The impact of traffic barrier geometric features on crash frequency and injury severity of non-interstate highways.

INTRODUCTION: The main objective of this research is to investigate the effect of traffic barrier geometric characteristics on crashes that occurred on non-interstate roads. METHOD: For this purpose, height, side-slope rate, post-spacing, and lateral offset of about 137 miles of traffic barriers were collected on non-interstate (state, federal aid primary, federal aid secondary, and federal aid urban) highways in Wyoming. In addition, crash reports recorded between 2008 and 2017 were added to the traffic barrier dataset. The safety performance of traffic barriers with regards to their geometric features was analyzed in terms of crash frequency and crash severity using random-parameters negative binomial, and random-parameters ordered logit models, respectively. RESULTS: From the results, box beam barriers with a height of 27-29 inches were less likely to be associated with injury and fatal injury crashes compared to other barrier types. On the other hand, the likelihood of a severe injury crash was found to be higher for box beam barriers with a height taller than 31 inches. Both W-beam and box beam barriers with a post-spacing between 6.1 and 6.3 inches reduced the probability of severe injury crashes. In terms of the crash frequency, flare traffic barriers had a lower crash frequency compared to parallel traffic barriers. Non-interstate roads without longitudinal rumble strips were associated with a higher rate of traffic barrier crashes.

Application of Quantile Mixed Model for modeling Traffic Barrier Crash Cost.

Run-off the road crashes account for a significant proportion of severe injuries to vehicle occupants. Traffic barriers have been installed with an objective to keep vehicles on the roadway, and prevent them from hitting natural obstacles like trees or boulders. However, still injuries and fatalities of barrier crashes account for high proportion of fatalities on roadway. Due to challenging geometrics characteristics of Wyoming's roadway, a high mileage of barriers has been installed in the state. The high mileages of barriers result in a high number of barrier crashes in terms of crash frequency and severity due to high exposure. Previous studies mainly focused on crash frequency or individual crash severity. However, it has been recognized the importance of accounting for both aspects of crash severity, and crash frequency. So, in this study, crashes are aggregated across different barriers, and those crashes were converted into costs by considering the impacts of both crash severity and frequency. However, one of the main challenges of this type of dataset is highly skewness of crash data due to its sparseness nature. An improper use of model distribution of crash cost would result in biased estimations of the covariates, and erroneous results. Thus, in order to address this issue, a semi-parametric method of quantile regression technique was implemented to account for the skewness of the response by relaxing model distribution parameters. Also, to account for the heterogeneity in the dataset due to barriers' types, a random intercept model accounting for the structure of the data was implemented. In addition, interaction terms between significant predictors were considered. Understanding what factors with which magnitude contribute to the barrier crash costs is crucial for the future barriers' optimization process. Thus, contributory factors to barriers crash cost with high, medium, and low values, corresponding to 95th, 70th, and 60th percentiles were considered, and a comparison was made across these models. It was found, for instance, that although factors such as rollover, driving under the influence, and presence of heavy truck all have contributory impacts on the cost of crashes, their impacts are greater on higher quantiles, or higher barriers' costs. These models were compared from various perspectives such as intra class correlation (ICC), and standard error of coefficients. This study highlights the changes in coefficient estimates while modeling crash costs.

Estimating the effect of geometric features of side traffic barriers on crash severity of interstate roads in Wyoming.

Past roadside safety studies mostly evaluated the impact of traffic barrier geometric features using simulation tools or by conducting field crash tests. While past simulation and field crash tests could present important findings for upgrading the geometric design of traffic barriers, there is still a gap regarding conducting an actual data analysis on side traffic barriers crashes with regards to their geometric dimensions. This paper aims at filling this gap by combining a statewide dataset of side traffic barrier geometric features with historical crashes on interstate roads in Wyoming. Therefore, geometric features including system height, post-spacing, lateral offset (from the edge of pavement), and side-slope of over 150 miles of side traffic barriers were inventoried by conducting a field survey on interstate roads in Wyoming. For the statistical analysis, a random-parameters ordered logit model was utilized to investigate variables impacting crash severity of side traffic barriers. It was found that system height could significantly impact the crash severity of side box beam barriers. Box beam barriers with a system height between 25 and 31 in. were identified to be less severe in comparison to other height categories, while showing minimum risks of severe crashes in the system height of 29-31 in.. On the other hand, box beam barriers with a height taller than 31 in. may increase crash severity.

Investigating the effect of geometric dimensions of median traffic barriers on crashes: Crash analysis of interstate roads in Wyoming using actual crash datasets.

INTRODUCTION: Despite the numerous safety studies done on traffic barriers' performance assessment, the effect of variables such as traffic barrier's height has not been identified considering a comprehensive actual crash data analysis. This study seeks to identify the impact of geometric variables (i.e., height, post-spacing, sideslope ratio, and lateral offset) on median traffic barriers' performance in crashes on interstate roads. METHOD: Geometric dimensions of over 110 miles median traffic barriers on interstate Wyoming roads were inventoried in a field survey between 2016 and 2018. Then, the traffic barrier data collected was combined with historical crash records, traffic volume data, road geometric characteristics, and weather condition data to provide a comprehensive dataset for the analysis. Finally, an ordered logit model with random-parameters was developed for the severity of traffic barrier crashes. Based on the results, traffic barrier's height was found to impact crash severity. RESULTS: Crashes involving cable barriers with a height between 30″ and 42″ were less severe than other traffic barrier types, while concrete barriers with a height shorter than 32″ were more likely involved with severe injury crashes. As another important finding, the post-spacing of 6.1-6.3 ft. was identified as the least severe range in W-beam barriers. PRACTICAL APPLICATIONS: The results show that using flare barriers should reduce the number of crashes compared to parallel barriers.

Ordered logistic models of influencing factors on crash injury severity of single and multiple-vehicle downgrade crashes: A case study in Wyoming.

INTRODUCTION: The state of Wyoming, like other western United States, is characterized by mountainous terrain. Such terrain is well noted for its severe downgrades and difficult geometry. Given the specific challenges of driving in such difficult terrain, crashes with severe injuries are bound to occur. The literature is replete with research about factors that influence crash injury severity under different conditions. Differences in geometric characteristics of downgrades and mechanics of vehicle operations on such sections mean different factors may be at play in impacting crash severity in contrast to straight, level roadway sections. However, the impact of downgrades on injury severity has not been fully explored in the literature. This study is thus an attempt to fill this research gap. In this paper, an investigation was carried out to determine the influencing factors of crash injury severities of downgrade crashes. METHOD: Due to the ordered nature of the response variable, the ordered logit model was chosen to investigate the influencing factors of crash injury severities of downgrade crashes. The model was calibrated separately for single and multiple-vehicle crashes to ensure the different factors influencing both types of crashes were captured. RESULTS: The parameter estimates were as expected and mostly had signs consistent with engineering intuition. The results of the ordered model for single-vehicle crashes indicated that alcohol, gender, road condition, vehicle type, point of impact, vehicle maneuver, safety equipment use, driver action, and annual average daily traffic (AADT) per lane all impacted the injury severity of downgrade crashes. Safety equipment use, lighting conditions, posted speed limit, and lane width were also found to be significant factors influencing multiple-vehicle downgrade crashes. Injury severity probability plots were included as part of the study to provide a pictorial representation of how some of the variables change in response to each level of crash injury severity. CONCLUSION: Overall, this study provides insights into contributory factors of downgrade crashes. The literature review indicated that there are substantial differences between single- and multiple vehicle crashes. This was confirmed by the analysis which showed that mostly, separate factors impacted the crash injury severity of the two crash types. Practical applications: The results of this study could be used by policy makers, in other locations, to reduce downgrade crashes in mountainous areas.

Truck safety evaluation on Wyoming mountain passes.

The Manual on Uniform Traffic Control Devices (MUTCD) for Streets and Highways recommends hill signs be placed in advance of downgrade descent of mountain passes. Mountain passes increase the risk of a runaway, or out of control trucks and so the advance warning signs inform the driver to take special precautions such as reducing speed or using lower gears during the descent. The Wyoming Department of Transportation has installed steep grade advance warning systems on Wyoming mountain passes. However, concerns for out of control trucks on the mountain passes persist. The objective of this study is to evaluate the safety effectiveness of steep grade advance warning signs for trucks on Wyoming mountain passes. The safety evaluation was carried out by implementing a zero-inflated negative binomial modeling technique for predicting truck crashes on mountain passes. The outcome was two models that showed the risk of runaway truck accidents were high at locations where either the grades were steep and long or the grades were long with multiple vertical curves. The analysis showed the current advance warning systems were not significantly impacting truck crash risks at the high risk locations. The study, therefore, recommends some improvements to the current advanced warning signs or implementing an improved Federal Highway Administration Grade Severity Rating System based warning system that will significantly improve truck safety at the hazardous locations. The study informs policy makers on the safety issues on Wyoming Mountain passes with regards to runaway trucks and makes recommendations for reducing the risk of runaway truck crashes on mountain passes.