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19 100% Percent of Temporary Lane Closure Hours 80% Performed at Night 60% 40% 20% 0% 0 50000 100000 150000 200000 Roadway AADT Figure 2. Percentage of temporary lane closures performed at night at each project. work zone upon safety and is referred to herein as the "index Results of change" observed. Researchers analyzed fatal and injury crashes separate from PDO so that possible daytime and Increases in Traffic Crashes nighttime differences in crash severity could be estimated as Occurring during Nighttime well (reference to "injury" crashes herein implies the combi- and Daytime Work Activities nation of both fatal and injury crashes). Additional details of Increases in Crash Risk the EB procedure employed for this study can be found in Appendix A. Appendix B provides the number of injury and PDO After the differences in crashes were estimated at each proj- crashes occurring at each project during each nighttime and ect location for each time period of interest, the crash costs daytime work period type (work activity or no work activity, associated with these differences were computed. Recent cost with or without lane closures) and those expected to have values for freeway crashes (51) were used: occurred during those same periods if the work zone were not present. Injury crash (fatality or injury)--$206,015, and In Figures 3 through 5, the index of change estimated by PDO crash--$7,800. the EB procedure is plotted against AADT for each project for each of the following scenarios: The differential crash costs per unit duration of work activ- ity or inactivity (with and without temporary lane closures) Project work was occurring (the work area was active), and per mile of work zone were computed and modeled as a func- temporary lane closures were in place (Figure 3); tion of AADT. Project work was occurring, but no temporary lane clo- Consideration was given to developing incremental crash sures were in place (Figure 4); and increase models for selected collision types (rear-end, sideswipe, The project was inactive, and no temporary lane closures run-off-road, etc.), but this level of dissection of the data was were in place (Figure 5). determined to be too fine to permit statistically significant conclusions to be drawn from the data. Therefore, a simple An index of change of 1.0 indicates that the number of comparison of the percentage involvement of these factors in crashes actually occurring is equal to the number of crashes the crashes before and during construction, aggregated across that were expected to have occurred based on the EB analysis. each state, was used to determine whether significant differ- Values greater than 1.0 reflect an increase in actual ences existed between the before project conditions and each crashes during construction relative to the number of of the work activity periods of interest in this study. crashes that would be expected if the work zone was not

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20 9. 0 8. 0 7. 0 6. 0 Index of Change 5. 0 4. 0 3. 0 2. 0 1. 0 0. 0 0 50000 100000 150000 200000 Roadway AADT Daytime Nighttime (a) Injury Crashes 9.0 8.0 7.0 6.0 I ndex of Change 5.0 4.0 3.0 2.0 1.0 0.0 0 50000 100000 150000 200000 Roadway AADT Daytime Nighttime (b) PDO Crashes Figure 3. Index of change for injury and PDO crashes during periods of work activity with temporary lane closures in place. present. A higher ratio indicates a greater increase in actual number (i.e., index of change is less than one). In still other crashes. cases, no crashes occurred during the project work period of Across the three figures, considerable variability is evident interest, so the index of change is zero. from project to project. The index of change is as high as eight Although it was initially hypothesized that the effects of for some projects (i.e., the actual number of crashes that oc- work activity and temporary lane closures would be higher curred is eight times greater than the crashes expected at that (i.e., the index of change would be higher) at higher AADT project location based on the EB analysis). In other instances, levels, the analysis results did not bear this out. The projects the actual number of crashes was less than the expected were stratified into three AADT regions (less than 50,000 vpd;

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21 9.0 8.0 7.0 6.0 Index of Change 5.0 4.0 3.0 2.0 1.0 0.0 0 50000 100000 1500000 200000 Roadway AADT Daytime Nighttime (a) Injury Crashes 9.0 8.0 7.0 6.0 Index of Change 5.0 4.0 3.0 2.0 1.0 0.0 0 50000 100000 1500000 200000 Roadway AADT Daytime Nighttime (b) PDO Crashes Figure 4. Index of change for injury and PDO crashes during periods of work activity but no temporary lane closures in place. 50,000100,000 vpd; and greater than 100,000 vpd) and ana- for either injury or PDO crashes during either nighttime or lyzed to determine the average index of change across the daytime work periods. Consolidated across the entire AADT projects in each region. Table 13 presents the results of the range, the index of change was essentially the same for both analysis for periods when the work area was active and tem- nighttime and daytime periods. Subtracting one from the porary lane closures were present. A weak trend of increasing index of change, expressed as a percent, defines the percent ratios at higher AADT levels is evident for the nighttime work increase in crashes that occurred overall across the projects periods, but this is not replicated for daytime work periods. for the work condition and time period of interest. Injury Furthermore, the fairly sizeable standard errors of these esti- crashes increased by 42.3 percent at night and by 45.5 percent mates indicates that there are no statistically significant dif- during the day. For PDO crashes, the increase was 74.8 per- ferences in the crash ratios between any of the AADT regions cent at night and 80.8 percent during the day. Researchers

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22 9.0 8.0 7.0 6.0 Index of Change 5.0 4.0 3.0 2.0 1.0 0.0 0 50000 100000 150000 200000 Roadway AADT Daytime Nighttime (a) Injury Crashes 9.0 8.0 7.0 6.0 Index of Change 5.0 4.0 3.0 2.0 1.0 0.0 0 50000 100000 150000 200000 Roadway AADT Daytime Nighttime (b) PDO Crashes Figure 5. Index of change for injury and PDO crashes when work area was inactive (no temporary lane closures in place). also combined all crash severities together and computed an no statistically significant differences were detected across the index of change. Total crashes increased 60.9 percent during different AADT levels during either nighttime or daytime daytime work activity with temporary lane closures and periods for either the injury crash or PDO crash indices of 66.3 percent at comparable work operations at night. change. Although the overall injury index of change for the In Table 14, analysis results are presented for periods when nighttime period appears to be higher than it is for the day- work activity was occurring but no temporary lane closures time period (indicating a 41.4 percent increase at night were in place. Overall, this was a fairly infrequent event dur- versus 17.4 percent increase during the day), they are, in fact, ing night operations. Consequently, the crash ratio estimates not statistically different from each other. Similarly, the index obtained were not extremely reliable (as indicated by the of change of PDO crashes in the nighttime period (indicating large standard errors associated with the estimates). Again, a 66.6 percent increase) is not statistically different than it is

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23 Table 13. Index of change for injury and PDO Table 14. Index of change for injury and PDO crashes by AADT range during periods of work crashes by AADT range during periods of work activity and temporary lane closures. activity but no temporary lane closures. AADT Index of Change (S.E.) AADT Index of Change (S.E.) Crash Level Crash Level Range Nighttime Daytime Range Nighttime Daytime 1.318 1.596 2.256 1.452 <50k <50k (0.227) (0.149) (1.302) (0.216) 1.335 1.166 1.341 1.189 50-100k 50-100k (0.151) (0.244) (0.338) (0.062) Injury Injury 1.491 1.261 1.395 1.132 >100k >100k (0.116) (0.224) (0.318) (0.057) 1.423 1.455 1.414 1.174 Overall Overall (0.085) (0.112) (0.229) (0.042) 1.630 1.899 1.359 1.371 <50k <50k (0.188) (0.126) (0.680) (0.147) 1.712 1.338 1.227 1.410 50-100k 50-100k (0.137) (0.213) (0.253) (0.056) PDO PDO 1.798 1.870 2.037 1.388 >100k >100k (0.103) (0.199) (0.293) (0.044) 1.748 1.808 1.666 1.398 Overall Overall (0.076) (0.096) (0.191) (0.034) 1.527 1.770 1.642 1.386 <50k <50k (0.147) (0.096) (0.622) (0.121) 1.569 1.262 1.285 1.323 All Crash 50-100k All Crash 50-100k (0.103) (0.161) (0.205) (0.042) Types Types 1.649 1.645 1.797 1.299 Combined >100k Combined >100k (0.076) (0.150) (0.215) (0.035) 1.609 1.663 1.577 1.314 Overall Overall (0.057) (0.073) (0.148) (0.027) S.E. = Standard Error Indices in italics are not significantly different than 1. Indices in italics are not significantly different than 1. than for the injury crashes, indicating that the additional for the daytime period (indicating a 39.8 percent increase). crashes that do occur while the work zone is present tend to Finally, total crashes during this condition increased 57.7 per- be less severe in nature. This trend exists regardless of cent during daytime periods and 31.4 percent during night- whether the work is performed during the day or at night time periods. and is consistent with previous studies that indicated that Interestingly, the only category in which statistically sig- crash rates may increase in work zones but that crash sever- nificant differences were found between nighttime and day- ity often decreases. time conditions was when the work area was inactive and no The magnitude of the change indices when work activity temporary lane closures were present. In Table 15, the differ- was occurring but no travel lanes were closed was a rather ences in nighttime and daytime injury and PDO crash ratios surprising finding from this analysis, especially for the night- across the AADT regions are not statistically significant. time period. These indices and those from when work was Overall, a slightly higher increase in injury crashes is seen for occurring and lanes were closed are compared directly in nighttime conditions (11.4 percent increase) than for daytime Table 16. Theoretically, the lack of temporary lane closures conditions (2.0 percent increase), but these are not statisti- when work is occurring means that motorists do not have an cally different from each other. For PDO crashes, the average obstacle (the lane closure) in their travel path that requires a increase at night (33.0 percent) was actually significantly driving reaction, and they do not have to deal with signifi- higher than during the day (19.6 percent). For total crashes, cant reductions in speed because of traffic congestion up- the average increases were 23.7 percent and 12.7 percent dur- stream of the closure. This would imply that the increase in ing nighttime and daytime periods, respectively. The greater crash risk when the work zone is active but lane closures are increases at night presumably reflect degraded geometric con- not present should be lower than when work activity is occur- ditions in the work zone (relative to a no-work zone condition) ring and temporary lane closures are present. Although the that--coupled with nighttime-specific issues such as limited crash ratios for daytime operations with and without tem- visibility, less attentive drivers, and so forth--raise nighttime porary lane closures are consistent with this hypothesis, the crash risk more substantially than during inactive times in results of this analysis indicate that working at night outside daytime periods. the travel lanes may have more substantial impacts on It is important to note that the indices of change in each motorist safety than was known previously. Unfortunately, work condition category are higher for the PDO crashes it is not clear from the data whether the increase in nighttime

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24 Table 15. Index of change for injury and PDO Comparison of Daytime and Nighttime Work crashes by AADT range during periods of no work Based on Increased Crash Costs Associated activity and no temporary lane closures. with Work Zone AADT Index of Change (S.E.) Figures 3 through 5 and Tables 13 through 16 provide es- Crash Level Range Nighttime Daytime 1.054 1.106 timates of the increased crash risk resulting from the presence <50k (0.087) (0.061) of a work zone under each of the different work period cate- 1.141 0.936 gories examined. The ratios (reflecting a percentage increase 50-100k (0.071) (0.038) Injury in actual crashes from what would have been expected had 1.106 1.051 >100k (0.063) (0.030) the work zone not been present) identify how the crash risk 1.114 1.020 Overall (0.042) (0.022) of individual drivers encountering these work zones is 1.133 1.271 affected. As noted above, drivers approaching a work opera- <50k (0.068) (0.050) tion at night where travel lanes are closed have a 42.3 percent 1.309 1.102 50-100k (0.067) (0.033) greater risk (on average) of being in an injury crash and PDO >100k 1.455 1.234 a 74.8 percent greater risk of being in a PDO crash than they (0.059) (0.025) would if the work zone were not there. Similarly, those same 1.330 1.196 Overall drivers traveling through that location at night when work is (0.039) (0.018) <50k 1.094 1.208 not occurring and no temporary lane closures are present have (0.054) (0.039) 1.240 1.042 an 11.4 percent greater risk of being in an injury crash and All Crash 50-100k a 33.0 percent greater risk of being in a PDO crash. In both (0.051) (0.025) Types 1.303 1.159 instances, the increase in risk to individual drivers does not Combined >100k (0.043) (0.019) 1.237 1.127 appear to depend upon the amount of traffic that the roadway Overall (0.029) (0.014) handles on a daily basis. From the perspective of the practi- Indices in italics are not significantly different than 1. tioner who has to decide whether or not to work at night, though, the issue is not simply of the effects upon individual crash risk during these work conditions is the result of the drivers, but on the entire driving population as a whole. Specif- following factors: ically, the question is whether completing a particular project or project task at night results in more or less additional crash Work area lighting glare that work crews do not mitigate consequences to motorists in total than doing the same project well when they are not located in travel lanes; or task during the day. Given that the severity of crashes nor- More frequent construction equipment and material de- mally differs between nighttime and daytime conditions, cal- liveries into and out of the work area at night that create culation of the effects of a standardized project task duration large speed differentials and subsequent crashes; or and length under a nighttime and daytime work scenario at a Other differences between daytime and nighttime work ac- given location is the appropriate basis of comparison. tivity behaviors when travel lanes are not closed, such as Theoretically, computation of the additional crash costs higher speeds, reduced driver expectancy of encountering expected for a particular project task duration and length a work zone, and more impaired and drowsy drivers. could be accomplished uniquely for any project location, as long as the analyst had the following data: Table 16. Index of change comparisons with AADT of the roadway segment and how that AADT is dis- and without temporary lane closures during tributed between the nighttime and daytime work periods periods of work activity. of interest, Crash AADT Index of Change (S.E.) Estimated duration of the project task to be completed, Severity Range Nighttime Daytime Length of the work zone or work area, and With lane 1.423 1.455 Normal or typical crash rates for the nighttime and daytime closures (0.085) (0.112) Injury Without lane 1.414 1.174 periods being analyzed (or models that allow the analyst to closures (0.229) (0.042) estimate the number of crashes normally expected on the With lane 1.748 1.808 closures (0.076) (0.096) facility). PDO Without lane 1.666 1.398 closures (0.191) (0.034) If the more traditional (but less accurate) crash rate per With lane 1.609 1.663 mvm of the roadway segment is available, the analyst esti- All Crash closures (0.057) (0.073) Types mates the total vehicular exposure that would be experienced Without lane 1.577 1.314 Combined closures (0.148) (0.027) if the work were done during the day (number of days of

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25 work required multiplied by the amount of traffic passing require travel lanes to be temporarily closed results in lower through the work area each day period multiplied by the length crash costs than the same work performed during the day over of the work zone) versus what it would be if the work were the entire range of AADT levels shown. On higher AADT performed at night. The appropriate percentage increases in roadways, there is actually a very sizeable overall economic crashes from Table 13 through Table 15 are then applied to benefit to the motoring public of doing this work at night the crash rate and multiplied by the estimated vehicle exposure from a safety standpoint; on lower AADT roadways, the ben- for each period to estimate the additional crashes anticipated efit may not be particularly large but still exists. For example, to occur because of the work task during that time period. the reduction in crash costs for 100 hours of work per mile of Finally, multiplying these crashes by the appropriate unit work zone at night versus doing the work during the day crash costs and summing across all severity levels (if severe exceeds $40,000 at a roadway AADT of 250,000 vpd. and PDO crashes are estimated separately) would allow an The differences between working at night versus working equivalent comparison of increased costs between the two during the day on a project task that does not require tempo- time periods. rary lane closures are less clear. Using the same California To illustrate the differences in daytime and nighttime work SPFs as before, researchers applied the appropriate percentage activity crash costs, SPF models for six-lane urban freeways in crash rate increases for this condition from Table 14 to estimate California (see Appendix A) were used to demonstrate how the total increased crash costs on a per 100 work hours per crash costs would be expected to increase for daytime and mile of work zone basis. Figure 7 provides the results of that nighttime work zones as a function of the AADT level of the analysis. Overall, the increased crash costs per 100 hours of roadway segment. Similar trends would be obtained if the work activity per mile at night are very close to what they were SPF models from the other states were used, although the ab- in Figure 6. However, the increased crash costs for this par- solute numbers would be different. The SPF model for free- ticular work condition are much lower for the daytime con- way segments within large interchanges (where crashes tend dition than they were in Figure 6. Ultimately, there is little or to be somewhat higher) was averaged with those segments no benefit for working at night when a lane closure is not between interchange areas. A comparison of the estimated in- present. While there is a small advantage for day work at creased crash costs for a project task that requires a temporary lower AADTs and a slight advantage for night work at higher lane closure to be used when work activity is occurring is pro- AADTs, these differences are too small to significantly impact vided in Figure 6. The values are computed assuming that a a decision of whether or not to work at night. project task requires 100 work hours to be completed regard- Finally, Figure 8 presents the estimate of increased crash less of whether it is done at night or during the day. The data costs during the day and at night when the work zone is in- are also normalized to a per-work-zone-mile basis. active and no temporary lane closures are required. For this Computed in terms of additional crash costs, it is apparent particular case, the increased crash costs at night are slightly from Figure 6 that working at night when work activities higher than during the day across the entire range of AADTs $70,000 Increased Crash Costs per 100 Work $60,000 Hours per Mile of Work Zone $50,000 $40,000+ $40,000 $30,000 $20,000 $10,000 $0 0 50000 100000 150000 200000 250000 Freeway AADT Daytime Nighttime Figure 6. Increased crash costs with active work and lane closure present.