Eric T. Donnell

To illuminate or not to illuminate: Roadway lighting as it affects traffic safety at intersections

A two-pronged effort to quantify the impact of lighting on traffic safety is presented. In the statistical approach, the effects of lighting on crash frequency for different intersection types in Minnesota were assessed using count regression models. The models included many geometric and traffic control variables to estimate the association between lighting and nighttime and daytime crashes and the resulting night-to-day crash ratios. Overall, the presence of roadway intersection lighting was found to be associated with an approximately 12% lower night-to-day crash ratio than unlighted intersections. In the parallel analytical approach, visual performance analyses based on roadway intersection lighting practices in Minnesota were made for the same intersection types investigated in the statistical approach. The results of both approaches were convergent, suggesting that visual performance improvements from roadway lighting could serve as input for predicting improvements in crash frequency. A provisional transfer function allows transportation engineers to evaluate alternative lighting systems in the design phase so selections based on expected benefits and costs can be made.

Median barrier crash severity: Some new insights

Median barrier is used to prevent cross-median crashes on divided highways. Although it is well documented that crash frequencies increase after installing median barrier, little is known about median barrier crash severity outcomes. The present study estimated a nested logit model of median barrier crash severity using 5 years of data from rural divided highways in North Carolina. Vehicle, driver, roadway, and median cross-section design data were factors considered in the model. A unique aspect of the data used to estimate the model was the availability of median barrier placement and median cross-slope data, two elements not commonly included in roadway inventory data files. The estimation results indicate that collisions with a cable median barrier increase the probability of less-severe crash outcomes relative to collisions with a concrete or guardrail median barrier. Increasing the median barrier offset was associated with a lower probability of severe crash outcomes. The presence of a cable median barrier installed on foreslopes that were between 6H:1V and 10H:1V were associated with an increase in severe crash probabilities when compared to cable median barrier installations on foreslopes that were 10H:1V or flatter.

Predicting the severity of median-related crashes in Pennsylvania by using logistic regression

Models of median-related crash severity were developed by using roadway inventory and crash records for Pennsylvania Interstate highways. Cross-median and median barrier crashes formed the sample of crash types considered. Data were collected to model crash severity, including cross-section, traffic volume, and environmental predictor variables. Logistic regression models were developed by using both an ordinal and a nominal response. The results indicate that modeling crash severity as an ordinal response provided appropriate results for cross-median crashes, whereas a nominal response was more appropriate for median barrier crashes. Explanatory variables such as pavement surface conditions, use of drugs or alcohol, presence of an interchange entrance ramp, horizontal alignment, crash type, and average daily traffic volumes affect crash severity. The analysis results may be used by practitioners to understand the trade-off between geometric design decisions and median-related crash severity. Approximately 0.7% median barrier crashes on the Interstate system resulted in a fatality, whereas 43% were property-damage-only crashes and about 56% were injury crashes. More than 17% of cross-median collisions were fatal, and 67% involved injury.

Factors affecting driver speed choice along two-lane rural highway transition zones

Two-lane rural highways attract trips between traffic generators such as residential villages or small commercial areas and are generally uninterrupted flow facilities that provide a high level of mobility with high posted speeds limits. When passing through developed areas, the posted speed limit may be lowered to encourage reduced vehicle operating speeds. No published guidelines currently exist for the design of transition zones that connect high- to low-speed operating environments on two-lane rural highways. The objective of this research was to collect operating speed and roadway characteristic data along two-lane rural highway transition zones in Pennsylvania to explore the roadway, roadside, and traffic control factors that are associated with driver speed differentials. Single- and multilevel models were estimated and compared. The posted speed limit reduction as well as a change in the paved shoulder width, total number of driveways, various advance warning signs, the transition zone length, and the presence of horizontal curves were shown to increase the expected speed reduction in transition zones in the models. Additionally, drivers entering transition zones at higher speeds were found to have greater speed reductions than drivers entering the transition zone at lower speeds.

Cross-Median Collisions on Pennsylvania Interstates and Expressways

Cross-median collisions (CMCs) on high functional class roadways are a concern because of the potential economic and social losses that often accompany these crashes. The Pennsylvania Department of Transportation (PENNDOT) design policy suggests protecting medians on divided highways where the median width is less than 10m (32.8 ft) and the average daily traffic is above 20,000 vehicles per day. However, CMC crashes have occurred where a barrier was not warranted by the policy. The methodology used to assess median safety on Interstates and expressways in Pennsylvania is reviewed, and the findings are reported. A critical review of literature and an assessment of various state department of transportation median safety practices were undertaken. An expert panel qualitatively assessed median safety practices and provided input for quantitative data collection. Two PENNDOT databases, police accident reports, photologs, and field data were analyzed to complement the qualitative findings. Qualitative results suggest that the three-strand cable system, strong-post W-beam guiderail, or the concrete safety shape are recommended median barriers in appropriate site conditions. Quantitative results show that CMC crashes are rare events; however, nearly 15% involve fatalities and 72% involve nonfatal injuries. In addition, CMC crash rates at earth-divided highways decrease as the median width increases, CMC crashes appear more likely to occur downstream of interchange entrance ramps, and CMC crashes are more likely to involve adverse pavement surface conditions (wet or icy) than are other crashes. Negative binomial regression models were used to model CMC crash frequencies on earth-divided highways.

Application of propensity scores and potential outcomes to estimate effectiveness of traffic safety countermeasures: Exploratory analysis using intersection lighting data

More than 5.5 million police-reported traffic crashes occurred in the United States in 2009, resulting in 33,808 fatalities and more than 2.2 million injuries. Significant funds are expended annually by federal, state, and local transportation agencies in an effort to reduce traffic crashes. Effective safety management involves selecting highway and street locations with potential for safety improvements; correctly diagnosing safety problems; identifying appropriate countermeasures; prioritizing countermeasure implementation at selected sites; and, evaluating the effectiveness of implemented countermeasures. Accurate estimation of countermeasure effectiveness is a critical component of the safety management process. In this study, a statistical modeling framework, based on propensity scores and potential outcomes, is described to estimate countermeasure effectiveness from non-randomized observational data. Average treatment effects are estimated using semi-parametric estimation methods. To demonstrate the framework, the average treatment effect of fixed roadway lighting at intersections in Minnesota is estimated. The results indicate that fixed roadway lighting reduces expected nighttime crashes by approximately 6%, which compares favorably to other, recent lighting-safety research findings.

Speed Prediction Models for Trucks on Two-Lane Rural Highways

In the design consistency literature, the development of speed prediction models has concentrated on passenger cars, with little attention given to truck speed models. On some two-lane rural highways, however, trucks represent a large enough percentage of vehicles that they may be considered the design vehicle. The objective of the study was to develop operating speed prediction models for trucks on two-lane rural highways. A series of regression models was developed using a combination of field data and simulated data. First, the “Traffic on Rural Roads” and “Two-Lane with Passing” (TWOPAS) simulation models were critically assessed, and TWOPAS was selected to be used in further analysis. Next, field data from 17 sites were used to evaluate TWOPAS’s capability in the prediction of passenger car speeds and truck speeds. On the basis of the results of this evaluation, 13 sites were selected for use in simulating truck operating speeds. These sites were used to generate additional speed data for inclusion in the database for model development. Finally, a series of regression models was developed to predict 85th percentile truck operating speeds upstream, along, and downstream of a horizontal curve. These models consider the effect of length and grade of approach tangent, horizontal curve radius, and length and grade of departure tangent.

EVALUATION OF EFFECTS OF CENTERLINE RUMBLE STRIPS ON LATERAL VEHICLE PLACEMENT AND SPEED

Transportation agencies are using a variety of rumble strip patterns to improve highway safety. Previous studies have documented the efficacy of rumble strips in reducing certain crash types. However, no research has been published on the operational effects of these low-cost safety treatments. An evaluation of the effect a treatment has on operational characteristics can be conducted and reported in much less time than an evaluation based on crash data. The effect of centerline rumble strips on lateral vehicle placement and vehicle speeds on two-lane highways was investigated with a before-and-after observational study. Data were collected using tape switches at two treatment sites and two comparison sites. The lane widths were 11 and 12 ft. Data analysis and statistical testing indicated the centerline rumble strips had a significant effect on the mean and variance of lateral vehicle placement at both treatment sites. The observed change in the mean lateral vehicle location was away from the centerline rumble strips at both treatment sites; the variance of lateral vehicle location decreased at both sites. Data collected at the corresponding comparison sites during the before-and-after periods indicated no change in the mean and variance of lateral vehicle placement. Additionally, the study found that lateral vehicle placement in travel lanes may not be normally distributed as was previously assumed. Further evaluation of lateral vehicle placement distribution should be conducted with larger sample sizes and different roadway cross sections. No relationship could be drawn between speed and the presence of rumble strips for 11- or 12-ft lanes.

A multivariate spatial crash frequency model for identifying sites with promise based on crash types

Many studies have proposed the use of a systemic approach to identify sites with promise (SWiPs). Proponents of the systemic approach to road safety management suggest that it is more effective in reducing crash frequency than the traditional hot spot approach. The systemic approach aims to identify SWiPs by crash type(s) and, therefore, effectively connects crashes to their corresponding countermeasures. Nevertheless, a major challenge to implementing this approach is the low precision of crash frequency models, which results from the systemic approach considering subsets (crash types) of total crashes leading to higher variability in modeling outcomes. This study responds to the need for more precise statistical output and proposes a multivariate spatial model for simultaneously modeling crash frequencies for different crash types. The multivariate spatial model not only induces a multivariate correlation structure between crash types at the same site, but also spatial correlation among adjacent sites to enhance model precision. This study utilized crash, traffic, and roadway inventory data on rural two-lane highways in Pennsylvania to construct and test the multivariate spatial model. Four models with and without the multivariate and spatial correlations were tested and compared. The results show that the model that considers both multivariate and spatial correlation has the best fit. Moreover, it was found that the multivariate correlation plays a stronger role than the spatial correlation when modeling crash frequencies in terms of different crash types.

Case–control and cross-sectional methods for estimating crash modification factors: Comparisons from roadway lighting and lane and shoulder width safety effect studies

PROBLEM While observational before-after studies are considered the industry standard for developing crash modification factors (CMFs), there are practical limitations that may preclude their use in highway safety analysis. There is a need to explore alternative methods for estimating CMFs. METHOD This paper employs case-control and cross-sectional analyses to estimate CMFs for fixed roadway lighting and the allocation of lane and shoulder widths. RESULTS Based on the case-control method, the CMF for intersection lighting is 0.886, while the cross-sectional study indicates a CMF of 0.881. The CMFs developed for lane and shoulder widths are also similar when comparing the two methods. CONCLUSIONS This paper suggests that case-control and cross-sectional studies produce consistent results if care is taken in the study design and model development. IMPACT ON INDUSTRY Case-control and cross-sectional studies may provide a viable alternative to estimate CMFs when a before-after study is impractical due to data restrictions.