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9 Table 1. Summary of NYSDOT work zone traffic crashes and construction accidents on freeways and expressways (2000-2005). Daytime Nighttime Nighttime Daytime Total Work Periods, Work Periods, Work Periods, Work Periods, Daytime Daytime Nighttime Nighttime Accidents Accidents Accidents Accidents Traffic 1762 9 316 102 2189 Crashes Construction 931 114 1045 Accidents Total 2693 9 430 102 3234 of work operation (day, night, or both). While most of the Findings crashes and accidents included in the database occurred dur- ing active work periods, some did occur during periods when Work Zone Traffic Crash Analysis the work zone was inactive. Statistical distributions of the Work Zone Conditions Where Crashes Occur other data fields were then computed separately for daytime incidents at daytime work operations and for nighttime inci- Daytime and nighttime work zone crashes on NYSDOT dents at nighttime work operations. Further stratification was freeways and expressways differed significantly in how they made to examine lane closure work operations as its own dis- were distributed among the common work zone traffic control tinct subset. As previously noted, temporary closure of one or operations utilized on these facilities. As shown in Table 2, more travel lanes on a freeway-type facility is often limited a substantially higher percentage of nighttime crashes occurred to night hours in order to avoid high traffic volume time during lane closure operations than of daytime crashes periods. Direct comparison of incident characteristics of this (57.6 vs. 50.4 percent, respectively). On the other hand, particular work zone situation during the day to the same a higher percentage of daytime crashes occurred where there situation at night was thus of primary interest to the research was only minor traffic control present and no work was occur- team. ring (17.2 percent vs. 7.6 percent of nighttime crashes). As Researchers then computed statistical distributions of the previously stated, the typical reason for working at night is various data field elements and used chi-square statistical that one or more travel lanes must be closed for several hours tests of independence to check whether the differences in the to perform the work, and doing so during the day would distributions between daytime and nighttime incidents were cause unacceptable traffic impacts. significant. As previously noted, exposure data were not The greater relative frequency of temporary lane closures available to allow traffic crash or construction accident rates at night is also a likely explanation of the higher percentage of to be calculated from these data. nighttime crashes during traffic control setup and takedown Table 2. NYSDOT work zone crashes by traffic control conditions. Type of Work Zone Traffic Control in Daytime Work Nighttime Work Use Operations, Operations, Daytime Crashes Nighttime Crashes (n = 1757) (n = 316) Lane Closure 50.4% 57.6% Minor Traffic Control, Work Inactive 17.2% 7.6% Minor Traffic Control, Work Active 8.3% 9.5% Flagging 8.0% 3.2% Shoulder Closure 5.4% 0.9% Median Crossover 3.3% 1.3% Lane Shift 3.2% 0.9% During Traffic Control Setup/Takedown 3.1% 14.2% Full Road or Bridge Closure 0.7% 3.8% Other 0.4% 1.0% Chi-Square Test Results Daytime and nighttime distributions are significantly different from each other a a X2 = 94.510 > X 2 Crit (6, 0.05) = 12.592

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10 relative to daytime crashes (14.2 percent vs. 3.1 percent, re- crashes are considered, no significant differences in severity spectively). Temporary lane closures require setup and take- are detected between daytime and nighttime crashes. As can down activities each night so that all travel lanes are returned be seen in the table, worker-involved crashes tend to be more to service for peak travel periods, which would imply a higher severe at night than during the day. Whereas 29.6 percent of relative frequency of these activities occurring during night- worker-involved crashes during daytime lane closure opera- time hours compared to daytime hours. Nevertheless, the tions resulted in fatalities or injuries, 50.8 percent of such percentage of such crashes during nighttime hours seems crashes at nighttime lane closure operations resulted in injuries rather high given that the amount of time typically required or fatalities. A similar trend is evident when worker-involved for such setup and removal is typically much less than the du- crashes are examined across all work zone types. Over one- ration of the actual temporary lane closure. It is also noted half of the worker-involved crashes at night resulted in in- that crashes at flagger-controlled sites are more frequent in juries or fatalities, and only about one-third of those types of daytime operations, which most likely reflects the relatively crashes during the day involved fatalities and injuries. lower usage of flaggers for traffic control at night. The designation of a crash as being worker involved does not automatically imply that the worker was actually hit by the vehicle and sustained the injury. In fact, for many of the Severity of Work Zone Crashes worker-involved crashes, the driver or other occupants of the The comparative effect of day versus night work on crash vehicle sustained the most serious injuries. In these instances, severity is illustrated in Table 3. Overall, work zone crash sever- the worker was involved but somehow managed to avoid ities trend slightly higher at night when consolidated across being struck. If the crash did involve a vehicle striking a all types of work zone situations and for lane closure traffic worker, the result was usually quite severe. Overall, 93 per- crashes specifically. However, if only non-worker-involved cent of those workers who were struck by a vehicle during the Table 3. NYSDOT work zone crash severity. Injury Severity All Work Zone Traffic Lane Closure Traffic Control Control Types Work Zones Only Daytime Nighttime Daytime Nighttime Work Work Work Work Operations, Operations, Operations, Operations, Daytime Nighttime Daytime Nighttime Crashes Crashes Crashes Crashes All Traffic Crashes: (n = 1762) (n = 304) (n = 886) (n = 182) Fatal 1.4% 3.3% 1.1% 2.7% Injury 36.4% 43.4% 32.5% 41.2% PDO 62.2% 53.3% 66.4% 56.1% Chi-Square Test Results Daytime and nighttime Daytime and nighttime distributions are significantly distributions are not differenta significantly differentd Traffic Crashes: (n = 1423) (n = 195) (n = 771) (n = 123) No Workers Involved Fatal 1.5% 3.6% 1.3% 2.4% Injuries 37.3% 40.0% 32.9% 38.2% PDO 61.2% 56.4% 65.8% 59.3% Chi-Square Test Results Daytime and nighttime Daytime and nighttime distributions are not distributions are not significantly differentb significantly differente Traffic Crashes: (n = 339) (n = 107) (n = 115) (n = 59) Workers Involved Fatal 0.9% 2.8% 0.0% 3.3% Injuries 32.7% 49.5% 29.6% 47.5% PDO 66.4% 47.7% 70.4% 49.2% Chi-Square Test Results Daytime and nighttime Daytime and nighttime distributions are significantly distributions are significantly differentc differentf a X2 = 12.609 > X 2Crit (2, 0.025) = 7.378 b X2 = 4.757 < X 2 Crit (2, 0.025) = 7.378 c 2 X = 12.068 > X 2 Crit (1, 0.025) = 5.024 d 2 X = 8.671 > X 2Crit (2, 0.025) = 5.024 e 2 X = 1.912 < X 2 Crit (1, 0.025) = 5.024 f 2 X = 7.596 > X 2 Crit (1, 0.025) = 5.024

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11 day sustained injury, as did 100 percent of workers who were area of work activities themselves. Add to this the fact that struck at night. drivers at night are more likely to be impaired than during the day, and it is fairly easy to understand why intrusions and im- pacts with TMAs would make up a greater proportion of Types of Traffic Crashes Occurring nighttime work zone crashes. Statistically, the differences be- Significant differences were detected in the types of collisions tween daytime and nighttime periods shown in Table 4 are that occurred on freeways and expressways during daytime highly significant when all work zone traffic control types are versus nighttime work zone operations. Table 4 illustrates the considered together. Limiting the analysis to lane closure traf- distribution of daytime and nighttime collision types for all fic control work zone crashes, differences between daytime types of work zone situations and also for lane closure work and nighttime conditions are still statistically significant but zone operations only. The percentage of the crashes that in- not by as large of an amount. volve rear-end collisions is substantially lower at night for all Table 4 also illustrates that the percentage of intrusion work zone traffic control types combined and for work zone crashes involving workers is substantially higher at night lane closure operations in particular. Presumably, the lower work operations than during daytime work operations. The traffic volumes present at night allow work activities to be amount of the increase is the same for lane closure crashes accomplished with fewer disruptions in traffic flow and fewer (0.7 percent daytime vs. 3.8 percent nighttime) and for all abrupt speed changes by vehicles. The decrease in rear-end work zone crashes (0.7 percent daytime vs. 3.8 percent night- crashes at night is offset by small increases in the percentage time). However, the absolute frequency of these crashes was of intrusion crashes, impacts with truck-mounted attenua- very low; totally only 12 crashes during daytime periods and tors (TMAs), and impacts with work equipment, materials, 11 crashes during nighttime periods over the 6-year period of and/or debris outside of the work area. analysis. One possible explanation for these increases is that in day- Intrusion crashes involving construction equipment and time conditions, the traffic congestion that is created migrates those involving construction debris or materials are also higher the majority of the crashes (most of them rear-end collisions) at night but to a lesser degree. Intrusion crashes still make up upstream away from the work area to where drivers first have only a small proportion of all types of traffic crashes in either to make significant adjustments in their speed. At night, this daytime or nighttime work zones. The percentages shown in crash impetus does not exist upstream, and so the conse- Table 4 are consistent with previously reported trends in quences of driver inattention congregate in and around the work zone intrusion crashes (47, 48). As stated previously, it Table 4. NYSDOT traffic crash types. Key Crash Types All Work Zone Traffic Lane Closure Traffic Control Control Types Work Zones Only Daytime Nighttime Daytime Nighttime Work Work Work Work Operations, Operations, Operations, Operations, Daytime Nighttime Daytime Nighttime Crashes Crashes Crashes Crashes (n = 1762) (n = 315) (n = 886) (n = 182) Rear End 49.0% 35.6% 59.1% 45.6% Other Multi-Vehicle 16.8% 14.3% 14.8% 13.2% Single Vehicle Run-Off- 9.4% 9.8% 5.3% 7.1% Road Intrusion Impacts: 0.7% 3.8% 0.7% 3.8% with Workers 3.7% 4.8% 4.7% 6.0% with Equipment 3.1% 3.8% 4.3% 4.4% with Debris/Other Non-intrusion Impacts: 0.4% 0.3% 0.2% 0.5% with Workers 3.6% 6.7% 2.5% 4.4% with Equipment 6.8% 8.6% 4.9% 7.7% with Debris/Other 2.7% 9.8% 2.5% 4.9% Impact with TMA 3.8% 2.5% 1.0% 2.4% Other Miscellaneous Types Chi-Square Test Results Daytime and nighttime Daytime and nighttime distributions are significantly distributions are significantly differenta differentb a X2 = 118.360 > X 2Crit (7, 0.025) = 16.013 b X2 = 21.828 > X 2 Crit (7, 0.025) = 16.013