Intersection and intersection related crashes in multilane non-limited access roads

As in section 4.1.2.2, the crash involvements were stratified for additional CMH statistic analysis by the number of lanes, urban/rural area and speed limit (high / low). Intersection crashes were not classified as being on a curve/straight section, due to the inadequacy of this general description to a point location such as an intersection. Moreover, the traffic way character variable lost about 80% of its previous measure of association value, which confirms that it is no longer an important variable for this particular crash type. In addition, traffic volume has been proven to be an important factor to predict crash severity in intersections, as previously discussed. This variable was not available in the original crash records databases, but was derived from additional databases and will be incorporated into the analysis Section 4.2. The result of the general association analysis for multilane road character combinations is presented next.

The characteristics of intersections and its influence area in arterial corridors are more complex and require more information to describe the driver injury severity trends. In the case of 4 or more lane road intersections in general more of the variables tested yielded significant interaction effect (association of two other variables when controlling for the variable being tested) This is probably due to more complex driving situations, as previously discussed. The most notable difference can be appreciated in Table 4-13 below, where the rural high speed road intersection case shows a predominance of the driver related variables interacting with the stronger variables (which are also mostly driver related). These results suggest that driver behavior at these intersections may be of greater concern relative to other cases.

Table 4-13 - Variables with significant association between injury severity and characteristics in roads with 4 or more lanes (intersection and intersection related crashes).

4 or more Lanes Straight Sections

Rural Urban

*- Denotes marginally significant general association (0.05≤p-value≤0.20)

In addition, some important differences between the intersection and non-intersection crashes for the high-speed roads can be seen. The format in Table 4-14 below clearly shows the contrasts between the different road entity characteristics. For crash involvements in rural areas,

the non-intersection straight sections had more statistically significant variables than for the intersections crashes, contrary to the urban areas. The most commonly significant variables were driver age group, lighting condition and time group. These variables were proved important in the injury severity models, including some interactions. The matrix also shows the importance of driver characteristics for intersections crashes; gender and driver fault are associated for non-intersection crashes.

In regards to roadway characteristics, location type (land use) and type of shoulder are associated with driver injury severity at intersections; while undivided highway is significant for rural non-intersection crashes. The type of shoulder could be an indirect effect of the intersection width (size), which will be discussed in Section 4.2. The higher operating speeds and design characteristics of rural segments certainly have an impact on the association between injury severity and highway divider. Road surface condition and weather are usually correlated and its use in the regression analysis was carefully tested to avoid multicollinearity. The day of week variable may correspond to trip purpose changes (weekday and weekend), and affect only straight sections in the rural areas.

Straight segments are more prone to higher operating speeds, increasing the risk of severe crashes. Urban intersections have more complex relationships with injury severity, judging from the amount of variables with significant association. Another clue is that urban intersections are the only locations where a crash related variable was found significant. This significance of the urban intersections was presumed when the research method was planned by including separate analysis by crash type, which is presented in Chapter 5. It is important to know what differences exists between intersection and non-intersection crashes that affect the driver injury severity.

Table 4-14 - Variables with significant association between injury severity and characteristics in roads with 4 or more lanes (intersection and intersection related crashes).

4 or more Lanes Straight Sections

Rural Urban

Important differences between rural and urban crashes can also be inferred. Urban intersection environments have more possible factors associated with driver injury severity. In particular, the resident variable is exclusive of these intersections, suggesting that the degree of complexity or uniqueness in design is a possible reason for this significant association with the driver injury severity. On the other hand rural straight sections have a significant association between driver age and injury severity, but not the urban sections. Meanwhile lighting conditions did not show a significant association with injury severity in rural intersections. Other driver behavior, such as speeding and alcohol use seem to dominate the associations. Rural intersections did not show significant associations between environment-related variables and driver injury severity, but land use and type of shoulder were significant, perhaps differentiating between suburban and rural areas. However, rural segments have significant associations

between the environment-related variables and driver injury severity. This points to the importance of weather conditions (related to visibility) in rural areas, similar to the freeway case.

Table 4-15 – Multilane road intersection crash driver involvements by injury severity (intersection crashes) 4 or more Lanes Roads

Evident Injury 473 2903 1473 2742 7591 14.39%

4 Incapacitating

Injury 216 1207 457 938 2818 5.34%

5 Fatal Injury 25 113 34 48 220 0.42%

Totals 3070 17883 11863 19927

Percent 5.82% 33.91% 22.49% 37.78% 52743 100.00%

The second important comparison between intersections and non-intersection crashes is the steady increase of the ratios of driver involvements between rural and urban high speed intersections as the injury level increases. The driver involvements by injury severity for intersection crashes are shown in Table 4-15 above. From the information presented, the rural to urban ratio for non-injury crash involvement is 0.78, but for incapacitating injury is 1.29 and fatal injury crash involvement it increases to 2.35. Another important observation for the high speed crashes is that while the total number of crash involvements (37,810) is larger than those occurring at straight sections, the number of fatalities is lower for the intersection crashes, the intersection (and intersection related) crashes resulted in almost double the number of incapacitating injuries. Special attention was given to these situations in the next stage of the investigation.