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ABSTRACT: 8 shown, for example, in Figure 9a of the work by Geroliminis and Sun (7), Figure 2 of the work of Endo et al. (8), and Figure 8c of the work by Buisson and Ladier (9). This scatter was attributed largely to data included from periods during which parts of each freeway system were congested while other parts were not. Thus, the suffi-ciency condition for an MFD's existence was violated. Ji et al. also found that freeway data from multiple regimes contribute to the scat-ter observed in data plots (10). The data in that study were not mea-sured on real freeways but were instead generated from computer simulation. The present work proposes that the sufficiency condition of Daganzo can be generalized for freeway networks (3, 4). The new condition is that all lanes of all links throughout the network be either congested or uncongested, even if vehicle speeds vary con-siderably in time and space. (This is less stringent than the suffi-ciency condition previously proposed for surface street networks but still is not easily satisfied for large freeway systems.) It is shown with real and highly detailed trajectory data, and for a range of congested conditions, that well-defined relations arise for freeway stretches of moderate physical length if the new sufficiency con-dition is satisfied. These relations can also be observed with data from ordinary loop detectors, and the relations are found to be repro-ducible across many days. All this is shown to be true even for free-way stretches that comprise links with inhomogeneous geometries, including on ramps and off ramps with variable flows. THEORY This paper proposes that any freeway network with homogeneous but different links that include on ramps and off ramps has a triangular MFD that relates VHT to VKT, that this MFD is the outer envelope of all VHT versus VKT data pairs that are measured on the network, and that the data points lie near the triangular MFD when all lanes within the entire network are either congested or uncongested.
Transportation Research Record Journal of the Transportation Research Board 01/2012; 2260:8-15. · 0.47 Impact Factor
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ABSTRACT: Real data show that reserving a lane for carpools on congested freeways induces a smoothing effect that is characterized by significantly higher bottleneck discharge flows (capacities) in adjacent lanes. The effect is reproducible across days and freeway sites: it was observed, without exception, in all cases tested. Predicted by an earlier theory, the effect arises because disruptive vehicle lane changing diminishes in the presence of a carpool lane. We therefore conjecture that smoothing can also be induced by other means that would reduce lane changing. The benefits can be large. Queueing analysis shows that the smoothing effect greatly reduces the times spent by people and vehicles in queues. For example, by ignoring the smoothing effect at one of the sites we analyzed one would predict that its carpool lane increased both the people-hours and the vehicle-hours traveled by well over 300%. In reality, the carpool lane reduced both measures due to smoothing. The effect is so significant that even a severely underused carpool lane can in some instances increase a freeway bottleneck's total discharge flow. This happens for the site we analyzed when carpool demand is as low as 1200Â vph.
Transportation Research Part A Policy and Practice 01/2010; 44(2):65-75. · 2.35 Impact Factor
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ABSTRACT: The paper explores some of the impacts of setting aside road lanes for the exclusive use of select vehicle classes. We examine
first the case of lanes that are reserved for carpools, and then extend the analysis to bus-only lanes. In doing so, the paper
makes three contributions. The first is methodological: it illustrates the importance of analyzing freeway data in full spatiotemporal
detail. The second is physical: data reveal that carpool lanes are not as damaging as previously reported. In fact, these
lanes are found to smooth traffic in adjacent lanes so much (by diminishing disruptive vehicle interactions near bottlenecks)
that even substantially underutilized carpool lanes can increase bottleneck discharge flows. The third contribution is theoretical:
it uses the smoothing phenomenon to show how the judicious deployment of bus-only lanes on freeways and city streets can favorably
affect not just buses, but also cars.
08/2009: pages 57-74;
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ABSTRACT: Traffic congestion is a growing problem in Nairobi, Kenya, resulting from rapidly increasing population and the crowding of motorized traffic onto a limited street network. This report includes analysis of the traffic conditions in Nairobi, the expected effects of further growth in demand, and a set of recommendations for how to improve the performance of the street network. Data describing motorized vehicle traffic was used to build a simulation model of Nairobi’s street network considering cars and matatus. This model was used to analyze traffic conditions at the city-scale under existing conditions and future growth scenarios. The results provide insights for improving the network performance and support recommendations for Nairobi. City-scale analysis of the street network was conducted with the use of the macroscopic fundamental diagram (MFD) which relates the number of vehicles circulating on the street network to the rate at which trips reach their destinations. The results of simulations with different demand patterns show that there is a consistent MFD relating vehicle accumulation to network flow in Nairobi’s central business district (CBD). Therefore, detailed knowledge of demand is not necessary to understand how the network performs, because the MFD depends on the properties of the street network itself. Monitoring and controlling the number of vehicles in the network is sufficient to maintain traffic flow on the city’s streets. As traffic demand grows in the future, the streets will quickly become more congested, so measures should be taken to improve the system. The first recommendations seek to control the accumulation of vehicles in the network so that traffic flow is maximized according to the MFD. One method is to meter the rate at which vehicles can enter the CBD in order to control accumulation so that everyone can reach their destinations sooner. Metering can be effective in the morning when more vehicles are entering the C
Institute of Transportation Studies, UC Berkeley, Institute of Transportation Studies, Research Reports, Working Papers, Proceedings. 01/2009;
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ABSTRACT: Spatiotemporal analysis of real freeway traffic reveals that carpool lanes are not as damaging as previously reported. To the contrary, the analysis unveils a surprising benefit of carpool lanes that should be even greater when special lanes are used to segregate very different vehicle classes, such as buses and cars. The paper pursues this finding and shows how reserving lanes on freeways and city streets for bus-use only can favorably affect not just buses, but also cars.
Institute of Transportation Studies, UC Berkeley, Institute of Transportation Studies, Research Reports, Working Papers, Proceedings. 01/2008;
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ABSTRACT: This paper shows that a macroscopic fundamental diagram (MFD) relating flow and average density must exist on any street with blocks of diverse widths and lengths, but no turns, even if all or some of the intersections are controlled by arbitrarily timed traffic signals. The timing patterns are assumed to be fixed in time. Exact expressions in terms of a shortest path recipe are given, both, for the street’s capacity and its MFD. Approximate formulas that require little data are also given. Conditions under which the results can be approximately extended to networks encompassing large city neighborhoods are discussed. The MFD’s produced with this method for the central business districts of San Francisco (California) and Yokohama (Japan) are compared with those obtained experimentally in earlier publications.
Institute of Transportation Studies, UC Berkeley, Institute of Transportation Studies, Research Reports, Working Papers, Proceedings. 01/2008;
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ABSTRACT: A macroscopic modeling approach is proposed for allocating a city’s road space among competing transport modes. In this approach, a city or neighborhood street network is viewed as a reservoir with aggregated traffic. Taking the number of vehicles (accumulation) in a reservoir as input, we show how one can reliably predict system performance in terms of person and vehicle hours spent in the system and person and vehicle kilometers traveled. The approach is used here to unveil two important results: first, that restricting access to a city’s congested areas can improve mobility for all travelers; and second, that dedicating street space to more sustainable modes like buses can improve accessibility for all modes, even if space is taken from cars. In this way, we show that this reservoir approach can determine the level of accessibility that can be sustained by a city of given structure, and can furnish insights into how city space should be allocated between various modes to improve accessibility for all travelers. We end the paper by discussing the value of expanding the approach so that neighborhood street networks can be modeled using systems of multiple, multimodal reservoirs.
Institute of Transportation Studies, UC Berkeley, Institute of Transportation Studies, Research Reports, Working Papers, Proceedings. 01/2008;
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ABSTRACT: Previous research on the effect of HOV (high occupancy vehicle) lanes on bottleneck flows is extended here to entire freeways using both theory and empirical evidence. The paper shows that if the flows of both high- and low-occupancy vehicles remain invariant before and after a freeway lane is converted to HOV use, then the freeway's overall traffic density upstream of its bottlenecks is reduced - albeit less than expected - if the HOV lane is underutilized. As a result, HOV lanes can extend queues over longer distances. These expansions can be problematic if the queues' expanded portions impede traffic on heavily traveled routes that do not pass through the bottleneck. To quantify this effect, the paper analyzes HOV lanes on long, multi-ramp freeways. Formulae are given for the changes in people-hours and vehicle-hours of travel induced by an HOV lane, both when there is uncongested freeway space upstream of the queue to accommodate its expansion, and when there is not. All the inputs to these formulae are either observable or easy to estimate. Hence, the recipes can help evaluate any freeway's existing, or planned, HOV lane installation. The HOV lanes at all the sites we have analyzed, which are quite typical, add less than 2% to vehicular delay and reduce people delay by more than 10%. These estimates assume no increase in car-pooling. More generally, the paper also suggests how to deploy HOV lanes on city-wide freeway systems and recommends steps to better plan city-wide systems of bus lanes.
Transportation Research Part B Methodological 01/2008; 42(10):861-872. · 2.86 Impact Factor
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ABSTRACT: A field experiment in Yokohama (Japan) reveals that a macroscopic fundamental diagram (MFD) linking space-mean flow, density and speed exists on a large urban area. The experiment used a combination of fixed detectors and floating vehicle probes as sensors. It was observed that when the somewhat chaotic scatter-plots of speed vs. density from individual fixed detectors were aggregated the scatter nearly disappeared and points grouped neatly along a smoothly declining curve. This evidence suggests, but does not prove, that an MFD exists for the complete network because the fixed detectors only measure conditions in their proximity, which may not represent the whole network. Therefore, the analysis was enriched with data from GPS-equipped taxis, which covered the entire network. The new data were filtered to ensure that only full-taxi trips (i.e., representative of automobile trips) were retained in the sample. The space-mean speeds and densities at different times-of-day were then estimated for the whole study area using relevant parts of the detector and taxi data sets. These estimates were still found to lie close to a smoothly declining curve with deviations smaller than those of individual links - and entirely explained by experimental error. The analysis also revealed a fixed relation between the space-mean flows on the whole network, which are easy to estimate given the existence of an MFD, and the trip completion rates, which dynamically measure accessibility.
Transportation Research Part B Methodological 01/2008; 42(9):759-770. · 2.86 Impact Factor
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Networks. 01/2007; 50:183-196.
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ABSTRACT: Simulations and field experiments in previous works suggest that a freeway’s general purpose lanes (those not dedicated to high occupancy vehicles) discharge vehicles from bottlenecks at an equal or higher average rate when one of the lanes is devoted to high occupancy vehicles than when it is not. This result was used in these previous works to develop formulae for the total discharge rate of bottlenecks, with and without dedicated lanes, as a function of the percentage of high occupancy vehicles in the traffic stream. This present paper extends these ideas by examining the effect of dedicated lanes on the density of traffic queues. We find that an underutilized dedicated lane reduces a queue’s density (in vehicles per km of freeway) when the downstream flow of both high occupancy and low occupancy vehicles is the same in both scenarios and exogenously determined; e.g., as would happen if the queue’s service rate is dictated by recurrent downstream congestion. A formula is given; and the reduction in density turns out to be small if the underutilization is small. Reductions in queue density without changes in bottleneck flows or traffic demand imply spatially longer queues, and this could be problematic. The paper also shows that the extra space consumed by a queue adjacent to a dedicated lane can contribute significantly to congestion, but only if heavily traveled routes that do not go through the bottleneck pass through this extra space. To quantify this effect, the paper analyzes dedicated lanes on multi-ramp freeways and beltways. Formulae are given for the changes in the people-hours and vehicle-hours of travel due to dedicated lanes both, when there is uncongested freeway space upstream of the queue for it to expand, and when there is not. The recipes are based on readily observable data and can be used to evaluate existing and planned installations of dedicated lanes. Building on these formulae, the paper finally presents qualitative pri
Institute of Transportation Studies, UC Berkeley, Institute of Transportation Studies, Research Reports, Working Papers, Proceedings. 01/2007;
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Annals OR. 01/2006; 144:153-160.
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ABSTRACT: An earlier empirical study of San Francisco Bay Area freeways concluded that HOV lanes unfavorably affect freeway traffic by creating congestion. That study attributed the observed congestion to HOV lanes and tentatively recommended their elimination over the full lengths of the freeways it examined; and even from all Bay Area freeways. It recognized, however, that its analysis is fragmentary and recommended further work to solidify its conclusions. This is logical since the study lacks a spatiotemporal analysis to pinpoint where and how congestion first forms (at bottlenecks). The present report re-examines the same set of freeway sites in spatiotemporal detail to understand more deeply how HOV lanes are affecting traffic. It enriches the data from the original study with data from neighboring detector stations, to identify: first the locations (bottlenecks) where queues are triggered; and second the role that HOV lanes play in this phenomenon. This study includes an even more detailed analysis of high-resolution video data from a bottleneck where the HOV lane initially seemed to be having an unfavorable effect. To our surprise, we found no compelling evidence that the HOV lanes were triggering delays and queues on the freeway sites in the earlier study. In all cases queues formed first at bottlenecks and, save for one questionable case, formed for reasons unrelated to the HOV lanes. This was true even on additional days that we studied. Moreover, data did not conclusively show that HOV lanes were reducing bottleneck flows or prolonging the queues; no adverse effects could be confirmed. To the contrary, and quite remarkably, the HOV lane seemed to increase the capacity of the bottleneck that was videotaped, even though that lane was underutilized. (The video data show that higher than normal discharge flows arose in the remaining lanes when the HOV lane was underutilized – enough even to compensate for that lane’s underutilization. Reassuringly, this
Institute of Transportation Studies, UC Berkeley, Institute of Transportation Studies, Research Reports, Working Papers, Proceedings. 01/2006;
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Transportation Science. 01/2004; 38:343-356.
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ABSTRACT: This paper presents related but qualitatively different results for single-commodity network flow problems with random inputs on undirected paths
06/2001;
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ABSTRACT: Network optimization problems with a "scalable" structure are examined in this report. Scalable networks are embedded in a normed space and must belong to a closed family under certain transformations of size (number of nodes) and scale (dimension of the norm.) The transportation problem of linear programming (TLP) with randomly distributed points and random demands, the earthwork minimization problem of highway design, and the distribution of currents in an electric grid are examples of scalable network problems. Asymptotic formulas for the optimum cost are developed for the case where one holds the scale parameter constant while increasing the size parameter, N. As occurs in some applied probability problems such as the Ising model of statistical mechanics, and the first passage of time of a random walk, the nature of the solution of linear problems depends on the dimensionality of the space. In the linear case, we find that the cost per node is bounded from above in 3+-dimensions (3+-D), but not in 1- and 2-D. Curiously, zone shape has no effect (asymptotically) on the optimum cost per point in 2 + -D, but it has an effect in 1-D. Therefore, the 2-D case can be viewed as a transition case that shares some of the properties of 1-D (unbounded cost) and some of the properties of 3-D (shape-independence). A simple formula for the 2-D, Euclidean TLP is given. Asymptotic results are also developed for a class of non-linear network problems. It is found that when the objective function is quadratic (as occurs in electric circuits), then the solution is always unbounded. 1.
12/2000;
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ABSTRACT: This paper shows, with experimental traffic data from a 4-mile long congested road, that traffic delays and vehicle accumulations between any two generic observers located inside the road section can be predicted quite accurately from the traffic counts measured at the extremes of the section. Predictions can be made with a streamlined version of the kinematic wave theory that does not rely on ambiguous traffic stream characteristics such as "density". The predictions were found to not require re-calibration on the day of the experiment, and to work well despite what appeared to be location-specific driver behavior.
12/1999;
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ABSTRACT: This paper shows, with experimental traffic data from a 4-mile long congested road, that traffic delays and vehicle accumulations between any two generic observers located inside the road section can be predicted quite accurately from the traffic counts measured at the extremes of the section. Predictions can be made with a streamlined version of the kinematic wave theory that does not rely on ambiguous traffic stream characteristics such as "density". The predictions were found to not require re-calibration on the day of the experiment, and to work well despite what appeared to be location-specific driver behavior.
12/1999;
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ABSTRACT: The arrival times of vehicles traveling southbound along a two-lane, bi-directional highway were recorded at eight neighboring locations upstream of a bottleneck caused by an oversaturated traffic signal. Cumulative curves constructed from these observations describe completely and in great detail the evolution of the resulting long queues. These queues formed directly upstream of the signal when the signal's service rate fell below the southbound arrival rates, and never formed away from the bottleneck. The predictability of bottlenecks like the one studied here can be exploited to manage traffic more effectively. The behavior of vehicles within the queue, however, was rather interesting. While the flow oscillations generated by the traffic signal were damped-out within one-half mile of the bottleneck, it was found that other oscillations arose within the queue farther upstream, at varied locations, and then grew in amplitude as they propagated in the upstream direction. Thus, the qu...
02/1970;
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ABSTRACT: Recent experimental work has shown that the average flow and average density within certain urban networks are related by a unique, reproducible curve known as the Macroscopic Fundamental Diagram (MFD). For networks consisting of a single route this MFD can be predicted analytically; but when the networks consist of multiple overlapping routes experience shows that the flows observed in congestion for a given density are less than those one would predict if the routes were homogeneously congested and did not overlap. These types of networks also tend to jam at densities that are only a fraction of their routes’ average jam density.This paper provides an explanation for these phenomena. It shows that, even for perfectly homogeneous networks with spatially uniform travel patterns, symmetric equilibrium patterns with equal flows and densities across all links are unstable if the average network density is sufficiently high. Instead, the stable equilibrium patterns are asymmetric. For this reason the networks jam at lower densities and exhibit lower flows than one would predict if traffic was evenly distributed.Analysis of small idealized networks that can be treated as simple dynamical systems shows that these networks undergo a bifurcation at a network-specific critical density such that for lower densities the MFDs have predictably high flows and are univalued, and for higher densities the order breaks down. Microsimulations show that this bifurcation also manifests itself in large symmetric networks. In this case though, the bifurcation is more pernicious: once the network density exceeds the critical value, the stable state is one of complete gridlock with zero flow. It is therefore important to ensure in real-world applications that a network’s density never be allowed to approach this critical value.Fortunately, analysis shows that the bifurcation’s critical density increases considerably if some of the drivers choose their routes adaptively in response to traffic conditions. So far, for networks with adaptive drivers, bifurcations have only been observed in simulations, but not (yet) in real life. This could be because real drivers are more adaptive than simulated drivers and/or because the observed real networks were not sufficiently congested.
Transportation Research Part B: Methodological.
