Relating Wildlife Crashes to Road Reconstruction
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6. Summary and Conclusions

The following chapter relates the conclusions reached as a result of this research effort. The first section provides a brief description of the reconstruction site candidate selection, as well as the projects that were ultimately selected. The second part of this chapter gives the general trends in crash rate observed for study sections. The third section gives the roadway design variables studied for significance, as well as the overall fit of an estimated model. The fourth part is an analysis of what can be said about the individual reconstruction sections. Finally, two recommendations for further study are proposed.

6.1 Project Selection

The first aspect of this research effort was the selection of candidates for study. ArcGIS proved to be a powerful tool in locating sections of roadway that experience either a high frequency or rate of reported wild animal crashes. Statewide maps showing the animal-vehicle crash rates and crash frequencies were created using this tool. With the aid of the State Transportation Improvement Programs (STIPs) created over the past decade, it was a straight-forward process to identify seven recent reconstruction projects on segments that experience a higher than normal number of wildlife crashes. The seven reconstruction projects were as follows:

  1. WY 130 Centennial East Section – between Centennial and Laramie from milepost 21.32 to 27.431. Reconstruction was started in November of 1996.
  2. US 14/16/20 Hanging Rock Section – between Yellowstone National Park and Cody from milepost 19.4 to 27.6. Reconstruction was started in June of 1998.
  3. US 189 Round Mountain Section – between Kemmerer and LaBarge from milepost 45.78 to 59.02. Reconstruction was started in April of 1999.
  4. US 14/16 Clearmont North Section – between Sheridan and Gillette from milepost 38.61 to 45.96. Reconstruction was started in November of 1999.
  5. WY 34 Morton Pass Section – between Bosler Junction and Wheatland from milepost 9.69 to 16.53. Reconstruction was started in March of 2001.
  6. US 89 Astoria Section – between Alpine Junction and Jackson from milepost 136.65 to 140.69. Reconstruction was started in March of 2000.
  7. US 26/85 Torrington West Section – between Torrington and Lingle from milepost 94.60 to 102.93. Construction was started in October of 1997.

6.2 Overall Trends

Using an analysis that compared the changes in crash rates for each of the seven sections, several trends were identified as to the changes in risk following the reconstruction.

6.2.1 Wildlife Crash Rate

The crash rate involving the animal-vehicle crashes is observed to increase. An increase in the animal-vehicle crash rate is observed in all seven of the reconstruction projects studied. The level of confidence for this claim is in excess of 97%. When changes in the size of the wildlife population are considered, the level of confidence is somewhat lower but still 96%.

6.2.2 Other Crash Rate

The crash rate for all crashes not involving wild animals (all crashes except animal-vehicle crashes) is observed to decrease. Five of the seven studied projects experienced this trend. The two that did not were the Hanging Rock section, located west of Cody, and the Torrington West Section, a roadway that was widened from two lanes to four lanes and connects Torrington to Lingle. The downward trend in the other crashes is confident to the 95% level.

6.2.3 Total Crash Rate

The overall crash rate (all crashes including animal-vehicle crashes) for the seven sections as a whole is observed to decrease. While this trend is observed on the whole, it is only seen on four of the seven sections. In addition to the two sections that experienced an increase in the non-wildlife related crash rate (see section 6.2.2), the Astoria Section south of Jackson can be included in this case. The confidence level that the overall crash rate decreased is 87%. The lower confidence in this statement may be due, in part, to the conflicting trends between the wildlife-related crash rate and the rate of all other crashes.

6.3 Roadway Attribute Analysis

An analysis was performed to gauge the effect of several roadway design variables. These included design speed, the design speed with shoulder and lane width speed reductions, lane width, shoulder width, and overall pavement width. The design speed for the reconstructed projects was taken from the construction documents, while for the previous sections it was estimated using the horizontal and vertical curvature of the roadway. An additional variable, animal density, was used to account for changes in animal population.

Through three different tests, the only variables deemed to have a statistically significant factor in the rate of animals being hit are animal population density and design speed. When the two variables are modeled using linear regression, the adjusted R2 value (a measure of actual data points fitting the model, as well as accounting for multiple variables) is 0.45. As the maximum value of this R2 value is 1.0, only 45% of the variation of the crash data are accounted for in these two variables. While more localized animal populations, as well as a more accurate measure of true driver speed, may account for more of the variation, it is likely that other variables are important to wild animal crash rates.

6.4 Individual Analysis of Sections

An attempt to quantify the changes in crash rates for each of the individual sections was made. This was performed using a variation of the Poisson distribution. The Poisson distribution is used in situations where the events, in this case, animal-vehicle crashes, occur independently. This assumption appears to be reasonable for this data.

Few of the study sections contain crash frequencies on their own high enough to state with confidence a noticeable trend. Concerning wild animal crash rates, only the Astoria section demonstrates a high probability (98.3%) that the crash rate increase is not due to chance. In the rate of all other crashes, the Morton Pass section (99.9%), the Clearmont North section (95.5%) and the Round Mountain section (approaching 100%) demonstrate high likelihoods the decrease in rate is not due to chance. Finally, only the Morton Pass section (99.6%) and the Round Mountain (approaching 100%) section show that the decrease in total crash rate is not due to chance.

6.5 Summary of Conclusions

6.6 Recommendations for Further Study

While this study indicates that design speed is the most important variable, only seven locations are considered as a part of this research effort. It is possible the additional conclusions could be drawn regarding the impact of roadway features with a larger sample size. Two directions have been identified that may help to further clarify the situation. The first would be a continuation of this research effort, increasing the number of reconstruction projects examined. The second would be to focus on the hypothesis that vehicle speed is the primary roadway factor contributing to the rate of wild animals being hit.

The most obvious direction to take in the continuation of this research effort would be to expand upon the data already collected. The addition of more reconstruction sections would allow for more certainty in the conclusions reached, and allow for the testing of further roadway attributes.

If this direction is taken, the selection of reconstruction sections might be better served by using wild animal crash frequency, rather than rate, as the primary selection factor. This may allow for more to be concluded about specific sections of highway. While the use of crash rate as the primary choice factor allowed for the sections selected in this effort to have a wide geographical distribution, the low volume nature of many Wyoming roads lowers the usefulness of these sections in a statistical sense. This is due to the fact that it may only take a change of only a few crashes to significantly raise or lower the rate of a specific section.

If the primary focus for further examination is deemed to be confirming the role that speed plays in the wild animal crash rate, a study should be developed that eliminates other variables from consideration. A possible direction in this fashion would be to look at changes in posted speed limit. As with design speed, posted speed limit does not directly quantify the actual speed of drivers, but may function as a suitable surrogate to gauge the effect of speed on the number of wild animal crashes.

While localized changes in speed limit are rare, one case presents itself as a possible opportunity to examine large portions of the roadway system in a reasonably controlled situation. Following the removal of a nationally mandated speed limit, Wyoming raised the speed limit of the majority of its rural highway system; rural Interstate Highway speed limits were raised from 65 mph to 75 mph, while most two-lane highway speed limits were raised from 55 mph to 65 mph. In the case of Wyoming, this occurred at a very specific time: December 8, 1995. For those sections that did not experience an increase in the posted speed limit, it may be possible to use these sections as a control of the study.

While this situation presents an opportunity to examine the effect of increased speed on the number of wild animal related crashes, several difficulties must be addressed to make this study practical in nature.

If the aforementioned challenges are addressed, this investigation may provide an opportunity to determine, with some degree of certainty, what the effects of speed on wild animal-vehicle crash rates are.

References


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