Showing papers by "Dominique Lord published in 2004"

Crash analysis of selected high-occupancy vehicle facilities in texas: methodology, findings, and recommendations

Abstract: In Texas, high-occupancy vehicle (HOV) lanes have emerged as an integral part of the state's current and future transportation system to aid urban mobility. As a result, the issue of HOV lane design and the influence of design on safety has become the focus of much attention in the transportation community. The objective of this research was to develop a better understanding of the safety issues associated with HOV lanes, particularly buffer-separated concurrent flow HOV lanes. The research team increased their understanding of these issues by surveying transportation professionals from across the United States on the topic of HOV lane safety for barrier-separated and buffer-separated facilities. Electronic crash data were analyzed and crash reports from Dallas, Texas, were reviewed to determine crash characteristics both before and after HOV lane implementation in selected corridors. The analysis of the IH-30 corridor with a moveable barrier-separated contraflow HOV lane did not indicate a change in injury crash occurrence. The IH-35E North and IH-635 corridors with buffer-separated, concurrent flow HOV lanes did show a change in crash occurrence with an increase in injury crash rates. Based on the key findings of the crash data analysis, the research team developed guidance for design when implementing HOV lanes similar to those in operation in Dallas, Texas. This guidance indicates desirable corridor characteristics when considering HOV lane implementation and recommends roadway cross-sections.

Statistical challenges with modeling motor vehicle crashes: Understanding the implications of alternative approaches

Abstract: There has been considerable research conducted over the last 20 years focused on predicting motor vehicle crashes on transportation facilities. The range of statistical models commonly applied includes binomial, Poisson, Poisson-gamma (or Negative Binomial), Zero-Inflated Poisson and Negative Binomial Models (ZIP and ZINB), and Multinomial probability models. Given the range of possible modeling approaches and the host of assumptions with each modeling approach, making an intelligent choice for modeling motor vehicle crash data is difficult at best. There is little discussion in the literature comparing different statistical modeling approaches, identifying which statistical models are most appropriate for modeling crash data, and providing a strong justification from basic crash principles. In recent years, for example, it has been suggested that the motor vehicle crash process can successfully be modeled by assuming a dual-state data generating process, which implies that entities (e.g., intersections, road segments, pedestrian crossings, etc.) exist in one of two states—perfectly safe and unsafe. As a result the ZIP and ZINB are two models that have been applied to account for the preponderance of “excess” zeros frequently observed in crash count data. The objective of this study is to provide defensible guidance on how to appropriate model crash data. We first examine the motor vehicle crash process using theoretical principles and a basic understanding of the crash process. It is shown that the fundamental crash process follows a Bernoulli trial with unequal probability of independent events, also known as Poisson trials. We examine the evolution of statistical models as they apply to the motor vehicle crash process, and indicate how well they statistically approximate the crash process. We also present the theory behind dual-state process count models, and note why they have become popular for modeling crash data. A simulation experiment is then conducted to demonstrate how crash data give rise to “excess” zeroes frequently observed in crash data. It is shown that the Poisson and other mixed probabilistic structures are approximations assumed for modeling the motor vehicle crash process. Furthermore, it is demonstrated that under certain (fairly common) circumstances excess zeroes are observed—and that these circumstances arise from low exposure and/or inappropriate selection of time/space scales and not an underlying dual state process. In conclusion, carefully selecting the time/space scales for analysis, including an improved set of explanatory variables and/or unobserved heterogeneity effects in count regression models, or applying small area statistical methods (observations with low exposure) represent the most defensible modeling approaches for datasets with a preponderance of zeros.

Guidance for future design of freeways with high- occupancy vehicle (hov) lanes based on an analysis of crash data from dallas, texas

Abstract: In Texas, high-occupancy vehicle (HOV) lanes have emerged as an integral part of the state's current and future transportation system to aid urban mobility. As a result, the issue of HOV lane design and the influence of design on safety has become the focus of much attention in the transportation community. This document provides guidance for future design of freeways with HOV lanes based on an analysis of crash data from Dallas, Texas. This guidance indicates desirable corridor characteristics when considering HOV lane implementation and recommends roadway cross-sections when implementing HOV lanes similar to those in operation in Dallas, Texas.

Development of ramp design procedures for facilities without frontage roads

Abstract: Based on a recent change in Texas Department of Transportation (TxDOT) policy, frontage roads are not to be included along controlled-access highways unless a study indicates that the frontage road improves safety, improves operations, lowers overall facility costs, or provides essential access. Interchange design options that do not include frontage roads are to be considered for all new freeway construction. Ramps in non-frontage-road settings can be more challenging to design than those in frontage-road settings for several reasons. Adequate ramp length, appropriate horizontal and vertical curvature, and flaring to increase storage area at the crossroad intersection should all be used to design safe and efficient ramps for non-frontage-road settings. However, current design procedures available in standard TxDOT reference documents focus on ramp design for frontage-road settings. The objective of this research project was to develop recommended design procedures for interchange ramps on facilities without frontage roads. This report describes the findings from the second year of a two-year project. Crash data from Texas interchanges were used to calibrate several ramp safety prediction models. These models form the basis for a procedure for evaluating the safety of alternative ramp configurations. Ramp design guidelines are developed. Design controls and elements routinely considered during the ramp design process are identified. The report titled "Recommended Ramp Design Procedures for Facilities without Frontage Roads" presents this guidance in a manner suitable for application by designers.

Developing an In-Service Performance Evaluation (ISPE) for Roadside Safety Features in Texas

Abstract: Current methodologies for the in-service performance evaluation (ISPE) of roadside safety features are not viable and practical for all state departments of transportation. The research team developed an ISPE process that is sensitive to the data and resource constraints of Texas Department of Transportation (TxDOT). The following methodology was used to develop The TxDOT-ISPE process: identify the objectives of the ISPE, estimate expected extent of data collection, evaluate existing DOT procedures and organizational structure, identify features to be included in the ISPE, develop framework for an ideal ISPE, identify system constraints within the DOT, identify alternative methodologies and data sources for the ISPE, design DOT-specific ISPE process, design and conduct a pilot test of the ISPE, evaluate the data collection methodology and data quality during the pilot test, and recommend an ISPE process for TxDOT. A two-phase ISPE process was developed and tested to meet the specific needs of TxDOT. In Phase I, the data collection form included basic information such as accident date, location, whether the vehicle rolled over or not, whether the crash was fatal or not, the specific device that was hit, and an evaluation of whether the system performed as intended or not. If the Phase I process identified a device having a high rate of failure, a Phase II investigation of that device was recommended. Phase II required detailed information regarding the layout and specific features of the particular device that was impacted, and a detailed investigation into the impact performance of the device. ISPE site inspection forms and training materials along with a recommended ISPE procedure were prepared as part of the project.

Improving safety at signalized intersections near vertical curves

Abstract: The research described herein documents an investigation into countermeasures that can be used to provide motorists with advance notification of traffic signals or queues from those signals that are located beyond the motorists' line of sight due to a crest vertical curve. Whereas standard roadway design ensures that stopping sight distance (SSD) is provided at all locations along a roadway, there is no standard established for when decision sight distance (DSD) is needed with respect to traffic signals. A "reduced decision zone" (RDZ) was identified in the research as the location along a roadway with a vertical curve and a traffic signal beyond the curve where SSD is provided but DSD is not. Essentially, motorists within the RDZ are provided with SSD for unexpected stopping but are not provided with the added decision-making and response time that DSD might otherwise provide as they approach the vertical curve and the downstream traffic signal. Contained within this report are techniques for determining whether an RDZ exists along an existing roadway or has the potential to exist in a proposed design. It is suggested that intersections not be located within the RDZ. In cases where a traffic signal or queue from a signal is located within the RDZ for an existing roadway, guidance is provided on both the type and location of countermeasures that can be used. Results of field tests conducted using both the suggested type and location of advance warning sign countermeasures are included within this report and its appendix.