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The flow on urban and suburban freeways is characterized typically by high densities, occupancies, and frequent breakdowns, particularly when the volume approaches capacity. This creates unstable flow conditions, resulting in recurrent as well as random congestion. The purpose of this study was twofold: (1) to develop models for evaluating flow characteristics on a busy suburban freeway, specifically speed versus flow and flow versus occupancy; (2) to understand the meaning of capacity by evaluating the actual capacity of a freeway section. The data were collected on Highway 1 in Israel and on Interstate 66 in Virginia, USA. Previous observations showed that both freeways operated at a saturated-flow condition almost daily and that breakdowns of the stream occurred often, though not daily. The paper presents a concept that suggests that freeway capacity is an actual-site specific term. This capacity can change frequently in space and time, depending on the section geometry, the time interval, the prevailing traffic and environmental conditions, and driving behavior. Determination of a representative value of actual capacity for level-of-service or planning analyses is suggested, based on one of three methods: parabolic curve-fitting for free-flow, dense congested stable (DCS) flow, and breakdown- flow data; the intersection of the best-fit stable-flow parabola and the equal-density adjusted line; and curve fitting of one regime models to flow-occupancy data. Also included is a comparison with the 2000 Highway capacity manual (HCM) models and with capacity estimates from other studies. The results showed similar values of actual capacity estimates for the three methods. Key words: freeway, flow rate, actual capacity, curve, breakdown, dense congested, time interval. La circulation sur les autoroutes urbaines et suburbaines est caracterisee normalement par de hautes densites, l'occupation et les bris frequents, particulierement lorsque le volume de circulation approche la pleine capacite. Cela cree des conditions de circulation instables, engendrant une congestion recurrente et aleatoire. La presente etude avait un objectif a deux volets : (1) developper des modeles devaluation des caracteristiques de la circulation sur une autoroute suburbaine occupee, plus particulierement la vitesse p/r a la circulation et la circulation p/r a l'occupation; (2) comprendre la signification de la capacite en evaluant la capacite reelle d'une section d'autoroute. Les donnees ont ete colligees sur l'autoroute 1 en Israel et sur l'autoroute interetatique 66 en Virginie, aux Etats-Unis. Des observations preecedentes ont montre que la circulation etait saturee sur les deux autoroutes presque quotidiennement et que la circulation arretait souvent, mais pas de maniere quotidienne. Cet article presente un concept qui suggere que la capacite de l'autoroute soit un terme specifique au site. Cette capacite peut changer frequemment dans l'espace et le temps, selon la geometrie de la section, l'intervalle de temps, les conditions de circulation et environnementales predominantes ainsi que le comportement des conducteurs. La determination d'une valeur representative de la capacite reelle des analyses aux fins de niveau de service et de planification est suggeree cette valeur est baseee sur l'une des trois methodes suivantes : (1) ajustement de la distribution parabolique pour la circulation libre, une circulation stable congestionnee et dense ainsi que les donnees de circulation congestionneee (2) l'intersection du meilleur ajustement de la parabole de circulation stable et de la ligne ajustee de densite egale (3) l'ajustement de la courbe de l'un des modeles a regime unique aux donneees de circulation. Une comparaison aux modeles HCM 2000 et aux estimations de capacites provenant d'autres etudes est egalement incluse. Les resultats ont montre des valeurs similaires des estimations de la capacite reelle pour les trois methodes. Mots-cles: autoroute, flot de circulation, capacite reelle, courbe, defaillance, congestions et dense, intervalle de temps. [Traduit par la Redaction], Introduction The flow on urban and suburban freeways is characterized typically by high densities, occupancies, and volumes, and thus, low speeds and low level-of-service. These parameters can be used to [...]

This study has three main objectives: (1) to develop new, theory-based, queuing relationships for the quality of flow on two-lane rural highways; (2) to estimate the relationships both from the empirical data that were collected on 15 two-lane rural highways and from the queuing models that were developed; and (3) to propose a new level-of-service variable that measures the quality of the flow both inside and between platoons. The paper presents five flow-characteristic measures for two-lane rural highways: the flow, the average platoon length, the traffic intensity, the percent-time-spent following, and the freedom of flow. It is shown that the five measures can be calculated from easily collectible data parameters and also from empirical models related to the two-way flow that are developed based on the field-data collected. It is proposed that the level of service be estimated from the freedom-of-flow parameter [eta], which is developed in the paper. The relationship between [eta] and the two-way flow is calibrated from traffic observations. Level-of-service thresholds, based on the flow and on [eta] are presented and discussed. CE Database subject headings: Traffic flow; Traffic volume; Highways and roads; Queueing.

Abstract: Urban and suburban freeways are designed to allow smooth traffic flow at high speed. However, when traffic demand is high or during irregular events, significant congestion may develop. Traffic [...]

The goal of the research was to study consistency of design on two-lane rural highways and to ascertain the existence of a relationship between consistency and safety level. The immediate objectives were to develop new, independent measures of consistency that could reflect the similarity (or lack thereof) of performance along an entire level or hilly section, to develop a new consistency model, and to find the relationship between the new model and crash rates on two-lane rural highways. Two consistency measures were developed: the first was the relative area bounded by the speed profile and the average weighted speed; the second was the standard deviation of operating speeds in each design element along the entire section investigated. Following an extensive sensitivity analysis of these two measures, thresholds that quantified the design quality were suggested. Based on the two independent measures, a consistency model was developed; and thresholds for good, acceptable, and poor design consistency of any section were proposed. Additional analysis was conducted on the relationship between the proposed consistency model and the safety level of two-lane highways. This was done initially on a limited data set of nine local, two-lane highway sections. It was found that as design consistency increased, crash rates decreased significantly. In a second phase, the analysis was expanded and the same consistency model was applied to a data set of 28 two-lane U.S. highways. It was found that crash rates decreased when the consistency value increased. DOI: 10.1061/(ASCE)0733-947X(2004)130:3(286) CE Database subject headings: Consistency; Models; Rural areas; Highways; Highway safety.

This paper presents a study of two important roundabout performance issues: (1) the impact of the waiting time of drivers on the approach leg, on the critical gap they accept before entering a roundabout and (2) the entry capacity of the approach. The main hypothesis is that an increase in drivers' average waiting times reduces the average accepted gap and, consequently, the critical gap. A model for a change in the critical gap at busy roundabouts, with long approach queues and long waiting times, is provided. The model for a change in the critical gap was found to be logistic; i.e., it had an s-shape between a maximum and a minimum critical gap. The parameters of each site studied in the present investigation were found to correlate in regard to geometric complexity and traffic intensity. The parameters were the outside diameter, pedestrian volume, and circulating flow of vehicles around the central island. A reduction in the critical gap because of increased waiting times results in a significant increase in entry capacity, which is particularly significant in high circulating volumes. A model for the entry capacity that depends on the circulating flow, the outside diameter, and the critical gaps (which, in turn, depend on the waiting times) was calibrated and presented. This model was found to be sensitive in high circulating volumes to the critical gap; and in low circulating volumes, to the geometry, represented by the outside diameter. The use of this model is recommended when estimating the entry capacity of either new or existing roundabouts. DOI: 10.1061/(ASCE)0733-947X(2003)129:5(504) CE Database subject headings: Intersection; Time factors; Traffic management.

Drivers' speed selection has been a great interest to road safety researchers. In this study, a driving simulator (STISIM) was used to explore drivers' speed choices, and how they are influenced by infrastructure, traffic, risk/benefit, and driver characteristics. Drivers also filled out a stated-preference survey, which included speed selection items and demographic characteristics. The experimental design included four scenarios of a specific risk or benefit to the driver: A daily trip, a higher speed enforcement scenario, a higher crash-risk scenario, and a scenario with high time pressure. The driver sample included 111 drivers from different ages between 20 and 65, with 44% women. The database included 9,768 observations. The largest effect on drivers' speed was under time-saving benefits with an average increase of 10 km/hr. Infrastructure effects included horizontal curves, longitudinal slope changes, and design speed. Age and gender also influenced speed selection. Thus, the most effective measures for speed reduction may be (1) enforcement, (2) design speed, and (3) horizontal curves, in contrast to (1) time-saving benefits and (2) a high average speed of close-by vehicles, which motivated drivers to increase their speed but may be reduced by counteracting policies. [ABSTRACT FROM AUTHOR]

To maximize the potential for success of an urban high-occupancy vehicle (HOV) lane, there are several design intricacies that require special attention, particularly at critical intersections. This paper is based on extensive research of HOV lanes, specifically their optimal setback planning and the resulting impact on delay, that was conducted recently in Israel. A setback, the termination of an HOV lane prior to a critical intersection--typically characterized by long delays and diverging queues--allows passenger vehicles, in addition to buses, to use the HOV lane. Throughput capacity is thus increased and delay for passenger vehicles reduced at the critical intersection, especially when the HOV lane is not utilized to its fullest capacity. The paper also presents the geometry of a setback, with and without a right-turn lane and, in addition, introduces the development of a model for determining the optimal length of a setback. Average delay to HOV vehicles and to non-HOV vehicles is given as a function of the degree of saturation for a design that both includes and does not include a setback. At critical intersections, the provision of an HOV lane with an optimal setback is shown to result in the least amount of total passenger delay.

Passing manoeuvres on rural two-lane highways significantly affect highway capacity, safety and level of service. This article presents an analysis of data on drivers' passing decisions on two-lane rural highways that were collected with an interactive driving simulator. Measurements of the speeds and positions of all vehicles in several different scenarios were collected and processed to generate observations of gap acceptance behaviour. In addition, participants responded to a questionnaire which collected information on their socio-demographic and driving styles characteristics. These data were utilised to develop a model that explains the decision whether to pass or not, using variables that capture the impact of the road geometry, traffic conditions and drivers' characteristics. It was found that while the traffic related variables had the most important effect on passing decision, factors related to the geometric design and the driver characteristics also had a significant effect on these decisions. [ABSTRACT FROM AUTHOR]