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26 CHAPTER 4 Results 4.1 General comparisons, the differences were not statistically significant. Recall that IS was defined in Chapter 2 as: The following chapter presents an overview of the data set developed under the current study. A brief comparison of the IS = 1 2 M ( V sin ) content of the 17-22 and TTI data set is presented below. Descriptive statistics for the combined data set are then pre- where: sented followed by a detailed evaluation of the impact con- ditions and comparison of the current data and historical IS = Impact Severity studies. Encroachment lengths from the combined data M = Vehicle mass set are then compared to historical studies and implications V = Vehicle velocity of the new data on the calculation of appropriate guardrail = Impact angle length is discussed. Additional tables and plots describing the Table 14 and Figures 1 through 4 clearly illustrate that basic characteristics of the combined data set are presented in injury rates and departure conditions from the TTI and 17-22 Appendix E. data are sufficiently similar to allow the data to be combined into a single database. As discussed in the prior chapter, the 4.1.1 Comparison of 17-22 and TTI Data similarity between the two data sets for the vast majority of important data elements is sufficient to justify combining A summary of the efforts to compare the 17-22 and TTI them into a single database. Nevertheless, database users data sets was presented previously in Section 3.5. As shown in should be cognizant of the differences in rollover rates and Table 13, differences between the two data sets were found to vehicle classes when developing data queries. Additional com- be statistically insignificant for the vast majority of the impor- parisons between the 17-22 and TTI data sets are presented in tant variables. Vehicle weight, highway speed limit, rollover Appendix E. frequency, and vehicle class were exceptions to this finding. The modest changes observed in vehicle weight and roadway 4.2 Descriptive Statistics speed limits could be explained by changes in the vehicle fleet and elimination of the national speed limit law. Unfortu- When combined into a single data set, the 17-22 and TTI nately, the magnitude of the change in vehicle class and the data included a total of 877 cases. The following sections pro- rollover rates between the 17-22 and TTI data could not be vide a basic description of the combined data set. adequately explained. Most other important variables correlated very well between 4.2.1 Characteristics of Sampled Cases the two data sets. As shown in Table 14, injury and fatality rates for the two studies are virtually identical. Departure As shown in Table 15, rural highways make up approxi- speeds and angles are also very similar as shown in Figures 1 mately 72% of the accident cases with the remaining 28% of and 2. Vehicle heading angle distributions were also found to cases located in urban areas. Table 16 shows that the data set be very similar, as shown in Figure 3. Although the IS distri- includes a significant representation of cases on Interstate butions, shown in Figure 4, were not as similar as the other highways, US routes, state routes, and county roads. The

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27 Table 14. Injury severity by study. 17-22 Data TTI Data Total Data Injury Type No. Percentage No. Percentage No. Percentage Fatal 55 14.0% 74 15.3% 129 14.7% A-injury 228 58.2% 279 57.5% 507 57.8% B-injury 40 10.2% 49 10.1% 89 10.2% C-injury 33 8.4% 42 8.7% 75 8.6% PDO 36 9.2% 41 8.5% 77 8.8% Total 392 100.0% 485 100.0% 877 100.0% Figure 3. 17-22 and TTI heading angle distributions. Note: Many of the figures in this report have been converted from color to grayscale for printing. The electronic version of the report (posted on the web at www.trb.org) retains the color version as submitted by the contractor. Figure 1. 17-22 and TTI departure velocity distributions. Figure 2. 17-22 and TTI departure angle distributions. Figure 4. 17-22 and TTI IS distribution.

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28 Table 15. Case distribution limits ranging from 45 to 75 mph, as shown in Table 17. by land use. Table 18 presents this distribution of speed limit by high- way class. As expected, most of the data collected from No. of Cases Percentage high-speed facilities involved Interstate highways and the Urban 235 27.94% majority of cases involving low-speed facilities were col- lected on county roads. Tables 19 and 20 show the number Rural 606 72.06% of lanes at the accident site for divided and undivided high- Total 841 100.00% ways, respectively. Surprisingly, even though a large proportion of crashes Table 16. Highway classification. involved Interstates and US routes, very few cases involved vehicles departing from a portland cement pavement surface. Hwy Class No. of Cases Percentage As shown in Table 21, the vast majority of the cases, 773 Interstate 195 23.16% (88.1%), occurred on asphalt with only 45 (5.1%) involving US Route 160 19.00% portland cement concrete. State Route 161 19.12% As shown in Table 22, winter months were significantly County Road 275 32.66% underrepresented in the data. Only 132 (15.1%) crashes City Street 43 5.11% occurred during the winter months from December through Other 8 0.95% Total 842 100.00% February. The low proportion of crashes during the winter provided an explanation for the low numbers of crashes with ice, 28 (3.2%), or snow, 25 (2.9%), on the roadway surface, as Table 17. Speed limit. shown in Table 23. This table also shows that almost 80% of Cases all of the crashes in the data set occurred on dry roadways. Speed Limit These findings correlated with the weather conditions at the No. Percentage 75 58 6.7% time of the crash, shown in Table 24. More than 85% of the 70 114 13.1% crashes occurred in clear weather and less than 10% occurred 65 75 8.6% in the rain. 55 361 41.4% A total of 529 of the 877 cases were recorded as having 50 68 7.8% struck an object on the roadside. As shown in Table 25, more 45 195 22.4% 100.0% than 37% of these fixed-object crashes involved trees and Total 871 another 7% involved utility pole impacts. More than 18% of the fixed-object crashes involved longitudinal barrier impacts. Thus, approximately 62% of fixed-object crashes involved largest number of cases, 275 (32.7%), occurred on county impacts that would be expected to significantly reduce vehicle roads and 195 (23.2%) cases were on Interstate highways. The speed or redirect it back toward the roadway. The remaining number of cases on US and state routes are approximately 38% of crashes involved fixed objects that would be less likely the same at 160 (19.0%) and 161 (19.1%) cases, respectively. to significantly reduce the speed of the impacting vehicle (e.g. As would be expected for crashes collected from these high- embankments, ditches, curbs, breakaway sign and luminaire way types, the data set includes a wide distribution of speed supports, fences, mailboxes and culverts). Table 18. Highway class vs. speed limit. Speed Limit (mph) 75 70 65 55 50 45 Hwy Class No. % No. % No. % No. % No. % No. % Interstate 57 98.3 63 56.3 25 34.2 41 11.9 2 3.2 5 2.7 US Route 0 0.0 46 41.1 34 46.6 50 14.5 11 17.5 18 9.7 State Route 1 1.7 2 1.8 13 17.8 95 27.5 20 31.7 29 15.6 County Road 0 0.0 0 0.0 0 0.0 153 44.3 27 42.9 94 50.5 City Street 0 0.0 0 0.0 0 0.0 2 0.6 3 4.8 38 20.4 Other 0 0.0 1 0.9 1 1.4 4 1.2 0 0.0 2 1.1 Total 58 100.0 112 100.0 73 100.0 345 100.0 63 100.0 186 100.0

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29 Table 23. Distribution by surface condition. Percentage Surface Condition No. of Cases Table 19. Number of lanes-- of Total divided highways. Dry 695 79.2% Wet 121 13.8% Number of Lanes Ice 28 3.2% Hwy Class 1-2 3-4 More than 4 Snow 25 2.9% Interstate 67 (38.3%) 86 (49.1%) 22 (12.6%) Other 8 0.9% US Route 29 (30.9%) 56 (59.6%) 9 (9.6%) Total 877 100.0% State Route 17 (37.8%) 25 (55.6%) 3 (6.7%) County Road 1 (33.3%) 2 (66.7%) 0 (0.0%) City Street 7 (58.3%) 5 (41.7%) 0 (0.0%) Table 24. Weather condition. Other 0 (0.0%) 1 (100.0%) 0 (0.0%) Weather Condition No. of Cases Percentage Clear 750 85.81% Table 20. Number of lanes-- 9.38% Rain 82 undivided highways. Snow 30 3.43% Fog 6 0.69% Number of Lanes Hail 3 0.34% Hwy Class 1-2 3-4 More than 4 0.23% Sleet 2 Interstate 18 (94.7%) 1 (5.3%) 0 (0.0%) Sandstorm 1 0.11% US Route 54 (81.8%) 7 (10.6%) 5 (7.6%) Total 874 100.00% State Route 101 (87.8%) 11 (9.6%) 3 (2.6%) County Road 237 (98.8%) 3 (1.3%) 0 (0.0%) City Street 20 (64.5%) 7 (22.6%) 4 (12.9%) Other 6 (100.0%) 0 (0.0%) 0 (0.0%) Table 26 presents the distribution of vehicle classes included in the data set. Almost 58% of vehicles included in the data set were classified as "car." Further, another 28% of vehicles Table 21. Distribution by roadway material. fell into the compact light truck class including compact pick- Percentage ups, compact utility vehicles, and minivans. Only 13% of Roadway Surface No. of Cases of Total vehicles included in the database were full-size pickups, util- Asphalt 773 88.1% ity vehicles, or vans. Portland Cement 45 5.1% Dirt 31 3.5% Gravel 28 3.2% Total 877 100.0% 4.2.2 Crash Severity As expected, the data set is biased toward higher severity crashes. As shown in Table 27, roughly 15% of the cases Table 22. Case distribution by month. Number of Percentage Month Occurrences Table 25. First impact. January 37 4.2% February 50 5.7% March 102 11.6% Object/Feature Struck No. Percentage April 87 9.9% Tree 197 37.2% May 82 9.4% Guardrail 71 13.4% June 101 11.5% Embankment 65 12.3% July 83 9.5% Sign and Luminaire Support 39 7.4% August 86 9.8% September 76 8.7% Utility Pole 37 7.0% October 71 8.1% Culvert 30 5.7% November 57 6.5% Concrete Barrier 25 4.7% December 45 5.1% Ditch 24 4.5% Total 877 100.0% Mailbox 18 3.4% Fence 13 2.5% Curb 10 1.9% Total 529 100.0%

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30 Table 26. Vehicle class. Percentage by Percentage of Vehicle Class No. of Cases Veh. Subclass Total Subcompact Car 145 28.7% 16.5% Compact 167 33.0% 19.0% Intermediate 117 23.1% 13.4% Car Full-Size Sedan 55 10.9% 6.3% Large Size 22 4.3% 2.5% Subtotal 506 100.0% 57.7% Compact Pickup 99 52.4% 11.3% Large Pickup 87 46.0% 9.9% Pickup Truck Other Pickup Type 3 1.6% 0.3% Subtotal 189 100.0% 21.5% Compact Utility 120 83.9% 13.7% Utility Large Utility 15 10.5% 1.7% Vehicle Stationwagon Utility 8 5.6% 0.9% Subtotal 143 100.0% 16.4% Minivan 27 69.2% 3.1% Large Van 10 25.6% 1.1% Van Full-Size Van 2 5.2% 0.2% Subtotal 39 100.0% 4.4% Total 877 100.0% involved a fatality (denoted "K") and approximately 73% of with a minimum of 69% for county roads and a high of 75% all cases involved either an A-injury or a fatality (A+K). A for US routes. recent study of single-vehicle crashes on controlled-access This same bias toward higher severity crashes is also freeways in Kansas found a fatality rate of only 0.73% and an evident in Tables 28 through 31. Table 28 presents the rela- A+K rate of only 3.8% (32). From the data in Table 27, the tionship between specific vehicle class and crash severity. fatality rate for Interstate highways in the data set was 17.9% There appears to be no consistent trend between vehicle and the A+K rate was 73.8%. These fatality and A+K rates size and crash severity. Table 29 condenses this information were 25 and 19 times higher, respectively, than the values for to produce crash severity by overall vehicle type. Again controlled-access freeways in Kansas. This degree of bias is there appears to be only modest differences in crash sever- associated with the original case-selection criteria used to ity as a function of overall vehicle type. Tables 30 and 31 identify the NASS CDS cases and therefore cannot be avoided. also illustrate that the severity bias masks the effects of This inherent bias toward increased severity may be masking rollover and the object struck on crash severity, respec- the relationship between highway functional class and crash tively. For example, fatality rates for tree and guardrail severity for this database. As shown in Table 27, the A+K rates impacts are found to be very similar at 13.2% and 12.7% for all highway functional classes is approximately the same respectively. Thus, Tables 27 through 31 clearly illustrate Table 27. Highway class vs. crash severity. Maximum Severity Fatality Injury Type A Injury Type B Injury Type C PDO Hwy Class No. % No. % No. % No. % No. % Interstate 35 17.9% 109 55.9% 15 7.7% 20 10.3% 16 8.2% US Route 19 11.9% 102 63.8% 11 6.9% 15 9.4% 13 8.1% State Route 26 16.1% 93 57.8% 18 11.2% 11 6.8% 13 8.1% County Road 40 14.5% 150 54.5% 36 13.1% 23 8.4% 26 9.5% City Street 7 16.3% 30 69.8% 3 7.0% 1 2.3% 2 4.7% Other 0 0.0% 4 50.0% 1 12.5% 3 37.5% 0 0.0% All 127 15.1% 488 58.0% 84 10.0% 73 8.7% 70 8.3%

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31 Table 28. Crash severity by vehicle class. Maximum Injury (%) No. of Cases Vehicle Class Fatal A-injury B-Injury C-Injury PDO Subcompact 145 13.1 53.8 11.0 12.4 9.7 Compact Car 167 16.2 52.1 10.8 10.2 10.8 Car Intermediate 117 13.7 63.2 9.4 6.0 7.7 Full-Size Sedan 55 14.5 54.5 12.7 10.9 7.3 Large Size 22 4.5 68.2 13.6 0.0 13.6 Compact Pickup 99 19.2 57.6 7.1 8.1 8.1 Pickup Truck Large Pickup 87 10.3 64.4 5.7 10.3 9.2 Other Pickup Type 3 33.3 33.3 0.0 0.0 33.3 Compact Utility 120 14.2 59.2 14.2 5.0 7.5 Utility Vehicle Large Utility 8 0.0 75.0 12.5 0.0 12.5 Stationwagon Utility 15 13.3 60.0 6.7 13.3 6.7 Minivan 27 22.2 63.0 7.4 3.7 3.7 Van Large Van 2 50.0 50.0 0.0 0.0 0.0 Full-Size Van 10 30.0 50.0 10.0 10.0 0.0 Table 29. Crash severity by vehicle type. No. of Maximum Injury (%) Vehicle Type Cases Fatal A-injury B-Injury C-Injury PDO Automobile 506 14.0% 56.1% 10.9% 9.5% 9.5% Pickup 189 15.3% 60.3% 6.3% 9.0% 9.0% Utility 143 13.3% 60.1% 13.3% 5.6% 7.7% Van 39 25.6% 59.0% 7.7% 5.1% 2.6% Table 30. Rollover and crash severity. Percentage by Percentage Maximum Injury No. of Cases Roll Result of Total Fatality 79 16.7% 9.0% A-injury 274 57.9% 31.2% B-injury 48 10.1% 5.5% Rollover C-injury 40 8.5% 4.6% PDO 32 6.8% 3.6% Subtotal 473 100.0% 53.9% Fatality 50 12.4% 5.7% A-injury 233 57.7% 26.6% No B-injury 41 10.1% 4.7% Rollover C-injury 35 8.7% 4.0% PDO 45 11.1% 5.1% Subtotal 404 100.0% 46.1% Total 877 100.0% Table 31. First impact vs. crash severity. Object/Feature No. of Fatal A-Injury B-Injury C-Injury PDO Struck Cases No. % No. % No. % No. % No. % Tree 197 26 13.2% 127 64.5% 16 8.1% 14 7.1% 14 7.1% Guardrail 71 9 12.7% 36 50.7% 7 9.9% 6 8.5% 13 18.3% Embankment 58 6 10.3% 34 58.6% 9 15.5% 4 6.9% 14 24.1% Vertical Support 37 6 16.2% 19 51.4% 6 16.2% 1 2.7% 5 13.5% Utility Pole 37 9 24.3% 17 45.9% 7 18.9% 3 8.1% 1 2.7% Concrete Barrier 27 5 18.5% 13 48.1% 2 7.4% 4 14.8% 3 11.1% Culvert 27 3 11.1% 20 74.1% 1 3.7% 2 7.4% 1 3.7% Ditch 25 2 8.0% 15 60.0% 2 8.0% 5 20.0% 1 4.0% Mailbox 18 2 11.1% 10 55.6% 2 11.1% 2 11.1% 2 11.1% Fence 13 2 15.4% 8 61.5% 0 0.0% 3 23.1% 0 0.0% Curb 10 0 0.0% 7 70.0% 1 10.0% 1 10.0% 1 10.0% Total 520 70 13.5% 306 58.8% 53 10.2% 45 8.7% 55 10.6%