Richard J. Porter

Safety effects of wider edge lines on rural, two-lane highways

Although it is generally expected that wider lines will have a positive effect on vehicle safety, there have not been any convincing evidence based on the crash data analysis, partly because of the lack of relevant data. In this paper, the safety effect of wider edge lines was examined by analyzing crash frequency data for road segments with and without wider edge lines. The data from three states, Kansas, Michigan, and Illinois, have been analyzed. Because of different nature of data from each state, a different statistical analysis approach was employed for each state: an empirical Bayes, before-after analysis of Kansas data, an interrupted time series design and generalized linear segmented regression analysis of Michigan data, and a cross sectional analysis of Illinois data. Although it is well-known that causation is hard to establish based on observational studies, the results from three extensive statistical analyses all point to the same findings. The consistent findings lend support to the positive safety effects of wider edge lines installed on rural, two-lane highways.

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.

Commercial Driver Medical Examinations: Prevalence of Obesity, Comorbidities, and Certification Outcomes

Objective: The objective of this study was to assess relationships between body mass index (BMI) and comorbid conditions within a large sample of truck drivers. Methods: Commercial driver medical examination data from 88,246 commercial drivers between 2005 and 2012 were analyzed for associations between BMI, medical disorders, and driver certification. Results: Most drivers were obese (53.3%, BMI >30.0 kg/m2) and morbidly obese (26.6%, BMI >35.0 kg/m2), higher than prior reports. Obese drivers were less likely to be certified for 2 years and more likely to report heart disease, hypertension, diabetes mellitus, nervous disorders, sleep disorders, and chronic low back pain (all P < 0.0001). There are relationships between multiple potentially disqualifying conditions and increasing obesity (P < 0.0001). Morbid obesity prevalence increased 8.9% and prevalence of three or more multiple conditions increased fourfold between 2005 and 2012. Conclusions: Obesity is related to multiple medical factors as well as increasing numbers of conditions that limit driving certification.

Geometric Design, Speed, and Safety

A performance-based approach to the interaction of geometric design, speed, and safety is considered given the availability of two key documents: the Highway Safety Manual and Modeling Operating Speed: Synthesis Report. A historical look at the concept of design speed shows that although the definition of design speed has changed on more than one occasion, the same basic philosophy that related design speed to a safe speed is still reflected in current policy in supplemental guidance related to the selection of design speed. A conservative approach to establishing design criteria, used to address the range of driver, vehicle, and roadway conditions and capabilities that a designer must consider, is demonstrated. Operating speeds are shown to be higher than design speeds for design speeds of approximately 55 mph or less. This outcome may be considered undesirable, but that categorization seems to be based more on subjective judgments of what is desirable than on actual safety findings. Finally, the idea of speed management through the use of roadway geometrics (i.e., geometric designs that influence driver selection of operating speed)—one component of self-enforcing, self-explaining roadway design—is explored. Findings uncover possible challenges to implementing this idea. Five related questions are addressed: (a) What is known about the relationships between road geometry and operating speeds? (b) To what degree does road geometry influence operating speeds? (c) How are safety and security influenced by road geometry? (d) What are the potential impacts on large vehicles? and (e) What is the nature of the speed–safety trade-off?

Understanding Speed Concepts: Key Definitions and Case Study Examples

The relationships between design speed, operating speed, and posted speed limits are important considerations in the geometric design of highways and streets. Highway designers establish geometric design criteria by using a designated design speed. The ultimate objective is compatibility between design speed, operating speed, and posted speed limit–-or speed harmony. The design speed concept is intended to result in operating speeds that are consistent with the intended function of the highway or street and are therefore favorable with respect to safety and mobility. Because AASHTOs A Policy on Geometric Design of Highway Streets recommends using design values greater than the minimum for a designated design speed and because drivers choose to operate their vehicles according to perceived physical and operational limitations present along a roadway, higher than minimum design values may result in operational inconsistencies–-or speed discord. Speed discord is a condition in which the design speed is lower than the posted speed limit, lower than various operating speed measures, or both. Highway designers may not necessarily perceive such a relationship as problematic, but the public and enforcement personnel may express compliance or safety concerns for highway segments that experience speed discord. This paper provides key definitions of speed concepts used in the geometric design process and presents several case studies that demonstrate the concepts of speed harmony and speed discord.

Comparison of safety effect estimates obtained from empirical Bayes before–after study, propensity scores-potential outcomes framework, and regression model with cross-sectional data

A variety of different study designs and analysis methods have been used to evaluate the performance of traffic safety countermeasures. The most common study designs and methods include observational before-after studies using the empirical Bayes method and cross-sectional studies using regression models. The propensity scores-potential outcomes framework has recently been proposed as an alternative traffic safety countermeasure evaluation method to address the challenges associated with selection biases that can be part of cross-sectional studies. Crash modification factors derived from the application of all three methods have not yet been compared. This paper compares the results of retrospective, observational evaluations of a traffic safety countermeasure using both before-after and cross-sectional study designs. The paper describes the strengths and limitations of each method, focusing primarily on how each addresses site selection bias, which is a common issue in observational safety studies. The Safety Edge paving technique, which seeks to mitigate crashes related to roadway departure events, is the countermeasure used in the present study to compare the alternative evaluation methods. The results indicated that all three methods yielded results that were consistent with each other and with previous research. The empirical Bayes results had the smallest standard errors. It is concluded that the propensity scores with potential outcomes framework is a viable alternative analysis method to the empirical Bayes before-after study. It should be considered whenever a before-after study is not possible or practical.

Safety Impacts of Design Exceptions on Nonfreeway Segments

Safety (in terms of expected crash frequency and severity) was compared on road segments where design exceptions had been approved and constructed and on similar road segments where no design exceptions had been approved or constructed. Data were collected for design exceptions in Utah from 2001 to 2006. Multiple data sources were used to identify and define road segments with and without design exceptions. Propensity scores were applied to assess similarities between treatment and comparison sites. Ultimately, 34 total nonfreeway segments with design exceptions and 80 nonfreeway segments without design exceptions were used for modeling. The relationship between the presence of design exceptions and crash frequency was explored with a negative binomial regression modeling approach. The relationship between the presence of design exceptions and crash severity was explored in three ways: (a) computation of severity distributions at locations with and without design exceptions, (b) estimation of separate negative binomial regression models by severity level, and (c) estimation of multinomial logit models to predict the severity outcome of a crash. The presence of design exceptions was represented in the regression models by an indicator variable (where 1 5 one design exception or more and 0 5 no design exceptions). Crash data from 2007 through 2010 were used for model estimation. No significant differences were observed in expected crash frequencies and crash severities between nonfreeway road segments with and without design exceptions. This overall finding was consistent with two previous related efforts in Kentucky and Indiana.

Repeated Cross-Sectional Assessment of Commercial Truck Driver Health.

Objective: To assess relationships and trends over time in individual conditions and multiple conditions among a large sample of independent, nonoverlapping truck drivers using a repeated cross-sectional study design. Methods: Commercial driver medical examinations were conducted on 95,567 commercial drivers between January 1, 2005, and October 31, 2012. Specific medical conditions that have been identified by the Federal Motor Carrier Safety Administrations Medical Review Board as possibly increasing crash risk were examined. Prevalence and trends over time were analyzed. Results: A total of 8 of the 13 conditions significantly increased from 2005 to 2012. Prevalence of multiple concomitant conditions also increased, with prevalence odds ratios as high as 7.39 (95% confidence interval, 3.92 to 13.98) for four or more conditions in 2012 as compared with 2005. Conclusions: Individual and multiple conditions thought to be associated with increased crash risk significantly increased between 2005 and 2012.

New Insights on Evaluations of Design Consistency for Two-Lane Highways

Operating speed prediction models along horizontal curves of two-lane rural highways have been the topic of a significant body of literature. Operating speed models for two-lane rural highway tangents and for horizontal curves on low-speed urban streets also exist but are less developed. These models, estimated primarily by means of ordinary least squares regression, generally use aggregate-level data to estimate the effect of geometric design variables on the mean or 85th percentile operating speed but do not explicitly consider measures of speed dispersion (e.g., standard deviation). Most existing models consider speeds of free-flow passenger vehicles; traffic flow effects on speed are generally not quantified. Finally, published literature shows that researchers have not converged on a consistent modeling practice about the use of posted speed limit as an explanatory variable in operating speed models. Collectively, these issues may lead to design consistency assessments that are not truly reflective of the operating conditions along a two-lane highway. These issues are explored in this paper with the use of operating speed data collected at multiple points along eight two-lane highway segments. A simultaneous equations approach is used to estimate a three-equation model of the posted speed limit, mean speed, and speed deviation. The predicted 85th percentile speed is compared with the observed 85th percentile speed along a high-speed, two-lane rural highway. Predicted 85th percentile operating speeds along the same high-speed highway determined with the Interactive Highway Safety Design Model design consistency module are also compared with observed 85th percentile speeds as a basis for evaluating the simultaneous equations approach. Finally, the paper proposes an alternative approach to evaluating design consistency that uses inferred design speed.

Safety Evaluation of Geometric Design Criteria for Spacing of Entrance-Exit Ramp Sequence and Use of Auxiliary Lanes

Geometric design decisions regarding interchange and ramp spacing on freeways have traditionally taken a nominal approach to safety. This approach oversimplifies driver behavior and complex interactions between roadway geometrics, traffic operations, and safety and tends to promote a one-size-fits-all approach to evaluating design alternatives. The objective of this paper was to quantify the relationship between ramp spacing and freeway safety. Data for this study included freeway geometric features, traffic characteristics, and crash counts for 404 freeway segments in California and Washington State. A negative binomial regression modeling approach was used to explore the relationship between ramp spacing and safety. Results indicated that expected crash frequency increased as ramp spacing decreased. The expected proportion of crashes resulting in a fatality or injury appeared to decrease as ramp spacing decreased. The presence of an auxiliary lane was associated with a lower expected frequency of crashes for any given ramp spacing; the safety benefits of providing an auxiliary lane diminished as ramp spacing increased. The model results related to ramp spacing and the presence of an auxiliary lane were transformed into crash modification factors and validated through comparisons with an independent research study on weaving areas in Texas. The safety findings were generally consistent with current Green Book design criteria related to ramp spacing and the presence of an auxiliary lane. The ability to quantify the expected impact on safety of ramp spacing associated with new or modified interchanges will assist transportation agencies in making well-informed assessments of the overall benefits, impacts, and costs of freeway access decisions.