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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.

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 breakdown occurs during the phase transition from dense congested stable (DCS) flow to breakdown flow. In this study, the process of freeway flow breakdown was investigated by calibrating models in the density–time plane using morning peak data from Interstate 66, a US highway connecting Washington, D.C., and Virginia. It was shown that the models, which describe the collective behavior of drivers using the mathematical property of the log-periodic oscillations (LPO) process, reflect suitably the phase transition in freeway traffic flow. The LPO process has been used in the past to model stock market crashes and the occurrences of large earthquakes. The cyclic properties of the LPO models developd in this study were found to identify the “critical transition period,” which triggers the traffic breakdown process. This period starts when the density rate of change reaches its maximum during the first cycle that follows the DCS flow regime. This triggers a breakdown of flow conditions, which occur 5–8 min after the density rate of change has achieved its maximum.

The main purpose of this study was to investigate the meaning of freeway capacity, in particular to explore its stochastic nature and to estimate its distribution. The evaluation is based on traffic data from three busy urban freeway sections over 3 full days. Momentary capacity is defined as the intersection of the best-fit regression lines calculated for the dense- and unstable-flow regimes close to the maximum flow. An algorithm was developed for the selection of the relevant pairs for each regression line and is discussed. It is argued that momentary capacity values are stochastic in nature and distributed according to the shifted gamma distribution. Estimation of the parameters of this distribution for the three urban freeway sections is studied. It is proposed that the 5th percentile of the distribution, found to be approximately 2330 vehicles per hour per lane for the prevailing conditions of the basic section, could be adopted as the representative design value of capacity. Another conclusion is that the average distribution of capacities for all three through lanes is relatively close to the distribution of capacities in the middle lane.Key words: freeway flow, capacity, flow breakdown, dense flow, unstable flow.

This paper presents a methodology for the development of a toll plaza design manual intended for a preliminary estimation of the configuration of an urban barrier toll plaza, such as plazas on toll highway facilities, toll bridges, and toll tunnels. The manual is meant for cases in which conventional methods of toll collections are combined with electronic methods of automatic vehicle identification. During the development of the manual, a new concept o the throughput equivalent volume o was established. This concept is used to convert different volume combinations, of various toll-collection methods, and different vehicle classes at a specific plaza, into a common iequivalenti volume. The result is a homogeneous volume of equivalent vehicles, termed throughput equivalent volume, which enables the summation of various volumes of vehicles bound for each lane category: manual, automated, and dedicated automatic vehicle identification lanes. The method that was developed makes it possible to determine the number of lanes needed for each specific category; this is done by dividing the sum of the throughput equivalent volumes by the throughput capacity of the equivalent lane. A flow chart showing the proposed process and examples of the design charts developed are presented and discussed. Further discussion of the sensitivities and complexities of the planning process for a toll plaza is provided. The paper concludes with a list of five principal recommendations for the planning process. Since the results obtained are preliminary and intended to be used in a planning manual, the need is emphasized to fine-tune the plaza configuration for each specific site conditions with a simulation model that considers the randomness of the arrival of vehicles and the throughput variability of each plaza lane.

The purpose of this paper is to evaluate the differences between two design alternatives for the pavement edge along the superelevation runoff. The highway literature usually recommends to design the edge as a combination of straight lines, i.e., linear edge design and nonlinear change of superelevation. It is possible, however, to construct another method by which the superelevation increases linearly within the pavement along the runoff. This paper analyzes the two design alternatives and examines the differences between the two methods. It was shown that on a spiralled highway, a linear change of the superelevation is more suitable for the motion equation on the spiral. This provides a more natural transition and an increase in driver's comfort. When a spiral does not exist, the linear change of the pavement edge, which causes a nonlinear change in the superelevation rate, leads to a reduced side friction at the critical section where the superelevation was not fully developed. It is shown that the difference between the two methods is relatively small and therefore for reasons of practicality, the recommendation is to adapt the linear edge of pavement design. Key words: highway, superelevation, edge of pavement, runoff, spiral.