Showing papers in "Transportation research circular in 1998"

Relationship between roundabout geometry and accident rates

Abstract: This paper describes relationships between roundabout geometry and accident rates for one hundred roundabouts from throughout Queensland, Australia. Regression analyses are undertaken on "single vehicle," "approaching rear end," and "entering/circulating" accident categories. After various trials, it was found to be most important that accident models incorporate appropriate explanatory variables. The models described here were based on the concept of exposure and propensity, and on observed driver behavior. The driver behavior model developed in this study uses the 85th percentile speeds on each geometric element and the location of vehicle paths through the roundabout. The single-vehicle accident model includes traffic flow, length of driver path on the geometric element, 85th percentile speed on the previous geometric element, and radius of the geometric element. This model demonstrates the importance of limiting the difference between the expected drivers' speeds through successive geometric elements. The approaching rear-end vehicle accident model includes approaching and circulating traffic flow, and approach speed. This model demonstrates the importance of limiting the approach speed. Similarly, the entering/circulating vehicle accident model includes approaching and circulating traffic flow, and relative speed between entering and circulating vehicles. This model demonstrates the need to minimize the relative speed between entering and circulating vehicles. A brief discussion on the application of this research to other intersection types and roadways is given.

Open-graded friction course: state of the practice

Abstract: Open-graded friction courses (OGFCs) have been used since 1950 in different parts of the United States to improve the frictional resistance of asphalt pavements However, the experience of states with this kind of mix has been widely varied This circular presents the results of a survey carried out by the National Center for Asphalt Technology (NCAT) on design and construction practices for OGFCs Briefly, it was found that significant improvements have been observed in the performance of OGFCs since their introduction in the 1950s Half of the states surveyed in this study indicated good experience with OGFCs More than 70% of the states that use OGFCs reported service life of 8 or more years About 80% of the states using OGFCs have standard specifications for design and construction The vast majority of states reporting good experience use polymer-modified asphalt binders Also, gradations of aggregates used by these states tend to be somewhat coarser than gradations used earlier and gradations used by the states that had bad experience with OGFCs It seems that good design and construction practice is the key to improved performance of OGFC mixes An improved mix design procedure is needed to help the states adopt these good practices

Highway capacity and induced travel: issues, evidence and implications

Abstract: This paper objectively addresses the induced travel issue by answering the following questions: First, if new highway capacity does indeed "induce" new travel, to what extent compared to other factors? Second, if new highway capacity (as one of many factors) induces new travel, how should induced traffic be accounted for in the objective evaluation of highway investments? For the purposes of this paper, induced travel is defined as any increase in daily travel (measured as passenger or vehicle miles of travel) resulting from a change in the transportation system. It is concluded that the role of highway capacity expansion in increasing highway travel has been small relative to other factors. Highway capacity expansion interacts with far more important variables such as population, household and employment growth, personal income and auto ownership increases, regional economic growth and fuel price changes as determinants of total travel demand. Planning processes must have the capability to objectively examine a thoughtful combination of strategies to assure that, while the population of a region grows, its access and mobility needs are sustained and its environmental quality is enhanced. Metropolitan areas are encouraged to consider combinations of synergistic strategies, including: balanced investment, covering both capacity expansion and better system operation and management for all modes; alternative urban forms and urban design; and appropriate pricing to maximize transportation system efficiency. The major determinants of travel demand are clearly socio-economic in nature. However, the inducement of travel due to any transportation system change is an issue that needs to be and can be addressed by considering behavioral and land use change mechanisms.

A new stopping sight distance model for use in highway geometric design

Abstract: Stopping sight distance is an important design parameter in that it defines the minimum sight distance that must be provided at all points along the highway. Thus, it influences geometric design values, construction costs, and highway safety. Stopping sight distance is defined as the sum of two components - brake reaction distance and the braking distance. The basic model for calculating stopping sight distances was formalized in 1940, and the model's parameters have been altered to compensate for changes in eye height, object height, and driver behavior over the past 50 years. Recent studies, however, question whether the model's parameters and assumptions represent real-world conditions. This paper presents a new model for determining stopping sight distance requirements for geometric design of highways. The new model is based on parameters describing driver and vehicle capabilities that can be validated with field data and defended as safe driving behavior. More than 50 drivers, 3,000 braking maneuvers, 1,000 driver eye heights, and 1,000 accident narratives were used in developing the recommended parameter values for the new model. The recommended values are attainable by most drivers, vehicles, and roadways. The new model results in stopping sight distances, sag vertical curve lengths, and lateral clearances that are between the current minimum and desirable requirements, and crest vertical curve lengths that are shorter than current minimum requirements.

A practical safety approach to highway geometric design international case studies: germany, greece, lebanon, and the united states

Abstract: A practical procedure, which considers safety rules and criteria for the safety evaluation of new designs, redesigns, and restoration, rehabilitation or resurfacing (RRR) projects, is presented in this paper The procedure is based on statistical investigations of speeds and design parameters in Europe, the Middle East, and North America The main components of the procedure are: operating speed backgrounds, relation design backgrounds, skid-resistance backgrounds, and driving dynamic backgrounds, for tangents, curves, and transitions to curves, for different roadway types and topography classes The procedure presented in this paper provides interrelationships between design parameters, driving behavior, and driving dynamics, in order to determine sound roadway alignments and/or to detect poor ones, and to positively influence the accident situation

Effects of increased highway capacity: results of a household travel behavior survey

Abstract: Few current transportation issues engender more controversy than the effects of adding new highway capacity on traffic and travel demand. The purpose of adding new highway capacity is to reduce traffic congestion and improve automobile travel times, and in some cases, air quality. These changes in turn affect travel behavior by affecting peoples' choice of modes of travel, their choice of destination, and their choice of travel route. Less well known is how travel time changes caused by capacity increases may affect total travel demand, especially trip generation. Estimating the magnitude of this effect on trip generation is particularly unclear. One of the primary purposes of this project was to examine the effects of new capacity on trip generation, since in most conventional North American travel forecasting models, trip generation is not sensitive to transportation supply variables. A travel behavior survey was developed and administered to isolate the various effects of new highway capacity and identify those not currently treated by conventional travel forecasting models. The survey and its results are described in this paper.

An operational and safety evaluation of alternative horizontal curve design approaches on rural two-lane highways

Abstract: This paper evaluates the effects of superelevation on 85th percentile speeds and accident experience as well as the effects of side friction demand on accident experience at horizontal curves on rural two-lane highways. These evaluations were conducted as part of research toward development of a design consistency evaluation model for the United States. The operating speed analysis verified previous models that used degree of curvature, length of curve, and deflection angle as independent variables for estimating 85th percentile speed on curves. The analysis also found superelevation to be a statistically significant independent variable. Independent variables in the accident analysis included degree of curvature, operating speed reduction, superelevation deficiency, and implied side friction demand. Operating speed reduction and superelevation deficiency were found to be significant accident predictors; however, implied side friction demand was the strongest accident surrogate. Comparisons of alternative horizontal curve design methods, with respect to which speed should be used for the design of curves, were made. The 85th percentile speed on a curve was the strongest performer of four curve design ideologies and is recommended for use in horizontal curve design. Superelevation has significant effects on 85th percentile speed on rural two-lane horizontal curves. Operating speed reduction, superelevation deficiency and side friction demand based on 85th percentile operating speeds have significant effects on the safety of horizontal curves. These findings provide further support of the adoption of an operating-speed based design procedure for two-lane rural highways in the United States.

Overview of cross section design elements

Abstract: This overview paper provided an introduction to the symposium session devoted to cross section elements. In providing an overview of this important topic, the paper addressed the following components: a brief discussion of the most prominent cross section elements; the results of a limited international survey to identify representatitve parameters and procedures for these elements; an introduction to roadside design, with an emphasis on safety and the clear recovery area concept; a synopsis of each of the papers that was presented during this session; and a summary emphasizing the emerging cross section design issues and research needs.

International sight distance design practices

Abstract: This paper reviews the geometric design practices related to sight distance of a number of countries The purpose of this paper is to present the sight distance design practices of a variety of countries as a resource to highway agencies in any country that may be considering possible modifications and updates to their own policies and practices It is hoped that this paper will serve as a resource presenting ideas and concepts that may be new to some, but are in actual use elsewhere in the world International exchanges of this type of information are valuable in that they provide practicing engineers and researchers with a perspective that goes beyond their own country and their own part of the world The ability to see ahead and observe potentially conflicting traffic is critical to safe highway operations Sight distance, an important element in the geometric design of highways, refers to the length of roadway over which a driver has an unobstructed view This paper deals with the criteria used in geometric design for three key aspects of sight distance: (1) stopping sight distance; (2) passing sight distance; and (3) intersection sight distance Each of these types of sight distance is discussed in this paper

A study to improve the safety of expressways by desirable combinations of geometric alignments

Abstract: Most of the information that drivers require is acquired through their eyes. Therefore, for safe high-speed driving, it is important to obtain accurate visual information about the road and its geometric features. There are, however, some cases in which the visual information is perceived incorrectly by drivers and this is one cause of accidents when complicated alignments are used. Two experiments have been conducted to determine which combinations of horizontal and/or vertical alignments would cause such visual illusions to be experienced on expressways. First, expressway patrol drivers who drive every day over a certain section of an expressway were questioned to determine if they experienced such illusions while they were driving there. Secondly, several ordinary drivers were asked to report everything that they saw while they were driving the same sections of the expressway. The drivers were being recorded continuously by a video camera. Analyzing the relationships between the locations and illusions, and the combinations of horizontal and/or vertical alignments, a few facts were derived in regard to alignments and a certain type of illusion. An attempt was also made to determine the relationship between the occurrence of visual illusions and traffic accidents by surveying traffic accident records. However, it does not seem that there is any significant correlation between the occurrence of illusions and the type and the likelihood of traffic accidents.

The relationship between geometric design standards and safety

Abstract: Geometric design standards are mainly based on logically derived relationships and engineering judgements which are seldom validated by accident studies. Consequently, it is difficult to assess the likely safety consequences of departures from standards. This paper compares the results of studies in different countries and summarizes current international knowledge of the relationships between safety and the principal non-intersection geometric design parameters. In general, there is broad international agreement on these relationships. Many studies have attempted to relate geometric elements and accident rates, but only a limited amount of reliable information is available and quantifying the safety impact of marginal changes in the values of geometric design parameters is difficult. It appears that significant changes in the value of many geometric design standard elements are unlikely to result in large increases in accident rates and it is concluded that the available international information provides a good indication of the differences in accidents which would result from departures from design standards or from alternative route alignments.

The traffic inducement effect: its meaning and measurement

Abstract: The "traffic inducement effect" of road improvements in urban areas is the subject of continuing controversy. Whether, to what extent, and under what conditions adding road capacity engenders traffic growth are, on the surface, empirical questions, but they have strong ideological overtones. These derive in part from the salience of the question to fundamental, highly contentious, issues in highway policy. The ideological dimension is enhanced because definitive answers to these questions have proved illusive, a consequence of our inability to conduct the relevant controlled experiments. This paper seeks to contribute to the ongoing debate about the relationship between road supply and traffic in several ways. First, the paper reflects upon the policy context of the debate. Second, it seeks to make the questions in dispute more precise by defining metrics that capture the impact of road supply on road traffic. Third, the paper reports on research that has attempted to measure these impacts. Fourth, and finally, the paper offers recommendations for improving our ability to monitor and document on an ongoing basis how road improvements, and perhaps other transportation investments as well, influence traffic and travel in urban regions.

Driving behavior and driver mental workload as criteria of highway geometric design quality

Abstract: The purpose of this study was to quantify the mental workload requirements and the limitations of human information processing capabilities of drivers in modern road traffic. In-car measurements using an instrumented car enable a detailed analysis of the behavior of the individual driver. The perceptible information of the outside environment (roadway, roadside environment) can be analyzed by measurements with video techniques. Such information was considered important in determining certain curve design criteria which would lead to effective safety and operational improvement at current curve sites. The data analyzed in this study included geometric, operational and physiological measures on road sections with different levels of difficulty. It was found that as curves become sharper, there is a proportionally greater increase in speed reduction and galvanic skin reaction. Other physiological measures, such as eye blink rate and variance of heart rate have shown a decrease in such curves. The study presents also an approach to combine operating-speed-based measurements and workload-based measurements for evaluating highway geometric design quality. The correspondence between the results of both measurements suggests that drivers oriented their driving behavior (e.g., speed, lateral guidance) besides others on their mental workload level. Appropriate curve design proposals for guidelines are discussed which may help to minimize potentially accident-related trends.

Worldwide review of alignment design policies

Abstract: This paper reviews highway alignment design policies and practices in a sample of countries throughout the world. The goal is to broaden our understanding and perspective of alignment design by highlighting similarities and differences between design philosophies and quantitative guidelines. It is hoped that accomplishing this goal will stimulate the continued improvement of highway geometric design. The information presented in this paper combines and builds upon recent studies in the United Kingdom, United States, and Germany that included reviews of alignment design policies and practices in a sample of countries. It was generally observed that there are many similarities in fundamental alignment design principles and philosophies and in quantitative guidelines on basic design parameters. These similarities may reinforce the reasonableness of countries' guidelines that fall within the norm of worldwide practice. What may be most interesting and important, however, are differences in policy emphasis and concern that have led to more advanced guidelines on certain geometric elements, and differences in local conditions and experience that have led to deviations from apparent worldwide norms for certain quantitative guidelines. It is the intent of this paper to gain insight by understanding the reasons for differences rather than to make judgments about deviations from the norm. Some countries combine their policies for rural and urban streets, whereas other countries have separate policies. This review focuses on alignment design for rural roadways. The review is divided into two major sections: first, a qualitative comparison of alignment design philosophies; and second, a quantitative comparison of design guidelines. The paper concludes with general observations about worldwide alignment design policy and practice.

A comparison and evaluation of the geometric design practices with passing lanes, wide-paved shoulders and extra-wide two-lane highways in canada and germany

Abstract: Passing lanes (also known as three-lane highways), wide paved shoulders, and extra-wide highways are designed to enhance traffic operations on two-lane highways. These low-cost improvements bridge the gap between conventional two-lane highways and four-lane highways. This paper compares design practices and policies as well as the operational experience regarding passing lanes, wide paved shoulders, and extra-wide two-lane highways. Canadian and German warrants for upgrading a two-lane highway to a passing lane system and ultimately a four-lane motorway are examined. Geometric design standards for cross-section elements such as lane width and shoulder width are compared. Design standards for the length and spacing of passing lanes as well as the length of merge and diverge tapers are examined. Design practices and policies concerning transition areas between, for example, conventional two-lane and extra-wide highways are discussed. The signing and marking practices for passing lanes, extra-wide highways and transition areas are also included in the paper. The paper discusses the operational experience in terms of overtaking rates, speed-volume relationships, platooning, level-of-service, accident rates, and driver attitudes. Research studies in both Canada and Germany show that passing lanes, wide-paved shoulders, and extra-wide highways increase the quality of service and decrease the accident rates compared to conventional two-lane highways. Both Canadian and German studies have concluded that passing lanes and extra-wide highways offer a cost-effective alternative of enhancing the quality of service and safety on those two-lane highways that do not meet the warrants for motorways or where cost and environmental concerns rule out the four lane option.

Review of international design speed practices in roadway geometric design

Abstract: Use of the classical design speed concept as a criterion for alignment consistency on rural highways originated in the United States in the 1930s in response to increasing accident rates at horizontal curves. As design practice and driver behavior have evolved, the concept has lost effectiveness at producing consistent alignments. The objective of this paper is to assess how design speed is being used in the United States and other countries internationally. Both a review of current design policies and a survey of transportation professionals from 18 countries were used to develop the evaluation.

Influence of vehicle parameters on horizontal curve design of rural highways

Abstract: The influence of several vehicle parameters (ie, vehicle weight, position of gravity center and type of wheel drive) was investigated for their impact on the curve design of rural highways; their impact has appeared to be significant for some combinations of horizontal curve radius, grade and speed when the vehicle motion in the driving mode has been examined The investigation was based on a total of 52 different passenger car models; 3 representative two-axle truck models; and one tractor semi-trailer The results, compared with the corresponding values for curve design accepted by the AASHTO-90 Design Policy (USA) and the RAS-L-95 guidelines (Germany) revealed that practically in most cases a cornering complying to the comfort or safety levels accepted by them is warranted, when the operating speed (85th percentile speed) coincides with the design speed and the positive grade is low However, in other cases, considerable deficiencies were discovered especially in high design speed levels and/or high positive grades

Operating speed approach to geometric design of low-speed urban streets

Abstract: Design speed is one tool used to attain a consistent design. It is also the most basic, fundamental criterion in highway and street design. The selection of design speed has evolved to be a method used for the design and correlation of design elements. This approach is reasonable in concept, but research has documented discrepancies between design speed and actual vehicle operating speed on existing roadways. The American Association of State Highway and Transportation Officials (AASHTO) Design Policy presents the design consistency approach. The determination of horizontal and vertical alignment is based on the selected design speed. For a given design speed, the AASHTO Design Policy presents design values for geometric elements such as minimum curve radius, stopping sight distance, and length of vertical curve. Designing geometric features using the same design speed should produce a segment of roadway with like characteristics. However, there are deficiencies in this design speed approach, especially when applied in the low-speed environment. This paper presents the analysis and interim findings from a Federal Highway Administration study on the "Relationship of Operating Speeds to Roadway Geometric Design Speeds." Under this study, data were collected on low-speed urban streets with varying roadway, roadside, and driver/vehicle characteristics. The objective of the study was to develop an "operating speed approach" that would result in design criteria yielding vehicle operating speeds more consistent with the intended vehicle speeds.

Safety effects of cross-section design on rural multi-lane highways

Abstract: Over 35,000 mi (56,315 km) of arterial highways in the United States are multi-lane, non-Interstate roads in rural areas. Fatality rates on rural federal-aid primary highways have been significantly higher compared to the fatality rates for urban and rural Interstate highways and urban primary highways. Unfortunately, very little is known concerning the effects of geometric design elements on the safety for rural, multi-lane, non-freeway highways since little past research has concentrated on these roads. This paper presents a study of the effects of the various cross-section related design elements on the frequency of accidents for rural, multi-lane, non-freeway roads. Data extracted from the Highway Safety Information System (HSIS) for four states were utilized for data exploration and descriptive analysis. Minnesota data were used for a statistical modeling due to the availability of accident, traffic, roadway inventory and supplemental inventory data for the selected data elements. Supplemental roadway variables which were needed included roadside condition and intersection/driveway access points. To collect those supplemental data elements, an advanced Photolog Laser Videodisc (PLV) data recording system was developed and applied for the study. These data were integrated into the HSIS database for the modeling analysis. The objective of the statistical modeling analysis was to identify cross-section related variables that were statistically associated with the occurrence of accidents on selected roadway segments and to estimate model parameters. A Poisson regression model was used to model the relationship between expected accident frequency and various roadway and traffic variables. The study results establish a quantitative relationship between accident frequency and various cross-section related roadway design elements on rural, multi-lane, non-freeway highways.

The effect of rural road geometry on safety in southern africa

Abstract: In this paper the data collection, analyses and results of three studies in Southern Africa on the effect of road geometry on accident rates are described. These studies used data from different periods for roads in the provinces of the Cape and Natal in South Africa and for roads in Malawi. The studies covered a combined total of 27,000 km of roads and included about 43,000 accidents. All the studies considered cross-sectional elements such as number of lanes, lane width, shoulder width and shoulder surface. In the first study the alignment was defined by indices such as hilliness and bendiness which are determined from the sum of vertical rise plus fall in m/km and the sum of horizontal deflections in degrees/km respectively. In the other two studies the alignment was described in terms of the terrain types of flat, rolling and mountainous. The results of the studies are compared to those of similar studies in the USA. The major difference was found to be in the effect of paved shoulders on the accident rate. In South Africa roads with such shoulders did not yield the safety benefit that was found overseas. This can be ascribed to the fact that these shoulders are used by both drivers and pedestrians - the former to allow other vehicles to overtake them.

Safety effects of road design standards in europe

Abstract: This paper deals with the result of a study carried out for the European Commission by the SWOV Institute for Road Safety Research, in cooperation with a number of other European institutes, and which was reported in 1994. The title of the study is "Safety Effects of Road Design Standards." The aims of this study were: Gathering information about existing knowledge on the design of road infrastructure elements by (a) drawing an inventory of international treaties and recommendations, with information about their legal status, and (b) drawing an inventory of national road design standards and the underlying knowledge; Analyzing the role safety arguments have played when road design standards were compiled; and Drawing a "best practice" for road design standards in which considerations, background information and assumptions concerning road safety have been made explicit. As a follow-up of this study, a new study will start by the end of this year (1995): SAFESTAR (Safety standards for road design and redesign). The task is "to develop safety standards for highway design and redesign on all classes of road, including tunnels and bridges, taking account of the proposals for technical standards made in the Trans-European Road Network (TERN) report." The second part of this paper contains a short description of the different so-called workpackages in this study.

Evaluation of roadside features to accommodate vans, mini-vans, pickup trucks and 4-wheel drive vehicles

Abstract: National Cooperative Highway Research Program (NCHRP) Project 22-11 began in June, 1994 and is scheduled to be completed in July, 1998. Project objectives are to evaluate current information on the safety performance of roadside features for each subclass of light trucks, to assess the significance of gaps in safety performance information, and to recommend priorities for future research, testing, and development needed to ensure that roadside features accommodate light trucks. This paper presents a summary of the tentative findings of the project to date.

Highway geometric design: the issue of driving behavior variability

Abstract: Highway geometric design is carried out on the assumption of a design speed, which implies the notion of a "design driver". However, it is well known that driving behavior generally displays considerable variability. In this paper, two different perspectives of driving behavior variability are dealt with. The former sees variability as a consequence of the stochastic nature of physical quantities relating to driver behavior, such as speeds, acceptable friction factors, and perception-reaction times. According to the latter approach, variability is attributed to systematic differences among categories of road users; a notable example in this area is the case of aging drivers. Next, the authors present the two main philosophies for coping with driving behavior variability: one is focused on reducing variability, mainly through policy measures, whereas the other calls for engineering-based measures aimed at accommodating variability within highway design. Within the context of the variability-accommodation approach, emphasis is placed on the improvement of highway design guidelines. The concept of "forgiving design" is proposed, referring to the specification of above-minimum design requirements for locations of increased complexity, where the safety of "non-design" road users (such as older drivers) may be severely reduced. Examples of candidate cases for forgiving design are presented by the authors, and suggestions are made regarding the future incorporation of the concept in highway geometric design guidelines.

Design considerations for passing sight distance and passing zones

Abstract: Several models have been developed to determine the minimum passing sight distance (PSD) required for safe and efficient operation on two-lane highways. The American Association of State Highway and Transportation Officials (AASHTO) has developed a model assuming that once the driver begins a pass, he/she has no opportunity but to complete it. This assumption is believed to result in exaggerated PSD requirements. Considerably shorter PSD values are presented in the Manual on Uniform Traffic Control Devices (MUTCD) and are used as the marking standards in Canada and the USA. However, the reasons for selecting these values are not stated. More appropriate models have been developed considering the driver's opportunity to abort the pass, and a critical sight distance is determined. The critical sight distance produces the same factor of safety whether the pass is completed or aborted. However, these models need to be revised to determine the PSD requirements more accurately and to closely match field observations. In this paper, a revised PSD model is used to develop design values for the minimum required PSD and is extended to determine the minimum length of passing zones and the profile of required PSD along the entire pass. The results show that the PSD requirements and minimum lengths of passing zones recommended in the MUTCD are sufficient for low speeds only. For design speeds higher than 70 kph, the deficiency in the MUTCD requirements increases with the increase in the design speed and subsequently the increase in the severity of potential collisions. Although the minimum lengths of passing zones used in Ontario are reasonable for passing maneuvers involving passenger cars, major revisions to the current MUTCD marking standards are needed.

Investigation of current and proposed superelevation design practices on roadway curves

Abstract: A common feature of curve design standards of the United States, Germany, and the United Kingdom is designers' freedom in applying above-minimum values for curve radii. On the basis of research findings suggesting a strong dependence of operating speed on degree of curve, it appears that, if consistency rules are not applied, this freedom may result in unrealistic assumptions for operating speed, with potential implications for safety. In addition, a comparison of the assumed values of the side friction coefficient (f) with empirical data for f-values "acceptable" by drivers confirms that, even for minimum-radius curves, actual operating speeds are underestimated for design speeds below 90 kph (56 mph). Innovative approaches to curve design suggest that, in addition to the incorporation of explicit consistency rules in design guidelines, the link between curvature and speed should be taken into account. A design procedure reflecting the association between curvature and speed, as well as enabling simplification of the superelevation - curvature relationship, is suggested in this paper and compared to existing design guidelines. It is suggested that the advantages and disadvantages of reconsidering design values for the f coefficient could be discussed at a future stage.

Relations between accident frequency and speed consistency in portuguese two-lane/two-way highways links

Abstract: During the period from 1988 to 1993 a total of 90,682 injury accidents were reported in the Portuguese main highway network, including more than 73% of which were on two-lane roads outside intersections. Overall, 26% of these accidents occurred on curves. More than 10% of the injury accidents usually occurred at accident black spots totaling less than 1% of the road length. Past attempts to directly relate road characteristics and accidents outside intersections met limited success. Mathematical and conceptual reasons can explain some of these failures. Different factors are known to influence traffic speed: driver population, road characteristics, traffic volume and composition, and prevailing lighting and weather conditions. Some of these factors are also known to be related to accident frequency. With a view to delineate standard procedures to help designers explicitly estimate the impact on safety of their design decisions, the Laboratorio Nacional de Engenharia Civil (LNEC) is presently carrying out a research project on the relations between highway characteristics and accidents using operating speed as a link variable. This paper contains a description of the study, a special emphasis being put on the collection of accident and speed data. Results of a first campaign of speed measurements on curves and tangents of national trunk roads are presented. Geometric characteristics and accident data for these road sections are introduced as well.

Design criteria for curves on two-lane rural highways

Abstract: Geometric design inconsistencies on two-lane rural roads are often characterized through strong operating speed reductions between successive design elements and great disparities between the selected design speed and the measured operating speed, as well as deficiencies between side friction demanded and side friction assumed. Critical driving maneuvers, especially on older roads and unexpected sharp curves, are the results of those operational features; consequently, the safety level decreases. Accidents are the measurable evidence of design, construction and signing deficiencies. Eight curved sites with high accident frequencies were selected through an exactly defined procedure. Three different techniques were used to collect traffic operational data on the selected curves and these are described. The obtained analyzed data reveal main safety problems on all curves. It is proved that an appropriate judgement of the sharpness of the curve from an approach tangent is difficult for drivers and additionally complicated by variations in the length of curvature, vertical alignment and the roadside. To improve the safety on critical locations without changes in the horizontal and vertical alignment, a new traffic warning device is proposed that would give the driver more accurate and effective information about the curve ahead. As shown in this paper, design criteria for curves on two-lane rural roads are related to roadside features, which should simplify a driver's decision of the adequate driving and speed behavior according to the curve characteristics.

Widening the roads: data gaps and philosophical problems

Abstract: One of the most fundamental tools for dealing with growing transportation demands is expanding the capacity of the transportation system, preferably by a little more than current needs, to allow for some future growth. New homes, new offices, and new stores require expanded transportation capacity. This can be accomplished by adding new routes, additional capacity to existing routes, or better operations to squeeze more output from the same facilities. Criticism of highway expansion as facilitating sprawl and generating more demand is troubling, because there are virtually no other choices if one cannot expand the supply. This paper offers some ideas on the topic based on the National Research Council report "Expanding Metropolitan Highways: Implications for Air Quality and Energy Use", analysis of 1990 regional data, and thoughts about pacing improvements in the highway system to underlying growth in population, the economy, and travel.

Innovative approaches to highway geometric design in poland

Abstract: A series of new requirements was introduced into policies on geometric highway design elaborated recently in Poland. Positive influence of these changes on traffic safety as well as on the operational quality of highways was proven by tests and traffic surveys on Polish roads. New elements of design policies in Poland, which are expected to help in improving the quality of Polish roads, are: determinant speed and its determination on the basis of such design elements, that has to be adjusted to the speed of passenger cars in free flow, with regard to traffic safety; procedures for the evaluation of road alignment design correctness; strict determination of visibility requirements on roads; and principles of planning for reconstruction of two-lane rural highways with paved shoulders. Thus, the requirements focus on roadway design control and sight distance.

Influence of vehicle dynamics on road geometrics

Abstract: Researchers in both Germany and the United States are developing safety models that will aid the designer in evaluating alternative designs from a safety standpoint. The current focus is primarily on the influence of vehicle dynamics on geometric design. In Germany, researchers at the University of Karlsruhe are working on a safety model that has three primary criteria: (1) Providing adequate side friction, (2) Harmonizing design speed and operating speed, and (3) Achieving design consistency in horizontal alignment. The current status of the safety model is discussed including a presentation on the software. This includes the role of vehicle dynamics in developing side friction factors based on alignment and specific vehicle characteristics. In the U.S., the Federal Highway Administration's Office of Safety and Traffic Operations R&D is developing the Interactive Highway Safety Design Model (IHSDM). One feature of the IHSDM is the ability of the designer to interface vehicle dynamics models with the geometric data stored in the Computer Aided Design (CAD) package. This will allow the designer to gather information on speed, lateral friction demand, and rollover potential as well as a visual view of the alternative based on the American Association of State Highway and Transportation Officials (AASHTO) design vehicles. The current status of this project is discussed, including demonstration of the software.