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30 Table 19. Percent of total crashes that involve fixed-object collisions. Time of Day Active Work Active Work No Active Work, No Work Zone with Lane without Lane No Lane Present Closures Closures Closures Daytime 20.3% 10.3% 15.9% 15.3% Periods Nighttime 22.8% 21.0% 31.9% 32.4% Periods as a function of roadway AADT in daytime, and to a lesser However, no clear trends exist in the percentages across the degree at nighttime. work conditions examined in either time period. During the The change in fixed-object crashes with work activity is ex- day, the percentages range between 14 and 21 percent; at amined in Figure 13 for daytime conditions and Figure 14 for night, the percentages range between 23 and 28 percent. nighttime conditions. Generally speaking, no clear trends are Neither of these ranges includes a statistically significant dif- evident with regards to the percentage of fixed-object crashes ference on the basis of work condition. that occurred at different AADT levels. As Figure 13 indicates, the percentage of such collisions during periods of daytime Summary work activity ranged between 10 and 25 percent across most of the AADT levels shown, with no clear trend evident. At night, The following is a summary of key findings from the analy- the percentage of fixed-object collisions and AADT levels sis of traffic crashes from 64 work zone projects across four during work activity also did not demonstrate any clear trends states: (see Figure 14). Overall, when work activity is occurring and travel lanes are temporarily closed, the risk of a crash to a motorist Other Vehicle Collision Types traveling through the work zone increased by about 66 per- The remaining crashes not previously categorized were con- cent during daytime conditions and by 61 percent during solidated into an "other crashes" category and examined for nighttime conditions, compared to the expected crash risk trends across the same time periods and work conditions as that would normally exist at a particular location. previously performed for the other categories. The results of The actual change in crash risk in these work zones varied this examination are shown in Table 20. Overall, the percentage substantially from project to project, even when stratified of crashes that fall into this remaining category is slightly less on the basis of time period (daytime or nighttime) and during daytime conditions than during nighttime conditions. work condition (no work activity, active work without lane Percent of Crashes That Involve a Fixed- 70% 60% 50% Object Collision 40% 30% 20% 10% 0% 0 50000 100000 150000 200000 Roadway AADT Daytime No Work Zone (Before) Daytime Work Zone Inactive Nighttime No Work Zone (Before) Nighttime Work Zone Inactive Figure 12. Comparison of roadway AADT to fixed-object collision percentages, no work zone versus inactive work zone conditions.

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31 70% Percent of Crashes That Involve a 60% Fixed-Object Collision 50% 40% 30% 20% 10% 0% 0 500000 100000 150000 200000 Roadway AADT Work Zone Active with Temporary Lane Closures Work Zone Active without Temporary Lane Closures Work Zone Inactive Figure 13. Comparison of work activity and roadway AADT to fixed-object collision percentages, daytime work periods. Percent of Crashes That Involve a Fixed- 80% 70% 60% Object Collision 50% 40% 30% 20% 10% 0% 0 50000 100000 150000 200000 Roadway AADT Work Zone Active with Temporary Lane Closures Work Zone Active Without Temporary Lane Closures Work Zone Inactive Figure 14. Comparison of work activity and roadway AADT to fixed-object collision percentages, nighttime work periods. Table 20. Percent of total crashes that involve all other collision types combined. Time of Day Active Work Active Work No Active Work, No Work Zone with Lane without Lane No Lane Present Closures Closures Closures Daytime 19.2% 20.6% 14.1% 17.7% Periods Nighttime 23.1% 24.4% 25.2% 28.3% Periods

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32 closures, or active work with lane closures). Crash risks The increased costs of work zone crashes, compared to increased on some projects and decreased on others, com- expected crash costs based on the pre-construction crash pared to the expected values. Furthermore, no relationship history, were consistently lower for nighttime work than appears to exist between the change in crash risk and road- daytime work when the work was active and a lane closure way AADT. was in place. This is true for the entire range of AADTs When work was active and lane closures were in place, examined, and the difference between day and night was severe crashes increased by 42.3 percent when the work substantial for higher AADTs. This means that the overall was done at night and by 45.5 percent during the day. For safety impacts to the motoring public of work activities PDO crashes, the increase was 74.8 percent at night and that involve temporary lane closures tend to always be less 80.8 percent during the day. at night, and the benefit of working at night increases as When work activity was occurring but no temporary lane AADTs increase. closures were used, the increase in severe crashes com- For work activities that do not involve a temporary lane puted at night was higher (41.4 percent) than during the closure, there appears to be little difference in working dur- day (17.4 percent). Similarly, the increase computed in ing the day or at night in terms of increased crash costs PDO crashes during work activity at night (66.6 percent) is generated. The increased crash costs at night are actually greater than that during the day (39.8 percent increase). slightly higher than during the day at lower AADT levels However, neither of these differences was found to be sta- but slightly lower at higher AADT levels. tistically significant due to the high project-to-project vari- The increase in crash costs when the work is inactive and ability in the results. Still, it appears that working at night no temporary lane closures are required is slightly higher with no lane closures in place may be affecting crash risks at night than during the day across the range of AADTs more than was previously known. It is not clear from the examined, although these differences are not statistically data whether the greater nighttime crash risk during these significant. For both daytime and nighttime periods, the work conditions is the result of the following factors: increased crash costs when work zones are present but with Work area lighting glare that work crews might not be no work activity are much less, at any AADT, than when mitigating well when they are not located in travel lanes; work is active, whether or not a lane closure is present. More frequent construction equipment and material In terms of work zone crash characteristics, the percent of deliveries into and out of the work area at night that cre- crashes involving rear-end collisions typically increases as a ate large speed differentials and subsequent crashes; or function of AADT in both daytime and nighttime periods, Issues that continue to plague drivers encountering a although the percentages remain substantially lower in the work zone at night (lack of expectancy, poorer visibil- nighttime periods for the higher AADT regions. Further- ity, increased levels of impairment, etc.) regardless of more, for both time periods, the percentage is very similar whether or not a travel lane is closed. between the before (no work zone) and work zone inactive When the work was inactive and no lane closures were conditions. present, the increase in injury crashes was slightly higher The effect of active work during the day with or without for nighttime conditions (11.4 percent) than for daytime lane closures on rear-end collisions is not consistent across conditions (2.0 percent), but this difference is not statisti- all AADT ranges. Rear-end collisions increase markedly cally significant. For PDO crashes, the increase at night during work activity on low- to moderate-volume road- (33.0 percent) was significantly higher than that during the ways, but not on higher-volume roadways. There may exist day (19.6 percent). The slightly greater increases at night an upper limit in terms of how much of the total crash presumably reflect somewhat degraded geometric condi- experience at a location will be the result of rear-end tions in the work zone relative to a pre-work zone condi- collisions. tion that--coupled with nighttime-specific issues such as At night, work activity resulted in an increase in the per- limited visibility, less attentive drivers and so forth--raise centage of crashes that are the result of rear-end collisions nighttime crash risk more than daytime conditions do across all roadway AADTs. The effect is somewhat greater when work is inactive. when temporary lane closures are in place than when they For each of the work conditions examined, the increases in are not, consistent with expectations. crash risk are higher for the PDO crashes than for the in- Overall, the percent of sideswipe collisions was not affected jury crashes, indicating that the additional crashes that do by time period, work activity, or lane closure presence. occur due to the work zone tend to be less severe in nature. Sideswipe collisions accounted for between 13 and 21 per- This trend exists regardless of whether the work is per- cent of crashes occurring in the work zone. formed during the day or at night. This is consistent with Fixed-object collisions consistently comprise a greater pro- previous studies that found similar results. portion of nighttime crashes than daytime crashes. Also,

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33 fixed-object collisions comprise an almost identical per- Overall, the percentage of all remaining crash types is centage of crashes between the before (no work zone) con- slightly less during daytime conditions than during night- dition and the work zone inactive condition. Fixed-object time conditions. No clear trends exist in the percentages collision involvement in crashes for both of those condi- across the work conditions examined in either time period. tions decreases significantly as a function of roadway During the day, the percentages range between 14 and AADT in the daytime period and to a lesser degree in the 21 percent; at night, the percentages range between 23 and nighttime period. 28 percent.