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NCHRP Project 17-61 iv LIST OF TABLES Table 1. Summary of Crash Causes Directly Related to Work Zone Presence ........................................... 9 Table 2. Location/Action of Work Zone Vehicles Involved in Crashes .................................................... 13 Table 3. Summary of Significant Crash Causes across Crash Types......................................................... 13 Table 4. Summary of NMVCCS and LTCC Crashes where the Work Zone Influence was Likely .......... 32 Table 5. Sample sizes ................................................................................................................................. 40 Table 6. Expected and Actual Crash Results ............................................................................................. 42 Table 7. Effects of Safety Treatments ........................................................................................................ 43 Table 8. Work Zone Database Elements .................................................................................................... 48 Table 9. Construction Project Descriptions ............................................................................................... 49 Table 10. Sample Sizes .............................................................................................................................. 50 Table 11. Geometric Variable Ranges ....................................................................................................... 50 Table 12. Crash rate comparison between pre- and during-construction periods ...................................... 51 Table 13. Results of Analysis for Model 1 ................................................................................................ 54 Table 14. Results of Analysis for Model 2 ................................................................................................ 55 Table 15. Results of Analysis for Model 3 ................................................................................................ 56
NCHRP Project 17-61 1 SUMMARY In this report, researchers have documented the results of multiple analyses focused on developing an improved understanding of work zone crash characteristics and countermeasure effectiveness. In-depth analyses of work zone crash narratives and other data sources above and beyond standard fields and codes in state and national crash databases yielded several useful insights into work zone factors associated with common types of crashes. Assessment of safety countermeasures used to combat rear-end collisions at interstate lane closure queues indicate that the countermeasures have a positive crash-reducing effect. Finally, an analysis of crashes at a national sampling of interstate work zones was not able to isolate the effects of individual roadway factors upon work zone crashes. However, general crash prediction models were computed that can assist practitioners in quantifying expected crash effects of work zones as a function of length, duration, and roadway traffic demand. As has been hypothesized in previous studies, congestion and queues due to work zones were shown to be a significant contributor to crashes. In addition, the analyses also suggested that congestion contributed to a proportion of sideswipe collisions and collisions with barriers or other objects, due to last-second swerving to avoid running into the back end of a vehicle. Queues were found to be a significant issue associated with crashes at interstate and freeway work zones, and with two-lane highway work zones as well. Most work zones on two-lane highways involve the temporary closure of one travel lane, and the use of alternating one-way traffic control via flaggers, automatic flagger assistance devices (AFADs), or portable traffic signals. Queues of varying length are created at these work zones, can be unexpected by motorists, and so can result in occasional rear-end collisions. Work vehicles entering/exiting the work zone were also found to be an issue in this assessment. Both rear-end collisions and sideswipe crashes could be attributed to this situation. In one of the databases analyzed in this research, 10% of the crashes on interstate/freeway facilities appeared to be the result of trucks attempting to enter or exit mainlane traffic to or from the work space. Furthermore, work zones on non-freeway/interstate facilities appear to be creating challenges for drivers that were previously unknown or underestimated based on past analyses. Data from several crashes indicate that drivers became confused when approaching and entering work zones on non- accessed controlled facilities at intersections and driveways, especially in urban areas. Work zones on facilities which were normally divided and then converted to two-way operation in one of the directional roadways while the other direction was repaired or rehabilitated appear to be particularly problematic. Sight distance challenges were also noted for several crashes occurring at these types of work zones. Two types of queue warning countermeasures, work zone ITS-based end-of-queue warning systems and portable transverse rumble strips, installed at interstate work zones when queues form indicate that the technologies can indeed reduce such crashes. Overall, the use of these countermeasures appeared to reduce crashes during periods of queuing and congestion by 53% to 60% from what would have been expected if the countermeasures had not been used. In addition, the crashes that did occur were significantly less severe when the countermeasures were deployed as compared to the no-countermeasure condition. Without the countermeasures deployed, 50% of the crashes occurring when queues were present involved injuries or fatalities; when the treatments were deployed, only 16% of the crashes involved injuries or fatalities. Despite an extensive project identification and data collection/reduction effort, analysis of the multi- state database of work zones on interstate facilities did not yield statistically significant crash modification factors (CMFs) for individual work zone features such as lane widths, shoulder widths, lane closures, shoulder closures, median widths, lane shifts, or barrier use. One of the main reasons for this
NCHRP Project 17-61 2 was the lack of sufficient variability in these features across the projects available for analysis. There was considerable confounding of many of these features (e.g., reduced lane and shoulder widths together with barrier placed at the edge of the shoulder, etc.), which kept the research team from being able to extract useful CMFs from the data. Although the multi-state project analysis failed to provide individual work zone feature CMFs, the effort was successful in developing generic work zone safety performance functions (SPFs) for four and six-lane interstates, based on the national dataset generated. These SPFs have been computed for a defined set of base characteristics, and so can be useful in planning-level analysis of possible work zone safety impacts to be expected as a function of project length, duration, and roadway annual average daily traffic (AADT). Based on the myriad of analyses undertaken on this project, it is recommended that research continue into ways to reduce the frequency and severity of rear-end (and sideswipe) collisions caused by queues and congestion at work zones, especially on higher-speed facilities. Research is also needed to develop better guidance, such as typical applications or improved signing, for work zone traffic control at intersections. Several of the crashes examined in detail under this study pointed to driver confusion and visibility issues when work zones were performed in the vicinity of intersections. New designs for barricades and channelizing devices may be necessary to further improve work zone safety at these locations. Increased emphasis should be made towards improving ingress and egress at work space access points within the work zone to reduce the frequency and severity of work vehicle/motorist crashes. Strategies such as eliminating work space access to and from high-speed travel lanes, innovative access point designs, and testing and evaluation of systems to warn approaching traffic when work vehicles are about to enter or exit the work space, should be pursued. Finally, agencies should begin to incorporate safety assessments into their work zone design and transportation management planning processes. An implementation guide developed as part of this research effort describes how this can be accomplished and provides guidance on the availability and applicability of available CMFs for this purpose.
