Papers and Publications by DiExSys^™
Relationship between Traffic Density, Speed and Safety and Its Implication on Setting Variable Speed Limits on Freeways	Speed-flow relationships for a typical basic freeway segment are well understood at present and are documented by the successive editions of the Highway Capacity Manual. All recent freeway studies show that speed on freeways is insensitive to flow in the low to mid range. Increase in flow and density without reduction in speed has a significant influence on safety. Constructive discussion of this influence, however, is largely absent from extant literature. Empirical examination of the relationship between flow/density, speed and crash rate on selected freeways in Colorado suggests that as flow/density increases crash rate initially remains constant until a certain critical threshold combination of speed and density is reached. Once this threshold is exceeded the crash rate rapidly rises. The rise in crash rate may possibly be explained by the fact that compression of flow without notable reduction in speed produces headways so small that it becomes very difficult or impossible to compensate for driver’s error to avoid a crash. In addition to calibrating corridor specific SPFs relating crash rate to hourly volume/density and speed this paper proposes a variable speed limit (VSL) algorithm intended to slow traffic down in real time in advance of a high speed-high density operational regime. Deployment of such an algorithm has the potential to improve safety and reduce travel time variability.
Relationship between Freeway Flow Parameters and Safety and Its Implication on Adding Lanes	The decision to add lanes to a freeway is motivated by the need to relieve congestion. Practicing engineers and planners generally believe that the decreased congestion that results from adding lanes is associated with some degree of improved safety, yet the majority opinion of researchers is that accident rates increase as the number of lanes increases. In more than 70 years of modern road building, these conflicting views have not been reconciled. This paper first examines the relationship of traffic flow parameters, such as volume, density, and speed, with safety by calibrating corridor-specific safety performance functions. On the basis of an understanding of this relationship, a possible explanation of the effect that adding lanes has on safety is formulated. An empirical examination of the relationship of flow, density, and speed to the crash rate on selected freeways in Colorado suggests that, as the flow increases, the crash rate initially remains constant until a certain critical threshold combination of speed and density is reached. Once this threshold is exceeded, the crash rate rapidly rises. The rise in the crash rate may be because an increase in density without a notable reduction in speed produces headways so small that it becomes difficult or impossible to compensate for driver error. This model suggests that, after the construction of additional lanes, crash rates initially decline because of the lower traffic volume and density per lane. However, as development and rerouting occur, freeways with more lanes are expected to have higher crash rates that are attributable to the increased opportunities for lane change–related conflicts.
Relationship between Freeway Flow Parameters and Safety and its Implications on Hard Shoulder Running	Decisions to run traffic on freeway shoulders during peak period are motivated by the need to relieve congestion. It is generally believed by practicing traffic engineers that decreased congestion resulting from hard shoulder running is associated with some unspecified degree of improved safety, yet the majority opinion among researchers is that accident rates increase with increase in the number of lanes even if full shoulders are provided. Despite many years of modern road building these conflicting views have not been reconciled. This paper first examines the relationship of traffic flow parameters such as volume, density, and speed to safety by calibrating corridor specific safety performance functions. On the basis of understanding this relationship a possible explanation of the effect of hard shoulder running on safety is formulated. Empirical examination of the relationship of flow, density, and speed to the crash rate on selected freeways in Colorado suggests that, as flow increases crash rate initially remains constant until a certain critical threshold combination of speed and density is reached. Once this threshold is exceeded, the crash rate rapidly rises. The rise in crash rate may possibly be explained by the fact that increase in density without notable reduction in speed produces headways so small that it becomes very difficult or impossible to compensate for driver’s error. This model suggests that during hard shoulder running crash rates decline because of lower traffic volume/density per lane. It also suggests that safety benefits of reducing volume or density per lane outweigh adverse effects of not providing a full shoulder.
Relating Flow, Speed, and Density of Urban Freeways to Functional Form of a Safety Performance Function 2011 Paper of the Year Award	Constructive discussion of the appropriate choice for the functional form of safety performance functions (SPFs) is generally absent from research literature on road safety. Among researchers who develop SPFs, there appears to be a consensus that the underlying randomness in accident counts is well described by the negative binomial (NB) distribution. The underlying phenomenon itself, however, is not well understood and is rarely discussed. The choice of the regression equation is usually not explained or documented. Researchers most commonly use the power function, possibly because most generalized linear modeling (GLM) statistical packages can accommodate the power function with little effort. The modeling process, however statistically rigorous, at times seems disconnected from the physical phenomenon that it is trying to describe. The disconnect, however, has attracted only limited interest from researchers to date. Accidents on an urban freeway are a by-product of traffic flow; therefore, changes in the flow parameters may give clues about the probability of accident occurrence and changes in accident frequency. This study related traffic flow parameters, such as speed and density, to the choice of the functional form of the SPF. It compared SPF models for urban freeways developed with sigmoid and exponential functional forms with the use of data from Colorado and California and contrasted the cumulative residual (CURE) plots of the models. SPFs developed around a sigmoid functional form through the use of neural network (NN) methodology suggested underlying relationships between safety and traffic flow characteristics. CURE plots for NN-generated SPFs generally showed a better-quality model fit when compared with power-function SPFs, which were developed in the GLM framework with an NB error structure.
Relationships Between Safety and Both Congestion and Number of Lanes on Urban Freeways	This paper first explores the relationship between safety and congestion and then examines the relationship between safety and the number of lanes on urban freeways. The relationship between safety and congestion on urban freeways was explored using Safety Performance Functions (SPF) calibrated for multilane freeways in Colorado, California and Texas. The focus of most SPF modeling efforts to date has been on the statistical technique and the underlying probability distribution with only a limited consideration given to the nature of the phenomenon itself. In this study Neural Networks have been used to identify the underlying relationship between safety and exposure. The modeling process was informed by the consideration of the traffic operations parameters described by the Highway Capacity Manual (HCM). The shape of the SPF is best described by a sigmoid reflecting dose-response-like relationship between safety and traffic demand on urban freeways. Relating safety to the degree of congestion suggests that safety deteriorates with the degradation in the quality of service expressed in terms of the Level of Service (LOS). It is generally believed by the practitioners that additional capacity afforded by additional lanes is associated with more safety. How much safety and for what time period is generally not considered. Comparison of the Safety Performance Functions (SPF) of multilane freeways suggests that adding lanes may initially result in a temporary safety improvement that disappears as congestion increases. As Annual Average Daily Traffic (AADT) increases, the slope of SPF, described by its first derivative, becomes steeper, reflecting that accidents are increasing at a faster rate than would be expected from a freeway with fewer lanes.
Level of Service of Safety Conceptual Blueprint and Analytical Framework	While some initial and significant progress has been made in the development of a highway safety manual, much remains to be done in the areas of conceptual development and the diagnostics of safety problems. The concept of level of service of safety (LOSS) in the framework of safety performance function is introduced, and the problem of diagnostics is addressed. LOSS reflects how the roadway segment is performing in regard to its expected accident frequency and severity at a specific level of annual average daily traffic. It provides a comparison of accident frequency and severity only with the expected norms; it does not, however, provide any information related to the nature of the safety problem itself. If the safety problem is present, LOSS will describe only its magnitude. The nature of the problem is determined through diagnostic analysis by direct diagnostics and pattern recognition techniques, which are also discussed.
Explicit Consideration of Safety in Transportation Planning and Project Scoping	The Transportation Equity Act for the 21st Century of 1998 required explicit consideration of safety in the transportation planning process. Although this government mandate is well intentioned, little is known about how to accomplish it. Despite 60 years of modern road building, there is still no consensus among transportation professionals about how to quantify the degree of safety or lack of safety of an existing transportation facility. It is even more difficult to anticipate the level of safety on highways not yet built. A methodology for the explicit consideration of safety in the transportation planning process is presented, followed by a review of two case histories illustrating its application.
How Best to Rank Sites with Promise	The starting point for a program of local safety improvements is the preparation of a list of sites with promise at which to conduct a detailed engineering study, by which cost-effective projects can be identified. Inclusion on the list depends on the criteria used for ranking sites. Five alternative ranking criteria were compared by the cost-effectiveness of the projects to which they lead. It was found that sites at which most accidents or most severity-weighed accidents are expected lead to most cost-effective projects.
Identifying Locations with Potential for Accident Reductions Use of Direct Diagnostics and Pattern Recognition Methodologies	Safety performance functions reflect the complex relationship between exposure, usually measured in annual average daily traffic, and accident count for a unit of road section over a unit of time. One of the main uses of the safety performance functions is to identify locations that experience more accidents than expected, thus exhibiting a potential for accident reduction. Overrepresentation in the number of accidents above the expected or normal threshold predicted by the safety performance function is only one of many indicators of a potential for accident reduction. Accident type, severity, road condition, spatial distribution of accidents, and lighting conditions are only a few of the many important symptoms of the accident problem. Two methodologies are introduced for identification of locations with potential for accident reduction: direct diagnostics and continuous pattern recognition analysis. Use of these methodologies revealed that existence of accident patterns susceptible to correction may or may not be accompanied by the overrepresentation in accident frequency detected by the safety performance functions.
Diagnostic Methodology for the Detection of Safety Problems at Intersections	There is an established consensus among traffic safety researchers that a nonlinear relationship exists between traffic exposure and safety. This relationship is reflected by the safety performance functions (SPFs) calibrated for various classes of roads and intersections. One of the main uses of SPFs is to identify locations with potential for accident reduction. While this application is certainly important, the use of SPFs provides no information related to the nature of the accident occurrence. Without being able to relate accident frequency and severity to roadway geometrics, traffic control devices, roadside features, roadway condition, driver behavior, or vehicle type, it is not possible to develop effective countermeasures. A methodology was developed to provide guidance in diagnostics of safety problems, recognition of accident patterns, and development of appropriate countermeasures. Considering that traffic accidents can be viewed as random Bernoulli trials, it is possible to detect deviation from the statistical process by computing observed cumulative probability for each of the accident characteristics. Detection of an accident pattern at an intersection suggests the presence of an element in the roadway environment that triggered a deviation from a random statistical process in the direction of reduced safety. Identification of such an element always provides a critical clue to accident causality.
Screening the Road Network for Sites with Promise	Network screening is the first step in the site safety improvement process. The product of network screening is a list of sites that are ranked by priority for the conduct of detailed engineering studies. In turn, cost effective projects are formulated from the studies. With the purpose of laying the foundation for improved network screening, the role of network screening is clarified, and how project cost and safety benefit can be anticipated at the time of screening is examined. The strengths and weaknesses of alternative assumptions on which the anticipation of safety benefit can be based are discussed. A way to guard against misallocation of resources due to the randomness of accident counts is suggested, and a method for finding peak sites within road sections is proposed.
Risk Analysis of Freeway Lane Closure During Peak Hour	This paper examines risks associated with peak period lane closure during construction or maintenance work on urban freeways. In accordance with recently implemented policy by the Colorado Department of Transportation, lane closure would be allowed if reserve capacity were available. A relatively minor accident in the work zone caused substantial delays during the peak period that virtually paralyzed traffic in the Denver, Colorado, metropolitan area. This occurrence caused reexamination of the existing lane closure policy. Generally speaking, if a contractor is allowed greater flexibility in establishing work schedules, including the ability to work through peak periods, a lower bid can be expected. This paper compares savings in the cost of construction related to allowing lane closure during peak periods with the cost of potential incident-related delays in the framework of a quantitative risk analysis.
Exploratory Analysis of Relationship between the Number of Lanes and Safety on Urban Freeways Accepted for Presentation at 2008 TRB Annual Meeting	The relationship between freeway capacity and the number of lanes is reasonably well understood at present. In contrast to capacity, the relationship between the number of lanes and safety is not fully understood or systematically considered when planning capacity improvements. During the planning and design phase the discussion is centered on the degree to which design alternatives comply with geometric design standards and what Level of Service (LOS) is provided. It is generally believed by the practitioners that additional capacity afforded by additional lanes is associated with more safety. How much safety and for what time period is generally not considered. Exploratory analysis of the Safety Performance Functions (SPF) for multilane freeways in Colorado, California and Texas suggests that adding lanes may initially result in a temporary safety improvement that disappears as congestion increases. As Annual Average Daily Traffic (AADT) increases, the slope of SPF, described by its first derivative, becomes steeper, reflecting that accidents are increasing at a faster rate than would be expected from a freeway with fewer lanes. This may possibly be explained by the fact that increase in the number of lanes is associated with increase in the number of potential lane-change-related conflict opportunities. Understanding of the relationship between number of lanes and safety should be used to inform the public involvement process in evaluating and selecting design alternatives. Additionally, high AADT on multilane freeways is associated with high crash frequency and consequently reduced mobility.
Exploratory Examination of the Functional Form of Safety Performance Functions of Urban Freeways Accepted for Presentation at 2008 TRB Annual Meeting	Safety Performance Functions are accident prediction models that relate traffic exposure, measured in Annual Average Daily Traffic (AADT) to safety, measured in the annual number of accidents per mile (accidents/mile per year). Review of literature on the development of Safety Performance Functions (SPF) suggests that the focus of most modeling efforts is on the statistical technique and the underlying probability distribution with only a limited consideration given to the nature of the phenomenon itself. In this study Neural Networks have been used to identify the underlying relationship between safety and exposure. The modeling process was informed by the consideration of the traffic operations parameters described by the Highway Capacity Manual (HCM). The shape of the SPF is best described by a sigmoid reflecting dose response - like relationship between safety and traffic demand on urban freeways. We observed that on un-congested segments the number of crashes increases only moderately with increase in traffic; however, once some critical traffic density is reached, the number of crashes begins to increase at a much faster rate with increase in traffic. This phenomenon is reflected by a steeper gradient of the SPF. Further examination of the SPF suggests that on segments with high AADT (LOS-F during peak period), the function begins to level off, reflecting decrease in accident rates related to a high degree of congestion and significant reduction in operating speeds. Relating safety to the degree of congestion suggests that safety deteriorates with the degradation in the quality of service expressed in terms of the Level of Service (LOS).
Safety Planning Study of an Urban Freeway - Proposed Methodology and Review of the Case History	Safety-conscious planning is a relatively new concept. It was developed in response to safety related provisions of the Transportation Equity Act for the 21st Century (TEA-21) of 1998 that required explicit consideration of safety in the transportation planning process. This paper will revisit the problem of using accident rates in transportation planning and review a case history of applying safety-conscious planning methods by the Colorado Department of Transportation (CDOT). A two-phase process that has been used to evaluate the safety impacts of multiple design alternatives is introduced. The evaluation process is based on the available Safety Performance Functions calibrated specifically for urban freeways in concert with diagnostic investigations, pattern recognition analysis and detailed accident diagramming. The critical importance of accident diagramming is discussed in reference to examining safety history at complex interchange locations.