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The objective of this study was to investigate the properties of the flexibility model in a network of freeways that conveys high volumes of traffic. A dynamic system flexibility model is defined, and its three major components are evaluated and discussed. The dynamic system flexibility model measures and estimates the options of drivers traveling between origin-destination pairs in a freeway network. The three models' components depend on the number of possible and feasible routes between a given origin-destination pair, on the common lengths of the possible routes, and on the amount of variability between the length of each route and the length of the shortest route. The lengths of the link in the model are perceived lengths and depend on the occurrence of congestion in them. The flexibility measure provides a good estimate of the number of options available to drivers in a given network. Moreover, this measure is sensitive to the amount of congestion in the freeway system, and therefore the model that ...

Traffic performance on two-lane rural highways is commonly characterized by percent time spent following (PTSF), a parameter that determines the level of service on a road segment. Although PTSF is frequently used by transportation engineers and researchers, there is no agreement about its estimation method and values. Therefore, a study was done to calibrate and compare models of PTSF with the data collected on 25 segments of 15 two-lane highways in Israel. The study showed large differences between the collected models and previous model values. Furthermore, the study estimated traffic intensity, which was an additional characteristic of the driver-following phenomenon; this variable did not approach its expected value of 1.0. This lack of agreement between actual and expected values may result from changing driver behavior, that is, impatience as platoon lengths and delays increase. To avoid long platoons, drivers prefer to travel between them and then to perform a passing maneuver sooner than expected. This phenomenon not only reduces platoon length and delays, but also increases the risk of serious and fatal crashes.

The aim of the study is to model the flow breakdown phenomenon on congested freeways. Because of the importance of the breakdown phenomenon, the present research focuses on analyzing, understanding, and modeling heavy-flow breakdowns when no apparent bottlenecks exist. It is hypothesized that breakdowns are caused by internal microscopic flow oscillations and the transition from dense flow to unstable flow on freeways. A new density-versus-time model was developed to describe this phenomenon. The model shows the density behavior over time, which increases sharply toward the peak period and then decreases and increases again toward the breakdown. This new revelation is described by a combination of an exponential model, a logistic model, and a weighting function. This density–time relationship is calibrated by using data collected from a congested freeway at 1-min intervals. The beginning of the unstable flow is defined as the drop in speed below 60 km/h; it is shown that breakdown is defined as a process that starts at the local minimum of the new model and ends at the beginning of the unstable flow. It was found that the model well describes the behavior of density over time, from the increase in density through the breakdown process and into the rapid density-fluctuation phase of the unstable flow.

Roundabouts are becoming increasingly popular in the United States and have been considered for use at intersections facing operational or safety problems. A comparative analysis was performed on all-way stop-controlled (AWSC) intersections and roundabouts on the basis of aaSIDRA software after calibration against local conditions in Delaware and Maryland. A decision support system was developed in the form of a knowledge-based expert system on the basis of the results of the analysis to evaluate the performance of AWSC intersections and their potential success if converted into roundabouts. The results of the comparative analysis are in line with some previous researchers’ findings that AWSC intersections always perform worse than signal-controlled or roundabout type intersections under the same set of traffic conditions and geometric limitations.

Considerable congestion may develop on urban and suburban freeways during peak day periods, especially when traffic demand is high. Traffic breakdown is identified when there is a steep rise in density levels and a simultaneous rapid drop in speeds during the phase transition from dense congested stable (DCS) flow to breakdown flow. The purpose of this study was to generate breakdown thresholds by modeling the process of freeway flow breakdown. This paper uses the mathematical property of log periodic oscillations (LPO) to model the traffic density over time. The LPO process has been suggested in the past for modeling stock market crashes and the occurrence of large earthquakes, but this is the first time it has been proposed for freeway breakdown modeling. The calibration results obtained threshold values of densities and corresponding speeds, which trigger the breakdown process. The critical point has been identified as the maximum density rate of change during the first cycle that follows the DCS flow regime. The resulting density thresholds are comparatively higher than the equal density line thresholds proposed by the Highway Capacity Manual (HCM) between levels of service E and F. The use of Athol's conversion factor between occupancy and density has a major impact on this finding, and further examination has shown a good match between the LPO and HCM density thresholds.

Prediction and estimation of speeds on two-lane rural highways are of enormous significance to planners and designers. Estimation of speeds on curves may be easier than prediction of speeds on tangent sections because of the strong correlation of speeds with a few defined and limiting variables, such as curvature, superelevation, and the side-friction coefficients between road surface and tires. On tangent sections, however, the speed of vehicles is dependent on a wide array of roadway characteristics, such as the length of the tangent section, the radius of the curve before and after the section, cross-section elements, vertical alignment, general terrain, and available sight distance. Few studies have dealt with this issue because a considerable database is necessary to identify any significant trends and substantial modeling effort is required. Research analyzed the variability of the operating speeds on 162 tangent sections of two-lane rural highways, and models were developed for prediction of operating speed based on the geometric characteristics available. A one-model approach was used initially. Because of the low R2values, a family of models was developed that better predicted operating speeds.

The main purpose of this research was to develop models to quantify the major components of the passing process. A second goal was to compare the results with existing highway-design models and to arrive at conclusions about the applicability of the existing models. Additional objectives were to evaluate several time elements of the passing process, such as the response time of drivers from the arrival of a proper gap until the start of the passing maneuver. The evaluation is based on an analysis of data that were collected by videotaping six tangent two-lane highway sections from high vantage points and from a helicopter hovering above one site. Normal driver behavior was not disturbed during the data collection process. About 1,500 passings were recorded; of these, 54 percent were characterized as “single passing,” in which one driver passed a single, slower vehicle. About half of all passing maneuvers were found to involve two cars; in the other half, at least one truck was overtaken. A model showing the relationship between the speed of the impeding vehicle and the passing distance was calibrated, and the implications for highway-design procedures were discussed. Several of the findings had unique safety implications, such as the very short headway before the start of the passing maneuver and very short driver-reaction times. The primary results of the analyses enabled determination of the required passing distances and, therefore, the sight distances needed for different design speeds and various traffic combinations. Additional safety-related aspects are evaluated and discussed.

Traffic-flow characteristics were studied at two work zones on suburban freeways and at a third freeway site where a police patrol car, easily noticed by drivers, was parked on the right shoulder. The purpose of the study was two-fold: ( a) to estimate the speeds, headways, and capacities of sections in which some lanes were temporarily closed to traffic because of relatively short construction operations, and ( b) to assess the impact of increased, visible police deterrents on traffic flow, capacity, and safety. Calculated average flow rates, obtained from 1-min observations, were similar to values suggested by the Highway Capacity Manual at Site 1 but considerably less at Site 2. The lower values at Site 2 are attributed to the transfer of vehicles from the northbound to the southbound lane and to the resulting “constrained” operation that resulted because of a temporary barrier near oncoming traffic. The presence of a police patrol car on the shoulder at Site 3 created a deterrent to drivers, resulting in reduced average headways (and therefore increased density) and reduced headway variability. Because the volumes remained about constant, the resultant space-mean speeds also dropped. The importance of the flow stability—obtained by warning and advisory signing, the gradual closure of lanes, and a visible deterrent such as a police presence—is emphasized. Further research is suggested on the optimal slope and length of lane-closure tapers on freeway work zone approaches, the capacity of other combinations of lane closures, and the safety effects of traffic deterrence caused by police presence.

High volumes at urban and suburban intersections may cause considerable delay to vehicles during the peak periods, particularly when the left-turning volume is combined with high through volumes in both directions. The operational impact resulting from converting a major conventional cross intersection into two smaller signalized intersections is analyzed and evaluated. The two intersections are constructed along the minor road, allowing the left-turn movement from the major road to operate simultaneously with the through movement and to be stored in more lanes on the minor road. The general advantages and disadvantages of a split intersection are discussed. It is shown that the split increases capacity because of better efficiency resulting from ( a) the smaller geometry of each intersection, which reduces “lost time,” compared to the geometry of a single larger intersection, ( b) a reduction in the number of signal phases from four to three and an increase in the effective green time for all movements, and ( c) an increase in the number of lanes available for storage of the left-turn movement. It is also shown that delay is reduced, particularly when the flow is close to saturation. The impact of the cycle length and the left-turn volume is evaluated. Further analysis ascertains the minimum distance between the two smaller intersections under two scenarios: ( a) a minimum distance for storage of the left-turn movement and ( b) a longer distance to reduce delays and to allow for the simultaneous start of the green time for the through movements in the two intersections. It is concluded that the longer distance is feasible mainly for new intersections in suburban areas where the right of way is available.

Roundabouts are replacing conventional unsignalized intersections in many parts of the world and could become more widespread in the United States, although there are some limitations as well as clear advantages. Models for entry capacity into the rotary were developed. Entry capacity depends on the geometric characteristics of the roundabout, particularly the diameter of the outside circle of the intersection. The geometric characteristics determine the speed of vehicles around the central island and, therefore, have an impact on the gap-acceptance process and consequently the capacity. Traffic conditions that impede entry capacity involve the flow around the roundabout. Flow and geometric data from six small to medium-sized roundabouts were analyzed. Individual and aggregated entry-capacity models were calibrated by using the diameter and circulating flows as explanatory variables. Very good fits to the data were obtained; the results also fit models developed in other countries. The Australian model resulted in slightly higher entry capacities for moderate to low circulating flows and lower entry capacities for high circulating flows. Very close proximity to the German model was obtained, although it does not depend on the geometric characteristics of the circle. The roundabout provides an advantage over a conventional unsignalized intersection. A faithful concurrence between the model developed and the latest Highway Capacity Manual model for right-turn capacity at an unsignalized intersection is obtained if the circulating flow is replaced by the conflicting flow. The advantage of entry capacities of the roundabout over the calculated capacities of the Highway Capacity Manual left-turn model is shown. Further research is proposed to study the effect on entry capacity of two circulating lanes rather than one and the effect of the increase in circulating flows on the gap-acceptance process, particularly the reduction in critical gap at high flows.

The concept of “shared streets” involves an area in which the street is shared by pedestrians and vehicles. The entire shared street area is paved, and no vertical elevation difference exists between the walking areas and the parking area and the “travel lane.” The concept of travel lane is developed in this paper, and it refers to that part of the shared street that is specifically intended for motorists to travel. The travel lane is often distinguished by blocks with different colors. Because of the narrow lane width, it is possible for only one vehicle to pass through a straight section; two vehicles can meet only in the diagonal section. The angle of the diagonal section and the length and width of the straight section are primary parameters that strongly affect the travel speed along a shared street. Some general planning considerations are discussed, such as creating a discontinuous alignment by inserting a diagonal section between two consecutive tangent sections. Unidirectional or alternating diagonals are discussed, and three general forms of diagonal sections are presented. Safety considerations are also discussed, along with the need to provide sufficient parking space for visitors and residents. It is shown that the geometry determines the path of vehicles that results in a possible safe speed for a given design. Specific design criteria are presented for the recommended length of a tangent section and the recommended minimum width ranges, depending on the length of street. The parameters determining the geometric characteristics of the diagonal section, the angle of turn, the offset between the centerlines of the two adjacent tangent sections, and the width of the road along the tangent and within the diagonal section are also presented and evaluated. It is concluded that shared streets provide an excellent, attractive accessibility solution for residential neighborhoods with low volume of residential traffic resulting from a low density of residential dwellings. If planned properly, a balanced design is achieved, and the reduced speed results in improved safety and enhanced environmental qualities.