2013 Session: 289

• Formulating Informative Priors and Effects on Bayesian Hierarchical Crash Models
  Abstract: The Bayesian inference method has been frequently adopted to develop safety performance functions. One advantage of the Bayesian inference is that prior information about the independent variables can be included in the models, which could benefit the inference conclusions from avoiding implausible results due to data fluctuations. However, there are few past studies discussing how to formulate the informative priors and what are the effects of having informative priors in developing Safety Performance Functions. This paper fills the void by introducing four approaches of developing informative priors for the independent variables based on historical data or general information. Merits of these informative priors have been tested along with two types of Bayesian Hierarchical models (Poisson-gamma model and Random effect Poisson model). Deviance Information Criterion (DIC), R-square values and standard errors were utilized as evaluation measures to select the best model(s). Comparisons across the models indicate that the Poisson-gamma model is superior with better model fitting and it is much more robust with the informative priors. Moreover, model fitting and coefficient estimation accuracies have been enhanced by the informative priors. Finally, based on the results, recommendations are made for the different informative prior development techniques.
  Authors: Yu, Rongjie; Abdel-Aty, Mohamed A.
  Authors: Yu, Rongjie; Abdel-Aty, Mohamed A.
  Year: 2013
  Document Type: Paper
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-0222
• International Transferability of Accident Modification Functions for Horizontal Curves
  Abstract: Studies of the relationship between characteristics of horizontal curves and accident rate have been reported in several countries. The characteristic most often studied is the radius of a horizontal curve. Functions describing the relationship between the radius of horizontal curves and accident rate have been developed in Canada, Denmark, Germany, Great Britain, New Zealand, Norway, Portugal, Sweden, and the United States. Other characteristics of horizontal curves that have been studied include deflection angle, curve length, the presence of transition curves, superelevation in curves and distance to adjacent curves. This paper assesses the international transferability of mathematical functions (accident modification functions) that have been developed to relate the radius of horizontal curves to their accident rate. The main research problem is whether these functions are similar, which enhances international transferability, or dissimilar, which reduces international transferability. Accident modification functions for horizontal curve radius developed in the countries listed above are synthesized. The sensitivity of the functions to other characteristics of curves than radius is examined. Accident modification functions developed in different countries have important similarities, but also some notable differences. The synthesized function appears to be a reasonable estimate of the typical relationship.
  Authors: Elvik, Rune
  Authors: Elvik, Rune
  Year: 2013
  Document Type: Paper
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-0321
• Gender Gaps in Crash Data: Statistical Look at Gender and Age Differences as Related to Crash Frequencies
  Abstract: This study was performed to calibrate safety performance functions (SPFs) to predict the number of injurious crashes per year per km per 10^8 vehicles on the horizontal homogeneous segment of two-lane rural roads. The crashes were analyzed from the perspective of driver gender for three main injurious crash types (head-on/side and rear collisions, tail crashes, and single-vehicle run-off-road crashes) as observed on the network. We analyzed more than 3,700 km of road network with 2,242 accidents recorded from 2003 to 2010, of which 1,597 were injurious, and 645 resulted in only damage to property. Generalized estimating equations with a negative binomial distribution and additional log linkage equations were implemented. A very exciting statistical variable was introduced in the models constructed according to plotted crash risk maps by varying the crash type, the number and gender of the drivers involved in the crash and the scenario represented by a particular combination of infrastructural and environmental conditions surveyed on the site at the time of the crash. We have also introduced lane width, horizontal curvature indicators and mean speed as consistent explanatory factors in the model. Countermeasures are suggested for reducing crash frequency such as awareness campaigns and road structural operations.
  Authors: Russo, Francesca; Biancardo, Salvatore Antonio
  Authors: Russo, Francesca; Biancardo, Salvatore Antonio
  Year: 2013
  Document Type: Paper
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-1313
• Safety Evaluation of Converting Traffic Signals from Incandescent to LED Bulbs
  Abstract: Data from 282 signalized intersections in Charlotte were used to examine the safety effect of converting the signals to composite light emitting diode (LED) bulbs. An empirical Bayes before-after method was used for the evaluation. Since this was a “blanket” installation by the city of Charlotte, a comparison group of stop controlled intersections were used to account for possible trends during the study period. Crash modification factors were estimated for 3 and 4 leg intersections for 8 different crash types including crashes during dawn, dusk, and dark conditions. For 3 leg intersections, all the CMFs were higher than 1.0 indicating a possible increase in crashes due to LEDs. However, none of these CMFs were statistically different from 1.0 at the 0.05 significance level. For 4 leg intersections, the CMFs associated with rear-end crashes were lower than 1.0 and statistically significant at the 0.05 level, indicating a reduction in these crash types due to the LEDs. There was substantial difference among the sites in terms of the effect of the LEDs. The reasons for these differences are not known at this time. Future research could investigate whether LEDs are more or less beneficial depending on the characteristics of the intersection including type of area, sight distance, traffic volume, and phasing scheme.
  Authors: Srinivasan, Raghavan; Carter, Daniel L.; Smith, Sarah; Lan, Bo
  Authors: Srinivasan, Raghavan; Carter, Daniel L.; Smith, Sarah; Lan, Bo
  Year: 2013
  Document Type: Paper
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-1373
• Monitoring and Analysis of Travel Speeds on the National Road Network Using Floating-Car Technologies
  Abstract: It is well known that many factors contribute sequentially and in parallel to the occurrence of road crashes. In order to address this multi-dimensionality, we are building a geographic information system (GIS) that will allow a visual as well as analytical monitoring and analysis of parameters and data related to traffic safety. In particular, the system will be capable of receiving data from various sources: infrastructure detailed data, traffic counts, travel speed data, data from road-installed sensors, camera data, driving events obtained from advanced technologies, as well as road crash and injury data. Among the main factors affecting injuries from road crashes - speed is considered a leading cause and contributing factor. There are numerous studies linking travel speeds and road crashes. Hence an essential part of every road safety plan is speed management. In order to manage speed – it has to be systematically and consistently monitored and analyzed. In this study we present a system for the collection and analysis of travel speeds at the nationwide level. The paper focuses on the collection and analysis of travel speeds on different road sections. The current research provides a comprehensive speed database in space and time by using the information gathered through advanced technologies combined with geographical data system that allows visual presentation of the data. This analysis can identify the road sections with significant excesses of travel speeds relative to the speed limit. It can also serve as baseline to evaluate current counter-measures employed to reduce speed.
  Authors: Bekhor, Shlomo; Lotan, Tsippy; Gitelman, Victoria
  Authors: Bekhor, Shlomo; Lotan, Tsippy; Gitelman, Victoria
  Year: 2013
  Document Type: Paper
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-1685
• Calibration Factor with the Consideration of Short-term Trend in Crash Occurrence
  Abstract: Crash prediction models are used to estimate the expected number of crashes for different highway facilities. These models re calibrated periodically, using short-term near-past crash data, regardless of the State data being used in the model development. For each facility type the calibration factor is estimated by dividing the observed number of crashes by the number of crashes predicted by the model over the same period. This factor is then used in the prediction of the expected number of crashes for near-future periods. One major short-coming of the current method in estimating the calibration factors is the lack of consideration of short-term trends in crash occurrences that are caused by factors that have no contribution in the models. Crashes observed in most States since 2006 show an example of such trend. Many of the safety improvements in vehicles and highways that are the major causes for this declining trend are not considered in the crash prediction models. This research proposes a methodology for estimating calibration factors that will consider the effect of such trends. Crash prediction models introduced in the Highway Safety Manual are used to examine this methodology. Data used in this study are from Washington State for rural two-lane, rural multilane and urban/suburban arterial highway segments. Crash data from 2006-2008 are used for estimating the calibration factors and 2009-2010 data are used to validate the hypothesis that calibration factors that are estimated by consideration of short-term trends are of better quality compared to the conventional calibration factors.
  Authors: Banihashemi, Mohamadreza
  Authors: Banihashemi, Mohamadreza
  Year: 2013
  Document Type: Paper
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-2634
• Severity Distribution Function For Freeway Segments
  Abstract: To date, the focus of modeling efforts for freeway safety has been on developing safety prediction functions (SPFs) and crash modification factors (CMFs), with only limited consideration for crash severity distributions. As a result, relatively little is known about the safety effects of design elements, such as lane width, rumble strips, and longitudinal barriers on crash severity. In some cases, countermeasures are implemented with the intent to reduce fatal crashes, but the effect of these treatments on less severe crashes is not well understood.Research was conducted to develop severity distribution functions (SDFs) to predict the proportion of crashes in each severity category as a function of roadway geometric design elements and traffic control features. The SDFs were calibrated using freeway segment data from California, Maine and Washington. The findings from this research show that barrier presence, increased traffic volume, increased lane width, and urban area type reduce the proportion of high-severity crashes. At the same time, rumble strip and horizontal curvature presence increase the proportion of high-severity crashes. These SDFs can be applied along with SPFs and CMFs to obtain more precise estimates of the safety effects of design decisions.
  Authors: Geedipally, Srinivas Reddy; Bonneson, James A.; Pratt, Michael Paul; Lord, Dominique
  Authors: Geedipally, Srinivas Reddy; Bonneson, James A.; Pratt, Michael Paul; Lord, Dominique
  Year: 2013
  Document Type: Paper
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-2873
• Network Safety Screening in the Context of Agency-Specific Screening Criteria
  Abstract: Network screening for identifying locations with specific safety needs is an important aspect of the safety management for any highway agency. Network screening is usually governed by specific criteria set by a particular agency. This study shows how an integrated safety management system can help achieve agency’s goal in identifying locations under various screening criteria. The safety framework can be interfaced with the agency’s crash database(s) and different support systems for implementing user-defined screening and project planning. The case study shows implementation of facility-level and segment-based network screening on a single route. The analysis route and associated crash information were fixed whereas the screening method and performance measures were varied. Results showed that both sliding window and peak search techniques showed comparable results and better performance than simple ranking technique in identifying hotspots with respect to different performance measures. In general, the sliding window technique shows more mileage of hotspots identified, whereas the peak search technique has better accuracy in terms of crash density. The coefficient of variation (CV) based sliding window on facility screening showed better performance in terms of mileage and crash density of identified hotspots. This study can help agency understand the underlying factors that affect their network screening process.
  Authors: Azam, Md. Shafiul; Manepalli, Uday; Laumet, Pascal
  Authors: Azam, Md. Shafiul; Manepalli, Uday; Laumet, Pascal
  Year: 2013
  Document Type: Paper
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-2887
• Spatial Effect on Zone-Level Collision Prediction Model
  Abstract: A recent study developed a set of zone-level negative binomial collision prediction models (CPMs) to investigate the relationship between various transportation and socio-demographic characteristics, and the overall roadway safety (1). The developed models used data from Metro Vancouver, British Columbia, and considered the Poisson variations and the heterogeneity (extra-variation) on collision occurrence of collisions. This study aims to evaluate the spatial effects on the occurrence of collisions and to check whether the inclusion of spatial variables can improve the goodness of fit and inference the capability of the previously developed CPMs presented in (1).The transit reliant and application based collision prediction models with spatial correlations were developed using the WinBUGS software. The convergences of the developed models were tested by the trace plots of the parameter estimated, the BGR statistics, and ratios of Monte Carlo errors relative to the standard deviations of the estimates. The results showed that the incorporation of the spatial correlation affected the parameter estimates, the values of dispersion parameters and intercepts, and also the t-statistics. The effect of the main exposure variable on all of the models for total, severe and property damage only collisions were found to be smaller under spatial models. The smaller values of the exponents of the main exposure variable asserted our assumption that spatial effects need to be considered in CPMs to mitigate any potential bias associated with model misspecification.
  Authors: Karim, Md. Ahsanul; Wahba, Mohamed; Sayed, Tarek
  Authors: Karim, Md. Ahsanul; Wahba, Mohamed; Sayed, Tarek
  Year: 2013
  Document Type: Paper
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-3145
• Safety Performance Functions Reflecting Categorical Impact of Exposure Variables for Freeways
  Abstract: The aim of this study is to develop safety performance functions (SPFs) reflecting the categorical impact of exposure variables that may vary with freeway segments. A four-step procedure is constructed including clustering analysis, distribution selection, model specification, and model integration. First, clustering analysis is employed to classify freeway segments into three similar groups. A goodness-of-fit test is performed to select suitable distributions for the SPFs of each of the three groups. We compare three forms of relationships between crash frequency and exposure variables and select the best one by using statistical indices. Two final models are proposed, one for fatal-injury (FI) crashes and one for total crashes, based on the evaluations performed by a test for taste variations and a paired asymptotic t-test. All of the coefficients and constants in the proposed models are statistically significant. In addition, both models show higher statistical significance than the models reflecting constant relationships between crash frequency and exposure variables. The proposed four-step procedure for SPF makes it possible to predict crash frequency more accurately, and it can be easily applicable to predict the number of crashes without any additional data or complex simulation procedures. If additional explanatory variables are available for criteria on the difference and similarity, the ability to explain the data may be enhanced. It would be necessary to select an appropriate tool for characteristics of targeted roads, which is a subject for further research.
  Authors: Kim, Ducknyung; Kim, Dong-Kyu; Lee, Chungwon
  Authors: Kim, Ducknyung; Kim, Dong-Kyu; Lee, Chungwon
  Year: 2013
  Document Type: Paper
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-3758
• Crash Causation in Statistical and Clinical Studies
  Abstract: This paper argues for two main points. First, in principle, the causal effect of a countermeasure as defined in the HSM, and counterfactual prediction as used in clinical analysis of individual crash events, can be viewed as different methods for estimating the relative fraction of crashes, preventable by the countermeasure, within a crash population. The accuracy of counterfactual prediction requires that the estimation of crash features produced in a crash reconstruction be consistent, which can be interpreted as a requirement that the reconstruction methodology be well-calibrated. Second, it is possible to make falsifiable predictions using microscopic crash simulation models, so the claim that these models are inherently unscientific is unfounded. Since, at the present state-of-art, microscopic models are better-suited to predicting characteristics of a population of crashes, rather than overall crash frequencies, testing and criticizing these models will require information at a finer level of detail than is typically available in computerized crash records. Since the effect of a countermeasure can vary depending on local conditions, using an aggregated, average estimate may be locally misleading. Structural models are better at predicting modification effects than they are at predicting crash frequencies, so one line of future research could be into the usefulness of a “mechanistic-empirical” strategy, where regression models, similar to the HSM base models, are used to predict event frequencies, while structural models are then used to classify the events
  Authors: Davis, Gary A.
  Authors: Davis, Gary A.
  Year: 2013
  Document Type: Paper
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-4162
• Evaluation of Remote-Sensing Technologies for Collecting Roadside Feature Data to Support Highway Safety Manual Implementation
  Abstract: Roadside feature data are critical inputs to highway safety models as described in the Highway Safety Manual (HSM). Collecting safety-related roadside feature data is an important step for HSM implementation. Many state DOTs routinely collect data on roadside objects using a variety of sensing methods, and these programs often incur significant cost. At present, it is unknown which of these methods or any combination of these methods is capable of efficiently collecting safety-related roadside feature data while minimizing cost and safety concern. The objective of this research is to identify required roadside feature data for various types of highway segments and to characterize the capability of existing sensing methods in contrast to required roadside feature data through literature review and a nation-wide survey, and large-scale field trials of selected sensing methods. The results of literature review and surveys are reported in this paper. The findings of this research suggest that either mobile LiDAR or the combination of video/photo log method with aerial imagery method is capable of collecting required HSM-related roadside information. However, due to the high data reduction effort, the current mobile LiDAR method needs significant improvement in the LiDAR data processing and feature extraction stage.
  Authors: Jalayer, Mohammad; Gong, Jie; Zhou, Huaguo; Grinter, Mark
  Authors: Jalayer, Mohammad; Gong, Jie; Zhou, Huaguo; Grinter, Mark
  Year: 2013
  Document Type: Paper
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-4709
• Full versus Simple Safety Performance Functions: A Comparison Based on Urban Four-Lane Freeway Interchange Influence Areas in Florida
  Abstract: The empirical Bayes (EB) approach adopted in the Highway Safety Manual (HSM) and the SafetyAnalyst application requires the use of Safety Performance Functions (SPFs). SafetyAnalyst adopts a form of SPF, known as simple SPF, which relates crash experience to only traffic volume. It is a flow-only model that is calibrated using all sites irrespective of their base geometric conditions. Full SPFs, on the other hand, relate crash occurrence to roadway geometric characteristics in addition to traffic characteristics. This study compares the simple SPFs provided in SafetyAnalyst with full SPFs in two safety applications: crash prediction performance and high crash locations’ (HCLs) identification. To compare the prediction performance, the simple and full SPFs were estimated using data collected on urban four-lane freeway interchange influence areas in Florida. Models were estimated for both total crashes and F+I (fatal and injury) crashes. The mean absolute deviance (MAD) and the mean square prediction error (MSPE) were used to assess and compare the prediction performance of the two models, and the variation in ranking the HCLs using each model was also examined. The results showed that the two models yielded very similar performance of crash prediction and network screening. This empirical result supports the use of the flow-only SPF model adopted in SafetyAnalyst, which requires much less effort to develop compared to full SPFs.
  Authors: Lu, Jinyan; Haleem, Kirolos M.; Alluri, Priyanka; Gan, Albert
  Authors: Lu, Jinyan; Haleem, Kirolos M.; Alluri, Priyanka; Gan, Albert
  Year: 2013
  Document Type: Paper
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-4828
• Safety of Channelized Right-Turn Lanesfor Motor Vehicles and Pedestrians
  Abstract: This paper presents the results of research undertaken to evaluate how the safety performance of intersection approaches with channelized right-turn lanes compares to that of intersection approaches with conventional right-turn lanes or shared through/right-turn lanes. Crash data for nearly 400 intersection approaches in Toronto, Ontario, Canada, including intersection approaches with channelized right-turn lanes, conventional right-turn lanes, and shared through/right-turn lanes, were analyzed to compare the safety performance of the three right-turn treatment types. The research results indicate that intersection approaches with channelized right-turn lanes appear to have similar motor-vehicle safety performance as approaches with conventional right-turn lanes or shared through/right-turn lanes. This was found to be the case both at the downstream end of the channelized right-turn lane (where the right-turning vehicle merges with the cross street) as well as at the upstream end of the channelized right-turn lane (where the right-turning vehicle begins the right-turn maneuver). Intersection approaches with channelized right-turn lanes also appear to have similar pedestrian safety performance as approaches with shared through/right-turn lanes. Intersection approaches with conventional right-turn lanes have substantially more pedestrian crashes (approximately 70 to 80 percent more) than approaches with channelized right-turn lanes or shared/through right-turn lanes.
  Authors: Potts, Ingrid B.; Bauer, Karin M.; Torbic, Darren John; Ringert, John F.
  Authors: Potts, Ingrid B.; Bauer, Karin M.; Torbic, Darren John; Ringert, John F.
  Year: 2013
  Document Type: Paper
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-4854
• Benefit-Cost Analysis Applied to Behavioral and Engineering Safety Countermeasures in San Francisco, California
  Abstract: The state of the practice in safety has advanced rapidly in recent years with the emergence of new tools and processes for improving selection of the most cost-effective safety countermeasures. However, many challenges prevent fair and objective comparisons of countermeasures applied across safety disciplines (e.g. engineering, emergency services, and behavioral measures). These countermeasures operate at different spatial scales, are funded often by different financial sources and agencies, and have associated costs and benefits that are difficult to estimate. This research proposes a methodology by which both behavioral and engineering safety investments are considered and compared in a specific local context. The methodology involves a multi-stage process that enables the analyst to select countermeasures that yield high benefits to costs, are targeted for a particular project, and that may involve costs and benefits that accrue over varying spatial and temporal scales. The methodology is illustrated using a case study from the Geary Boulevard Corridor in San Francisco, California. The case study illustrates that: 1) The methodology enables the identification and assessment of a wide range of safety investment types at the project level; 2) The nature of crash histories lend themselves to the selection of both behavioral and engineering investments, requiring cooperation across agencies; and 3) The results of the cost-benefit analysis are highly sensitive to cost and benefit assumptions, and thus listing and justification of all assumptions is required. It is recommended that a sensitivity analyses be conducted when there is large uncertainty surrounding cost and benefit assumptions.
  Authors: Greene-Roesel, Ryan; Washington, Simon; Wier, Megan L.; Bhatia, Rajiv; Haque, Md. Mazharul; Wemple, Elizabeth
  Authors: Greene-Roesel, Ryan; Washington, Simon; Wier, Megan L.; Bhatia, Rajiv; Haque, Md. Mazharul; Wemple, Elizabeth
  Year: 2013
  Document Type: Paper
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-4805
• Safety Evaluation of Discontinuing Late Night Flash Operations at Signalized Intersections
  Abstract: This study examined the safety impacts of converting late nighttime flash (LNF) to normal phasing operation at signalized intersections by means of the empirical Bayes (EB) before-after method, and the univariate and multivariate Full Bayesian (FB) before-after methods. Data were obtained from the North Carolina Department of Transportation and included 61 treatment sites and 395 reference intersections that remained on LNF operation from 2000 to 2007. The results from the EB method are almost identical to those of the univariate FB. The FB method offered more flexibility in selecting the functional form of expected crashes at similar sites (similar to the SPF in the EB), and addressing uncertainty in the data. Compared to the univariate FB, the multivariate FB using the multivariate poisson lognormal model (MVPLN) provided better results based on much lower deviance information criterion (DIC) values. The MVPLN model was favored and the recommended CRFs are 48% (±6%), 53% (±8%), and 57% (±7%) for night, injury and fatal, and frontal impact crashes, respectively.
  Authors: Lan, Bo; Srinivasan, Raghavan
  Authors: Lan, Bo; Srinivasan, Raghavan
  Year: 2013
  Document Type: Paper
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-0988
  Practice-Ready: Yes
• Safety Effects of Horizontal Curve and Grade Combinations on Rural Two-Lane Roads
  Abstract: The safety effects of horizontal curves and grades on highways have been quantified separately, but it is not currently known whether and how the safety performance of horizontal curves and that of grades interact.While the first edition of the HSM provides CMFs for the safety effects of horizontal curvature and percent grade on rural two-lane highways, it does not have any method for accounting for the interactions between these effects. In other words, in the HSM procedures for rural two-lane highways, the safety effect of a horizontal curve is the same whether it is located on a level roadway, a straight grade, or a vertical curve. Similarly, the safety effect of a straight grade is the same whether it is located on a tangent roadway or on a horizontal curve. Researchers have always supposed that there are interactions between the safety effects of horizontal and vertical alignment, but this has not been demonstrated in a form useful for safety prediction.This paper summarizes the results of research undertaken to quantify the safety effects of five types of horizontal and vertical alignment combinations based on Washington HSIS data and crash records from 2003 to 2008. The outcome is a set of safety prediction models for fatal-and-injury and PDO crashes. To present the results in a form suitable for incorporation in the AASHTO Highway Safety Manual, crash modification factors representing safety performance relative to level tangents were developed from these models for each of the five combinations.
  Authors: Bauer, Karin M.; Harwood, Douglas W.
  Authors: Bauer, Karin M.; Harwood, Douglas W.
  Year: 2013
  Document Type: Paper
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-3056
  Practice-Ready: Yes
• Safety Performance Function Calibration and Development for the State of Alabama: Two-Lane Two-Way Rural Roads and Four-Lane Divided Highways
  Abstract: The Highway Safety Manual (HSM) published by the American Association of State Highway Transportation Officials provides procedures and statistical tools for estimating the expected number of crashes for different roadway facilities. One critical component of the HSM method is the Safety Performance Function (SPF). SPFs are essentially regression models that correlate quantitatively the expected number of crashes with traffic exposure and geometric characteristics of the road. Since these models are developed using data from other states, its transferability is not gauranteed. As part of a project peformed by researchers from the University of Alabama to facilitate the implementation of the new HSM procedures in the State of Alabama, this study aims to evaluate the applicability of HSM predictive methods to Alabama data, and to develop state-specific statistical models for two facility types, namely two-lane two-way rural roads and four-lane divided highways. This study first calibrates the HSM base SPFs using two approaches. Besides the method recommended by HSM, this study also proposes a new approach that treats the estimation of calibration factors as a special case of a negative binomail regression. In addition, new forms for state-specific SPFs are further investigated to identify the best model using Poisson-gamma regression techniques. Four new functional forms are studied in this project. The prediction capabilities of the two calibrated models and the four newly developed state-specific SPFs are evaluated using a validation data set. Five performance measures are considered for model evaluation. They are the mean absolute deviance, the mean squared prediction error, the mean prediction bias, the log likelihood value, and the Akailke’s Information Criterion. The study is able to identify a particular state-specific SPF that fits the Alabama data well and outperforms other models, including the calibrated SPFs. The best model describes the mean crash frequency as a function of annual average daily traffic, segment length, lane width, year, and speed limt. The study finds that the HSM-recommended method for calibration factor estimation also performs well. Although it is not as good as the best state-specific SPF, it is still a good alternative considering the approach is very straightforward and can be easily applied.
  Authors: Mehta, Gaurav; Lou, Yingyan
  Authors: Mehta, Gaurav; Lou, Yingyan
  Year: 2013
  Document Type: Paper
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-4221
  Practice-Ready: Yes
• Safety-Effectiveness of Various Types of Shoulders on Rural Two-Lane Roads in Winter and Nonwinter Periods
  Abstract: There has been growing recognition of the quantitative effects of various roadway designs and traffic control strategies on safety. Meanwhile, there is increasing interest in measuring the variances of safety effectiveness in different periods of the year for similar roadway designs or similar traffic control strategies. This study tried to address the variances of safety effectiveness between the winter and non-winter periods for the ten most common shoulder designs in Kansas. Traffic and geometric data were collected on 6,510 miles (10,477 km) of rural two-lane highways in Kansas. A cross-sectional approach was applied to develop winter period safety performance functions (SPFs), non-winter period SPFs and SPFs aggregated at an annual level in which shoulder designs were treated as independent variables. A variance test was conducted based on these SPFs to investigate the variances of safety effectiveness between the two different periods. It was found that wider and upgraded shoulders offer significant less safety benefit in reducing total crash number during winter periods than during non-winter periods. The indexes of safety effectiveness for the winter period are larger than those for the non-winter period by between 13 to 25 percent. However, winter weather appears not to significantly diminish wider and/or upgraded shoulders’ safety benefit in reducing crash severity and the number of shoulder related crashes. The results demonstrate that treating the winter and non-winter data equally is likely to bias a shoulder’s estimated safety effectiveness in total crashes.
  Authors: Zeng, Huanghui; Schrock, Steven D.
  Authors: Zeng, Huanghui; Schrock, Steven D.
  Year: 2013
  Document Type: Paper
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-4943
  Practice-Ready: Yes
• Estimating Annual Average Daily Traffic for Local Roads for Highway Safety Analysis
  Abstract: Annual average daily traffic (AADT) is a required input to the newly released SafetyAnalyst software application. Further, AADT is also required to calculate crash rates. Traditionally, AADTs are estimated using a mix of permanent and temporary traffic counts collected in the field. Because field collection of traffic counts is expensive, it is usually performed for only the major roads. The mandate by the Federal Highway Administration (FHWA) to report the top 5% of high crash locations on all public roads underscores, for the first time, the need for state Departments of Transportation to acquire AADTs for also the non-state local roads. However, many local jurisdictions either do not have any AADT data, or they do not have them in sufficient quality. This paper presents a method to estimate AADTs for local roads using the travel demand modeling method. A major component of the method involves a parcel-level trip generation model that estimates the trips generated by each parcel. The generated trips are then distributed to existing traffic count sites using a parcel-level trip distribution gravity model. The all-or-nothing trip assignment method is then applied to assign the trips between the parcels and the traffic count sites onto local roadway network to yield estimates of AADTs. The estimated AADTs were compared with those from an existing regression-based method using actual traffic counts from Broward County, Florida. The results show that the proposed method produces significantly lower mean absolute percentage errors.
  Authors: Wang, Tao; Gan, Albert; Alluri, Priyanka
  Authors: Wang, Tao; Gan, Albert; Alluri, Priyanka
  Year: 2013
  Document Type: Paper
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-3490
  Practice-Ready: Yes
• Comprehensive Predictive Model of Interstate Highway Crash Severity
  Abstract: Highway safety is a topic that is at the forefront of most, if not all, state agendas, and roadway safety will become increasingly more important as the volume of traffic on roadways increases over time. However, currently not enough work exists in regard to modeling the combined effect of a wide variety of variables have on crash severity. This research uses crash data and supporting spatial data from Alabama to define the relationship between crash severity, broken down into nine distinct severity outcomes, and a comprehensive set of independent variables of: roadway infrastructure data, highway traffic and demand data, and spatially related socieconomic and demographic data, via an ordered probit regression model. This study identifies the most important links between crash-occurrence spatial specific variables (infrastructure, roadway demand, connected urban areas land use/demographics) driver/passenger specific, meteorological data and crash severity. Among the results, roadway infrastructure and spatial environments are some of the most important factors influencing crash severity. The comprehensive predictive model presented in this paper can be applied to a number of statewide settings and assist in identifying critical areas for improvements both today and in the future.
  Authors: LaMondia, Jeffrey J.; Morgan, Noah
  Authors: LaMondia, Jeffrey J.; Morgan, Noah
  Year: 2013
  Document Type: Paper
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-3584
  Practice-Ready: Yes
• Effect of Removing Freeway Mainline Barrier Toll Plazas on Safety
  Abstract: Toll plaza safety is a critical issue. Toll plazas induce motor vehicle crashes and also put workers such as toll collectors at risk. Therefore, enhancing safety at a toll plaza is crucial to improving safety on tolled roadways. This study aims to evaluate the safety effect of removing mainline barrier toll plazas on highways using Empirical Bayesian (EB) methodology. Recent removals of barrier toll plaza on the Garden State Parkway in New Jersey were used as a case study. Multiple-year traffic and crash data before and after the removals of the barrier toll plazas were used for analysis. Toll plaza crash frequency models as a function of traffic flow and other factors were developed, with the modeling results suggesting that there is a nonlinear relationship between toll plaza crash occurrences and both traffic flow as well as toll booth configurations. The EB approach is also used to predict crash frequency assuming that the barrier toll booths were not removed. These EB-based estimates were compared with the observed number of crashes after the removals of the toll plazas. Individual comparisons show reductions in crash frequency at almost all of the toll plazas and an estimated reduction of 47.2 percent overall at all toll plazas due to the removal of the barrier toll booths. The estimated crash cost was reduced by 43.2 percent. These estimated reductions demonstrate that the removal of barrier toll plazas is a very beneficial step towards improving safety of toll roads.
  Authors: Yang, Hong; Ozbay, Kaan; Bartin, Bekir; Ozturk, Ozgur
  Authors: Yang, Hong; Ozbay, Kaan; Bartin, Bekir; Ozturk, Ozgur
  Year: 2013
  Document Type: Paper
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-5001
  Practice-Ready: Yes
• Investigating Influence of Segmentation in Estimating Safety Performance Functions for Roadway Sections
  Abstract: Safety performance functions (SPFs) are crucial to science-based road safety management. Success in developing and applying SPFs depends fundamentally on two key factors: the validity of the statistical inferences for the available data and on how well the data can be organized into distinct homogenous entities. The latter aspect plays a key role in the identification and treatment of road sections or corridors with problems related to safety. Indeed, the segmentation of a road network could be especially critical in the development of SPFs that could be used in safety management for roadway types, such as motorways (freeways in North America), that have a large number of variables that could result in very short segments if these are desired to be homogeneous. This consequence, from an analytical point of view, can be a problem when the location of crashes is not precise and when there is an over abundance of segments with zero crashes. Lengthening the segments for developing and applying SPFs can mitigate this problem, but at a sacrifice of homogeneity. This paper seeks to address this dilemma by investigating five approaches for segmentation for motorways, using sample data from Italy. The best results were obtained for the segmentation based on two curves and two tangents within a segment and the segmentation with fixed length. The segmentation characterized by a constant value of all original variables inside each segment was the poorest approach by all measures.
  Authors: Cafiso, Salvatore; D'Agostino, Carmelo; Persaud, Bhagwant
  Authors: Cafiso, Salvatore; D'Agostino, Carmelo; Persaud, Bhagwant
  Year: 2013
  Document Type: Paper
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-4372
  Practice-Ready: Yes
• Investigating Safety Impact of Raised Pavement Markers on Freeways in Louisiana
  Abstract: Raised pavement markers (RPM) are intended as safety devices on roadways. Intuitively convinced by its safety benefits Louisiana Department of Transportation and Development (LADOTD) has been using RPM for many years on all freeways in the state. This paper evaluates the safety benefit of RPM along with pavement striping on freeways with nine years of data. The analysis results from three methods indicate that RPM has significant benefit in reducing nighttime crashes on rural freeways and there are no safety benefits on urban freeways.
  Authors: Das, Subasish; Sun, Xiaoduan; Wang, Fan; Rasel, S.
  Authors: Das, Subasish; Sun, Xiaoduan; Wang, Fan; Rasel, S.
  Year: 2013
  Document Type: Paper
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-4630
  Practice-Ready: Yes
• Influence of Land Use and Driveway Placement on Safety Performance of Arterial Highways
  Abstract: Characterizing driveway safety is a relevant and relatively complex topic in transportation safety research. Current literature proposes many factors to consider for design and evaluation of safety performance of roadside elements. This research studied the safety link of driveways in Oregon highways. This work is based on two probability samples from arterial state highways at both urban and rural designations. The primary goal of this research is to provide alternative safety performance functions (SPFs) to evaluate the safety impacts of various driveway-related configurations in more detail than average driveway density. The statistical models and methodologies in this paper are comparable to those in the Highway Safety Manual (HSM). The proposed models exhibited different ranges effects for urban and rural conditions, but type of land use proved a prominent factor for both models. In addition, the rural model uncovered a safety relationship of clusters of driveways that are within 1.5 seconds or less of each other.
  Authors: Avelar, Raul E.; Dixon, Karen K.; Brown, Lacy; Mecham, Megan; Van Schalkwyk, Ida
  Authors: Avelar, Raul E.; Dixon, Karen K.; Brown, Lacy; Mecham, Megan; Van Schalkwyk, Ida
  Year: 2013
  Document Type: Paper
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-5331
  Practice-Ready: Yes
• Safety Evaluation of Discontinuing Late Night Flash Operations at Signalized Intersections
  Authors: Lan, Bo
  Authors: Lan, Bo
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-0988
• Formulating Informative Priors and Effects on Bayesian Hierarchical Crash Models
  Authors: Yu, Rongjie
  Authors: Yu, Rongjie
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-0222
• Gender Gaps in Crash Data: Statistical Look at Gender and Age Differences as Related to Crash Frequencies
  Authors: Russo, Francesca
  Authors: Russo, Francesca
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-1313
• Monitoring and Analysis of Travel Speeds on the National Road Network Using Floating-Car Technologies
  Authors: Bekhor, Shlomo
  Authors: Bekhor, Shlomo
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-1685
• Calibration Factor with the Consideration of Short-term Trend in Crash Occurrence
  Authors: Banihashemi, Mohamadreza
  Authors: Banihashemi, Mohamadreza
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-2634
• Severity Distribution Function For Freeway Segments
  Authors: Geedipally, Srinivas Reddy
  Authors: Geedipally, Srinivas Reddy
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-2873
• Development of Safety Versus Congestion Relationships for Urban Freeways
  Authors: Potts, Ingrid
  Keywords: poster presentation; poster design; poster template
  Authors: Potts, Ingrid
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-2952
• Spatial Effect on Zone-Level Collision Prediction Model
  Authors: Karim, Md. Ahsanul
  Authors: Karim, Md. Ahsanul
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-3145
• Estimating Annual Average Daily Traffic for Local Roads for Highway Safety Analysis
  Authors: Alluri, Priyanka
  Authors: Alluri, Priyanka
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-3490
• Safety Performance Function Calibration and Development for the State of Alabama: Two-Lane Two-Way Rural Roads and Four-Lane Divided Highways
  Authors: Lou, Yingyan
  Authors: Lou, Yingyan
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-4221
• Investigating Safety Impact of Raised Pavement Markers on Freeways in Louisiana
  Authors: Das, Subasish
  Authors: Das, Subasish
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-4630
• Investigating Safety Impact of Raised Pavement Markers on Freeways in Louisiana
  Authors: Sun, Xiaoduan
  Authors: Sun, Xiaoduan
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-4630
• Evaluation of Remote-Sensing Technologies for Collecting Roadside Feature Data to Support Highway Safety Manual Implementation
  Authors: Gong, Jie
  Authors: Gong, Jie
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-4709
• Full versus Simple Safety Performance Functions: A Comparison Based on Urban Four-Lane Freeway Interchange Influence Areas in Florida
  Authors: Alluri, Priyanka
  Authors: Alluri, Priyanka
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-4828
• Safety of Channelized Right-Turn Lanes for Motor Vehicles and Pedestrians
  Authors: Potts, Ingrid
  Keywords: poster presentation; poster design; poster template
  Authors: Potts, Ingrid
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-4854
• Safety-Effectiveness of Various Types of Shoulders on Rural Two-Lane Roads in Winter and Nonwinter Periods
  Authors: Zeng, Huanghui
  Authors: Zeng, Huanghui
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-4943
• Effect of Removing Freeway Mainline Barrier Toll Plazas on Safety
  Authors: Yang, Hong
  Keywords: Secondary Crash; Traffic Incident; Sensor Data; Incident Management; Highway Operation; Freeway
  Authors: Yang, Hong
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-5001
• International Transferability of Accident Modification Functions for Horizontal Curves
  Authors: Elvik, Rune
  Authors: Elvik, Rune
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-0321
• Safety Effects of Horizontal Curve and Grade Combinations on Rural Two-Lane Roads
  Authors: Bauer, Karin
  Keywords: poster presentation; poster design; poster template
  Authors: Bauer, Karin
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-3056
• Benefit-Cost Analysis Applied to Behavioral and Engineering Safety Countermeasures in San Francisco, California
  Authors: Wemple, Elizabeth
  Authors: Wemple, Elizabeth
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-4805
• Full versus Simple Safety Performance Functions: A Comparison Based on Urban Four-Lane Freeway Interchange Influence Areas in Florida
  Authors: Lu, Jinyan
  Authors: Lu, Jinyan
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-4828
• Safety Performance Functions Reflecting Categorical Impact of Exposure Variables for Freeways
  Authors: Kim, Dong-Kyu
  Authors: Kim, Dong-Kyu
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-3758
• Development of Safety Versus Congestion Relationships for Urban Freeways
  Abstract:

  This paper documents the development of safety/congestion relationships using traffic volume, speed, and crash data by 15-min periods for urban freeways in the Seattle and Minneapolis-St. Paul areas. The safety/congestion relationships were developed to provide a basis for estimating the safety effects of design treatments intended to reduce nonrecurrent congestion and increase travel time reliability. The relationships were developed with traffic volume and speed data from detectors in individual lanes for 564 urban freeway segments during study periods up to three years. Traffic crash records were obtained for the same time periods as the volume and speed data. U-shaped relationships between crash rate per million veh-mi of travel and traffic density, the level of service measure for freeways, were found, with the highest crash rates at low and high traffic densities and the lowest crash rates in the middle range of traffic densities. The high crash rates at lower traffic densities represent predominantly single-vehicle crashes, while the high crash rates at higher traffic densities represent predominantly multiple-vehicle crashes. Quantitative safety/congestion relationships were developed for the range of traffic densities from 20 to 78 passenger cars per hour per lane, corresponding to the level of service range from LOS C to LOS F.

  Authors: Harwood, Douglas W.; Bauer, Karin M.; Potts, Ingrid B.
  Authors: Harwood, Douglas W.; Bauer, Karin M.; Potts, Ingrid B.
  Year: 2013
  Document Type: Paper
  Subject: Safety and Human Factors
  Session: 289
  Paper Number: 13-2952