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NCHRP Project 17-61 62 SUMMARY AND RESEARCH RECOMMENDATIONS 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. The following is a summary of the key findings in this report: ï· As has been hypothesized in previous studies, congestion and queues due to work zones were shown to be a significant contributor to crashes. The presence of queues and congestion was found to be a key contributor to rear-end collisions in each of the three different databases described in Chapter 2. 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. ï· Although queues were found to be a significant issue associated with crashes at interstate and freeway work zones, the VDOT analysis also found them to be associated with two-lane highway work zones as well. Most work zones on these types of roadways involve the temporary closure of one travel lane, and the use of alternating one-way traffic control via flaggers, AFADs, or portable traffic signals. Queues of varying length are created at these work zones, can be unexpected by motorists, and likely contribute to the rear-end collisions that occur. ï· Work vehicles entering/exiting the work space were also found to be an issue in this assessment. Both rear-end collisions and sideswipe crashes in the three databases could be attributed to this situation. In the LTCCS database, 10% of the crashes on interstate/freeway facilities appeared to be the result of trucks attempting to enter or exit main lane traffic to or from the work space. ï· Work zones on non-freeway/interstate facilities do also appear to be creating challenges for drivers. Data from several crashes examined in these analyses indicated 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. Obstructions created by the presence of work equipment too close to an intersection or driveway were cited as a contributor in a number of
NCHRP Project 17-61 63 crash narratives, as was the presence of certain temporary traffic control devices. In one instance, type 3 barricades placed in the opposing left-turn lane at a signalized intersection limited the ability left- turning traffic coming from the other direction to see approaching through lane traffic during the permissive green phase. ï· The analysis of EOQWS and PRS as countermeasures to reduce collisions 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. ï· Interestingly, the data indicates that the use of the PRS alone resulted in the same crash reduction as the combined EOQWS + PRS treatment, suggesting that there was little additional benefit to using both countermeasures together. However, it is important to reiterate that there were differences in where the PRS only and EOQWS + PRS treatments were deployed which may be partially responsible for these results. The two technologies also have significantly different capital and operational costs associated with their use. Practitioners should carefully weigh these differences when deciding whether to use either (or both) of these countermeasures. ï· 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 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 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 CMs from the data. ï· Although the multi-state project analysis failed to provide individual work zone feature CMFs, the effort was successful in developing generic work zone SPFs for four-lane 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 AADT. ï· In lieu of using the work zone SPFs directly, the analyses also yielded a basic work zone CMF for the base characteristics that can be used where an estimate of crashes normally occurring on a segment of roadway is already available. Research Recommendations The findings from this research effort have also yielded the following recommendations: ï· Research should 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. The analysis of the EOQWS and PRS indicate that they may be quite effective countermeasures, but the evaluation performed under this study was limited to a single interstate corridor. In addition, the effects of EOQWZ separate from PRS could not be evaluated at this time.
NCHRP Project 17-61 64 ï· It is recommended that the adequacy of existing training for work zone traffic control relative to arterials and intersections be examined, and improvements to that training be made where appropriate. 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. Efforts should also be made to ensure that the improved training be readily available to municipalities, counties, and contractors who typically have primary responsibility for this on the job site. ï· 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 where they cannot be avoided, 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. ï· Agencies need to 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. Examples highlighting how those analyses might be performed are also included in the guide.
