone congestion is a major contributor to rear-end collisions, as previous research has found or hypothesized.
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NCHRP Project 17-61 34 The fact that these trends were observed in all three databases, despite the fact that they were created for different purposes and cover very different levels of detail and geography, is particularly noteworthy. Such correlations across the databases adds credibility to the significance of these issues in work zones nationally.
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