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26 $30,000 Increased Crash Costs per 100 Work Hours per Mile of Work Zone $25,000 $20,000 $15,000 $10,000 $5,000 $0 0 50000 100000 150000 200000 250000 Freeway AADT Daytime Nighttime Figure 7. Increased crash costs with active work and no lane closure present. examined. Of course, the increased crash costs are much lower whole by including differences in traffic volume during night across the entire range of AADTs for both daytime and night- and day periods. Another relevant question is whether the time conditions when compared to the previous figures when types of work zone crashes differ significantly between night- work is occurring (either with or without lane closures present). time and daytime periods. If so, such differences could lend insight into improved work zone safety policies, procedures, and practices that may produce an overall reduction in work Types of Crashes Occurring during zone crash risk. Nighttime and Daytime Work In this section, an analysis of the distribution of different The preceding section examined the differences in work zone crash types/manners of collision is presented. Specifically, crash risk and crash costs between daytime and nighttime crashes were subdivided into one of four collision types: work for comparable periods of work activity and inactivity with and without lane closures present. The analysis consid- Rear-end collisions, ered both individual drivers and the driving population as a Sideswipe collisions, $10,000 Work Zone Inactivity per Mile of Work Zone Increased Crash Costs per 100 Hours of $5,000 $0 0 50000 100000 150000 200000 250000 Freeway AADT Daytime Nighttime Figure 8. Increased crash costs with inactive work zone and no temporary lane closures present.

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27 Fixed-object (generally single-vehicle) collisions, and nearly consistent and is actually lowest when active work ac- other remaining crash types. tivity and lane closures are present. In other words, there does not appear to be a strong association between the capacity re- The results of the analysis of collision types are provided in ductions associated with lane closures and the likelihood of a the following section. rear-end crash. In fact, the percentage of rear-end crashes in the work zone is actually a little lower than in the before work zone condition at these sites. Rear-End Collisions The effects of work zone conditions on rear-end crashes As noted in the background section, several studies have were further examined by stratifying the crash data by AADT. indicated that rear-end crashes tend to be overrepresented in In Figure 9, rear-end crash percentages are provided by work zones. This was previously confirmed in the NYSDOT AADT level before the work zone was present and in the work crash data analysis in Chapter 2, especially when lane closures zone during periods of inactive work with no lane closures. are present in the work zone. Typically, it is hypothesized that For these conditions, rear-end crashes typically increase as a the overrepresentation of such crashes occurs because of in- function of AADT in both daytime and nighttime periods creased traffic congestion and queues associated with the re- and are lower at night than during the day across the entire duction in roadway capacity in the work zone. The HSIS data range of AADTs. Also, the difference between daytime and collected in this project allow for a more thorough investiga- nighttime rear-end crash percentages increases with AADT. tion of this hypothesis. Furthermore, for both day and night, the percentage of Table 17 presents the percentage of crashes that involved a rear-end crashes in the work zone is very similar to the before- rear-end collision by work condition and time of day across construction condition. At higher AADTs, a small increase the entire range of projects contained in the four-state (approximately 5 percent) in rear-end crashes is seen when dataset. The second column (active work with lane closures) the work zone is present. represents the greatest work zone capacity reduction and thus The rear-end crash percentages during periods of active the highest potential for congestion and queuing. It would be work with and without temporary lane closures are provided expected to have the highest percentage of rear-end crashes in Figure 10 and Figure 11 for daytime and nighttime, re- associated with it. The third column (active work with no spectively. During the day, a significant increase in rear-end lane closures) represents the next most significant capacity crashes is evident at AADT levels below 100,000 vpd when the reduction, due to driver rubber-necking, work vehicle inter- work zone is active regardless of whether or not a lane closure ference, and lesser geometric restrictions. The fourth column is present. At higher AADTs, however, rear-end crashes when (no work activity and no lane closures) would be expected to the work zone is active are about the same or even lower than produce the least capacity reduction and thus the lowest per- when work is inactive. This may be partly attributable to small centage of rear-end crashes. Finally, the percentage of rear- sample sizes at higher AADTs for the active work with lane end crashes across all project locations prior to the start of closure condition. work is presented in the fifth column as an indication of non- These results may indicate that there is an upper limit in work conditions for comparison purposes. terms of how much of the total crash experience at a location The above expectations are generally confirmed in Table 17 will be rear-end crashes. These high-volume locations may for night work crashes. Active night work with lane closures re- already experience so much congestion and stop-and-go sulted in 38.4 percent rear-end crashes, compared to 33.6 per- traffic (which lead to rear-end crashes) that further degradation cent during active work without lane closures and 26.0 percent in operating conditions associated with the work zone simply with no active work or lane closures. The percentage of rear- results in the same distribution of crash types that normally end crashes during periods of no active work at night was iden- exist on that facility. At lower AADT levels, rear-end colli- tical to the corresponding pre-construction percentage. sions do not normally comprise the majority of crashes that However, this trend is not exhibited by crashes during day- occur, and so the introduction of capacity reductions and time periods. Instead, the percentage of rear-end crashes is other turbulence on the roadway leads to more congestion Table 17. Percent of rear-end crashes. Time of Day Active Work Active Work No Active Work, No Work Zone with Lane without Lane No Lane Present Closures Closures Closures Daytime 46.9% 54.4% 48.7% 52.8% Periods Nighttime 38.4% 33.6% 26.0% 26.0% Periods

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28 80% Percent of Crashes That Involve 70% a Rear-End Collision 60% 50% 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 9. Comparison of roadway AADT to rear-end collision percentages, no work zone versus inactive work zone conditions. and unexpected traffic, resulting in a greater proportion of Rather, this close similarity in rear-end crashes between lane rear-end collisions. closure and no lane closure conditions appears to indicate Authors of past studies have commonly assumed that the that other work zone features and conditions also affect traf- increase in rear-end crashes in work zones is primarily asso- fic flow during periods of work activity. These features and ciated with unexpected congestion and traffic queues created conditions may include construction traffic entering or exit- by lane closures. As was shown in Figure 10, however, rear- ing the workspace, actions by workers or equipment near the end crash percentages during work activities without lane travel lanes that cause nearby motorists to brake unexpect- closures are almost identical to when a temporary lane edly, overall changes to roadway geometry, etc. These condi- closure is present. While it is possible that there are a few in- tions and work zone features appear to contribute as much to stances in which the project diaries failed to note a temporary the increased crash risk of the work zone as a temporary lane lane closure, resulting in incorrectly coding the work zone closure. as having no lane closure present, these instances are not The rear-end crash trends by AADT for nighttime work are believed to be frequent enough to explain the close agreement more consistent with expectations. Active work lane closures between the lane closure and no lane closure conditions. result in more rear-end crashes than during periods of inactive 80% Percent of Crashes That Involve a 70% Rear-End Collision 60% 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 10. Comparison of work activity and roadway AADT to rear-end collision percentages, daytime work periods.

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29 80% Percent of Crashes That Involve a 70% 60% Rear-End 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 11. Comparison of work activity and roadway AADT to rear-end collision percentages, nighttime work periods. work over the entire AADT range. Further, the percentage of Sideswipe Collisions rear-end crashes during active work but without a lane closure Sideswipe crashes are summarized in Table 18, which is also somewhat higher than during periods of work inactivity shows that daytime versus nighttime work, work activity, and over the same AADT range. Finally, rear-end crashes during lane closure presence did not dramatically influence the per- active work without lane closures are less than active work centage of sideswipe crashes, especially during daytime. Side- with lane closures, except at the lower range of AADT. Simi- swipe collisions comprised between 13 and 16 percent of lar to the discussion for daytime conditions, there appear to crashes at the project locations under all conditions except for be many sources of traffic disruptions during periods of work nighttime active work without lane closures. For that group, activity with and without temporary lane closures at night sideswipe crashes comprised 21 percent of the total. How- that contribute to an increase in rear-end crashes. While other ever, none of these differences are statistically significant. effects--such as construction vehicle and equipment access and egress, distractions due to workers or equipment near the travel lanes, and so forth--cannot be determined with the Fixed-Object Collisions current dataset, it is reasonable to expect that driver inatten- tion, which is believed to be a greater concern at night, also is Table 19 illustrates that fixed-object collisions consis- a factor. Overall, it seems reasonable to believe that capacity tently comprise a greater proportion of nighttime crashes reductions associated with lane closures contribute some to than daytime crashes. Also, fixed-object crashes were almost the increase in rear-end crashes in active nighttime work identical for the no work zone present (before) condition zones, but other factors may also reasonably be believed to and the inactive work zone condition both during the day contribute. All of these contributing factors should be con- and at night. Figure 12 presents fixed-object crashes com- sidered when work zone designers look for opportunities to pared to AADT, and it is apparent that fixed-object crashes improve work zone traffic safety. for both the before and inactive periods decrease markedly Table 18. Percent of total crashes that involve sideswipe 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 13.6% 14.8% 14.8% 14.1% Periods Nighttime 15.8% 21.0% 15.0% 13.3% Periods