3. Results
There was a total of 1071 SVROR-injury crashes in the area studied between 2013 and 2016. Out of these, 555 SVROR-injury crashes involved a driver only. Out of the crashes involving a driver only, 71 (12.79 percent) resulted in fatalities. Forty-nine percent (271 cases) of these crashes occurred in urban areas, while 51.17 percent (284 cases) occurred in rural areas. Twelve percent of all urban-, and 13.73 percent of all rural-area crashes resulted in fatal injuries.
Table 2 shows the crash severity distribution by most harmful object struck. As can be seen, poles, trees, barriers, and curbs accounted for almost 80 percent (see cells in gray) of all most harmful objects struck. Barriers were found to be the most often most harmful object struck, accounting for 31.53 percent (see bold number) of all SVROR-injury crashes. This significant barrier involvement in SVROR crashes may stem from the fact that barriers are not only placed in front of other fixed roadside obstacles, but they also tend to be longer than the shielded hazards (i.e., trees and poles) [
33,
34]. Poles appeared to be the most severe object struck, as 16.22 percent (see underlined number) of the pole injury crashes resulted in fatalities. This compares to 15.58, 9.71, and 11.11 percent (see double-underlined numbers) for tree, barrier, and curb crashes, respectively. Sixty-one out of 188 (32.44 percent) tree/pole crashes involved side-impact collisions. Out of all side-impact collisions, 10 (16.39 percent) resulted in fatalities. Fifty-four out of the 77 tree crashes involved palm trees, which had a mean width of about 70 cm. Out of the barrier crashes, 111 involved WB guardrails, 51 involved concrete barriers, 10 involved plastic barriers, and 3 involved cable barriers. Due to the very small number of plastic and cable barrier crashes, these barriers were not considered in the statistical models developed in this study. Finally, 42 out of the 111 (37.84 percent) WB guardrail crashes, and 19 out of the 51 (37.25 percent) concrete barrier crashes resulted in rollovers. Six out of the 42 (14.28 percent) WB guardrail crashes and 5 out of 19 (26.31 percent) concrete barrier crashes involving rollover events resulted in fatalities.
Seventy percent of the curb crashes involved curbs higher than 15 cm, which is the curb height recommended by roadside design guidelines [
1]. In fact, 67 percent of all curb locations on lower-design-speed roads (i.e., ≤80 kph) had curbs higher than 15 cm while 81 percent of all curb locations on higher-design-speed roads (i.e., ≥100 kph) had curbs higher than 15 cm. Fifty-two percent of all curb crashes produced rollovers, while 7 out of the 8 fatal curb crashes involved rollovers. Lastly, 65 percent of all higher-curb crashes that occurred on higher-design-speed roads resulted in rollovers, while only 41 percent of all higher-curb crashes that occurred on lower-design-speed roads resulted in rollovers. Thus, traveling speeds may have played a role in an increased rollover occurrence in curb collisions. These are striking numbers, showing not only that curb height in the studied area is mostly non-compliant to roadside design guidelines [
1], but also that there may be a high rollover likelihood upon curb collisions.
Table 3 shows the results from the univariate analysis. As can be seen, driver fatality risk associated with crashes involving trees or poles is no different from that associated with crashes involving non-barrier objects such as curbs, fences, and walls (
p-values = 0.50 and 0.37), as well as concrete barrier crashes (
p-values = 0.77 and 0.68). On the other hand, the odds of a fatal injury occurring is 2.4 and 2.5 times higher for tree and pole crashes, as compared to WB guardrail crashes (
p-values = 0.07 and 0.04). The odds of a fatal injury occurring is 2.0 times higher for concrete barrier crashes as compared to WB guardrail crashes, though this finding was statistically significant only at the 19 percent significance level. However, there are a number of factors (e.g., posted speed limit, seat belt use, and vehicle class) that may significantly affect the odds of fatal injuries besides the fixed-object type. These factors were not controlled for in this univariate analysis. Thus, a multivariate model represented by Equation (2), and estimated based on the procedures described in the model building section, is needed.
Table 4 shows the results from the multivariate logistic regression analysis. All five variables shown in
Table 1 were initially inserted into the multivariate model, since all of them were found to be statistically significant to crash severity at the
p-value threshold of 0.25 discussed in the model building section. However, as all of the variables were considered simultaneously, some of them became insignificant, qualifying them to be excluded (through backward elimination) from further consideration. The first model (from top to bottom) shown in
Table 4 compares the odds of a driver fatality due to “Other hazards” (i.e., hazards other than barriers, trees and poles) crashes and the odds of a driver fatality due to tree/pole crashes. These hazards included fences, traffic signal poles, traffic signs, building walls, and curbs. In this case, the binary dependent variable is crash severity (i.e., fatal or not fatal), while the variables most harmful object struck, design speed, and seat belt usage were kept in the final, selected model. The variables vehicle class and rollover turned out to be insignificant, likely due to data sparseness. Because barrier cases were excluded, this particular model made use of 380 observations. Under the “# Non-Baseline Observations”, the number of tree and pole crashes, for example, were found to be 77 and 111, respectively. As a result, the total number of hazard crashes (i.e., excluding barrier, pole and tree crashes) was 192 (i.e., 380 minus 77, minus 111). Results from the first model shown in
Table 4 indicate that the odds of a fatal injury occurring due to a tree collision are comparable (i.e., 1.04) to those due to other hazard collisions (i.e., excluding barriers and poles), as well as that the odds of a fatal injury occurring are 1.46 times higher for pole collisions compared to other collisions (i.e., excluding barriers and trees), while controlling for design speed and seatbelt use. However, none of these findings were found to be statistically significant (i.e.,
p-values = 0.92 and 0.28, respectively). In sum, there is no statistically significant difference in fatal injury risk among non-barrier roadside hazards. Looking at the same model, it can also be concluded that the odds of a fatal injury occurring are 1.93 times higher (
p-value = 0.03) for crashes that occurred on roads with design speeds no higher than 80 km/h, as compared to crashes that occurred on roads with design speeds no lower than 100 km/h, and 2.21 times higher (
p-value = 0.01) for crashes that involved unbelted occupants compared to crashes that involved belted occupants. Lastly, the Hosmer-Lemeshow, goodness-of-fit test indicated that this model presented an acceptable fit based on a p-value of 0.475, which is significantly higher than the critical value of 0.05.
The other three multivariate models were built to investigate the in-service safety performance of WB guardrail and concrete barriers. Based on the model building process, these models ended up containing only two independent variables (i.e., most harmful object struck and design speed). The second model included in
Table 4 indicates that the odds of a fatal injury occurring are 3.1 and 4.7 times higher for tree and pole collisions, respectively, as compared to WB guardrail collisions (
p-values = 0.02 and 0.00). The third model indicates that while the odds of a fatal injury occurring are 1.1 and 1.9 times higher for tree and pole collisions, respectively, as compared to concrete barrier collisions, these findings were not found to be statistically significant (i.e.,
p-values = 0.81 and 0.23). Therefore, concrete barrier collisions did not appear to be any safer than tree or pole collisions, while controlling for design speed.
The last model shown in
Table 4 indicates that the odds of a fatal injury occurring are 2.5 times higher for concrete barrier collisions as compared to WB guardrail collisions, while controlling for design speed. This finding was found to be statistically significant at the 12 percent significance level only, likely due to sample size. That is, if seating position is not controlled for, such that crash severity relates to vehicle-occupant and not just driver injury, the total number of observations to be used in the last model shown in
Table 4 would be 316, rather than only 162. In this case (though not shown in
Table 4), the odds of a fatal injury occurring are 1.9 times higher for concrete barrier crashes as compared to WB guardrail crashes, while controlling for design speed. This finding was found to be statistically significant at the 8 percent significance level.
Another approach to assess the in-service safety performance of these roadside barriers (i.e., WB guardrails and concrete barriers) is to compare their vehicle containment performance. Roadside barriers should be designed to meet crash testing criteria [
35]. Thus, roadside barriers should safely contain and redirect errant vehicles, which means vehicles should remain upright during the containment/redirection event.
Table 5 shows the crash severity distribution by barrier type and vehicle containment outcome. Since controlling for seating position is irrelevant in assessing roadside barrier performance in terms of vehicle containment,
Table 5 also shows data from crashes involving vehicles with more than one occupant (see numbers in the right corner of each cell). As such, barrier performance can be assessed based on a larger sample. As can be seen, crashes involving lack of vehicle containment were associated with higher percentages of fatal outcomes. That is, the overall percentage of fatal crashes involving vehicle containment was 6.73 and 8.08 for the driver-only and all data, respectively. On the other hand, the overall percentage of fatal crashes involving a non-containment outcome was 13.79 and 22.88 for the driver-only and all data, respectively (see numbers in bold). Thus, these numbers may suggest that roadside barrier crash severity may be sensitive to vehicle containment outcome. In addition,
Table 5 also shows that 41.78 percent of all WB guardrail crashes and 26.37 percent of all concrete barriers involved non-containment outcomes (see underlined numbers). This may be due to the fact that concrete barriers are much more rigid than WB guardrails.
Data contained in
Table 5 may also be used to study the impact of vehicle containment outcome on crash severity, while controlling for barrier type. In this case, since crash severity is of concern, one may opt to rely on the driver-only data (shown in the left corner of each cell). As can be seen, out of the WB guardrail and concrete barrier crashes involving vehicle containment, 7.46 and 5.41 percent, respectively, resulted in fatal injuries. On the other hand, out of the WB guardrail and concrete barrier crashes involving non-containment, 6.82 and 35.71 percent, respectively, resulted in fatal injuries (see bold, underlined numbers). If the impact of vehicle containment outcome on crash severity is analyzed using the entire data (i.e., not controlling for seating position) instead for the sake of relying on a larger sample size, results are also striking. That is, out of the WB guardrail and concrete barrier crashes involving vehicle containment, 7.63 and 18.09 percent, respectively, resulted in fatal injuries. On the other hand, out of the WB guardrail and concrete barrier crashes involving non-containment, 8.96 and 41.67 percent, respectively, resulted in fatal injuries (see double-underlined numbers). Finally, overall, almost 90 percent of all non-containment events occurred on higher-design-speed roads (i.e., ≥100 kph). In summary, these numbers suggest that: (i) lack of vehicle containment (upon a barrier crash) tended to occur on higher-design-speed roads and resulted in a higher percentage of fatal crashes, (ii) lack of vehicle containment was more prevalent with the less rigid barrier type, and (iii) given that a vehicle had not been contained, a consistently higher percentage of fatal crashes was observed under the concrete barrier category.
Table 6 shows the results of an analysis intended to investigate the driver fatality risk between: (i) tree/pole/barrier crashes and no-fixed-object-hazard (except curbs) crashes and ii) tree/pole/barrier crashes and no-fixed-object-hazard (not even curbs) crashes. As can be seen, the odds of driver fatality occurring are 1.95 times higher (
p-value = 0.09) for tree/pole crashes as compared to curb-/collision-free crashes, while controlling for design speed and seatbelt use. If curb crashes are excluded, it can be seen that the odds of driver fatality occurring are 8.84 times higher (
p-value = 0.04) for tree/pole crashes as compared to collision-free crashes, while controlling for design speed. Likewise, the odds of driver fatality occurring are 2.44 times higher (
p-value = 0.08) for barrier crashes as compared to curb-/collision-free crashes, while controlling for design speed and rollover. If curb crashes are excluded, it can be seen that the odds of driver fatality occurring are 7.29 times higher (
p-value = 0.06) for barrier crashes as compared to collision-free crashes, while controlling for rollover events. Thus, fatalities were significantly more likely to occur when errant vehicles hit trees, poles, or barriers, as compared to vehicles that either hit curbs only or were involved in collision-free events. These results reinforce the importance of being compliant with design priorities defined by state-of-the-art roadside design guidelines [
1]. These guidelines recommend that fixed-object hazards should be (in order of preference): (i) removed, (ii) redesigned or relocated outside the minimum suggested clear zone distance, or (iii) made traversable. If, and only if, none of these design priorities can be implemented, shielding should be considered. Unfortunately, research has shown that roadside design compliance has been lacking [
7].
4. Discussions and Conclusions
The present study analyzed 555 SVROR-injury crashes that occurred in the Emirate of Abu Dhabi between 2013 and 2016. All crashes involved vehicles occupied by a driver only. Twelve percent of all crashes resulted in driver fatality. About half of the crashes occurred in urban areas and half in rural areas. Eleven percent of all urban-, and 14 percent of all rural-area crashes resulted in fatal injuries. Trees, poles, barriers, and curbs accounted for almost 80 percent of the most harmful object struck. Seventy percent of all tree crashes involved palm trees, which were found to be very robust fixed-objects with a mean width of 70 cm. Roadside barriers (i.e., WB guardrails and concrete barriers combined) were found to be the most often most harmful object struck, accounting for 31 percent of all crashes. Almost 10 percent of all roadside barrier crashes resulted in fatal injuries, which was found to be lower than nearly 15 and 16 percent of all tree and pole crashes. None of the barrier crashes resulted in secondary collisions involving other vehicles. About one-third of all tree/pole crashes involved side-impact collisions, while 16 percent of these collisions resulted in fatalities. These numbers may confirm the potential for reduction of fatalities in vehicle-guardrail collisions, considering that tree and pole locations may be treated by guardrail installation [
36].
Based on the multivariate logistic regression models developed, there was no statistically significant difference between the odds of driver fatality occurring among crashes involving roadside hazards (other than barriers), nor among those involving tree, pole, and concrete barrier crashes. Roadside fixed-objects considered included trees, poles, barriers, curbs, fences, building walls, traffic signal posts, and traffic signs. No traffic signs or poles were equipped with breakaway devices or any other form of energy-absorbing mechanism. On the other hand, the odds of driver fatality occurring were found to be 3.1, 4.7, and 2.5 times higher for tree, pole, and concrete barrier crashes, respectively, as compared to WB guardrail crashes. Therefore, these findings may suggest that road safety may be improved by having fixed roadside hazards shielded with WB guardrails instead of rigid barriers, wherever possible. When comparing the odds of driver fatality in pole and WB guardrail crashes to that of tree and WB guardrail crashes, differences in lateral offset distances between poles and trees may have contributed to the increased odds of driver fatality occurring among crashes involving poles, as compared to those involving trees. That is, about 50 percent of the trees were located farther than 5 m from the roadway edge, while only 30 percent of the poles were located farther than 5 m from the roadway edge. Furthermore, while only 6 percent of the trees were located within lateral offset distances shorter than 2 m from the roadway edge, one-quarter of the poles were located within that distance range. In addition, fatalities were significantly more likely to occur as errant vehicles hit trees, poles, or barriers, as compared to vehicles that either hit curbs only or were involved in collision-free events. These results reinforce the importance of being compliant with design priorities defined by state-of-the-art roadside design guidelines [
1]. These priorities, in order of preference, are: (i) remove obstacle, (ii) redesign or relocate obstacle, or iii) make obstacle traversable. If, and only if, none of these design priorities can be implemented, shielding should be considered.
Roadside barriers presented a less-than-desirable performance in terms of vehicle containment. That is, 41 and 26 percent (considering all data, and not just driver-only data) of all WB guardrail and concrete barrier crashes, respectively, involved a non-contained vehicle. In addition, 95 and 71 percent of all non-containment events involving WB guardrail and concrete barrier crashes, respectively, occurred on higher-speed-limit roads (i.e., ≥100 kph). These findings not only suggest that vehicle speeds may play a role on barrier containment performance, but also that barrier installations may need to be inspected in order to determine whether their design may be credited for meeting crash-testing criteria [
35], as well as whether they are installed and maintained in order to function as they were intended to. Barrier design, installation, and maintenance/repair may have an impact on barrier crash severity. Previous research showed that injury rates produced by guardrail installations may be lower if guardrail installations that may be obsolete, improperly installed, or inadequately maintained are accounted for. Previous research also indicated that injury rates produced by guardrail installations may be lower if guardrail installations submitted to crash conditions that may be outside the practical design range of modern guardrail systems are accounted for [
37]. However, more recent research showed that impact angles adopted in crash-testing criteria [
35] should be revised upwards if barrier installations are to properly accommodate real-world impacts [
38].
It is also important to point out the difference between the most harmful object struck and the most harmful event. A previous study showed that many injury-producing events, such as rollovers, occur after barrier impacts [
39]. Findings from the current study confirm findings from Viner’s study. That is, while barrier crashes accounted for 70, 87, and 93 percent of all first, second, and third crash events, respectively, rollovers accounted for 7, 36, and 39 percent of all first, second, and third crash events, respectively. However, while a crash may involve a sequence of events, the database used in the current research study does not identify the most harmful event. However, the most harmful object struck was identified by manually reviewing all crash descriptions and diagrams. For example, a crash may have involved a vehicle hitting a barrier and rolling over. In this case, the most harmful object struck was the barrier, but it was not obvious whether the rollover was the most harmful event. As a means to address this issue, the variable rollover was considered during the model building phase of the multivariate analysis, though rollover ended up being included in only two of the final models selected (see
Table 6). Nevertheless, data were segregated based on collision type preceding a rollover, and it was found that 72 percent of all rollovers were preceded by no impact, as well as by barrier and curb impacts. This suggests that: (i) causes of rollovers preceded by barrier collisions (i.e., 25 percent of all rollover events) may need to be investigated further, as barriers are expected to contain/redirect errant vehicles while keeping them upright, (ii) curb design may also need to be reviewed/inspected, as a significant portion (i.e., 28 percent) of all rollovers were preceded by curb collisions, fifty-two percent of all curb crashes produced rollovers, and 7 out of the 8 fatal curb crashes involved rollovers, and (iii) causes of rollovers in collision-free events (i.e., 19 percent) may need to be investigated further, since roadside terrain should keep errant vehicles on a stable trajectory.
Finally, it is also important to discuss the potential impact underreporting may have on the results of this study. Previous research showed that there may be a significant amount of barrier crashes that are not reported [
40,
41]. Thus, since underreporting is more recurrent among lower-severity crashes, and WB guardrail crashes were found to be less severe than tree, pole, and concrete barrier crashes, it may be reasonable to assume that underreporting may be more prevalent among WB guardrail crashes. If this is the case, then the real odds of driver fatality occurring in tree, pole, or concrete barrier crashes, versus WB guardrail crashes, should actually be higher than the estimated odds shown in
Table 4. Hence, the safety benefits of shielding, fixed, roadside obstacles with WB guardrails versus leaving these obstacles unprotected may be underestimated if the estimated odds presented in the current study are not adjusted for underreporting. Thus, the need for adjustment of underreporting is relevant, as the findings from this study may be helpful in the development of severity indices and in the evaluation of proposed roadside safety improvements [
22].