Papers by Michael Cassidy
Transportation Research Part B: Methodological, 2021
Transit vehicles operating under skip-stop service visit only a subset of the stops residing alon... more Transit vehicles operating under skip-stop service visit only a subset of the stops residing along a corridor. It is a strategy commonly used to increase vehicle speeds and reduce patron travel times. The present paper develops a continuous approximation model to optimally design a select form of skipstop service, termed AB-type service. The model accounts for spatially-heterogeneous demand patterns. An efficient heuristic is developed to obtain solutions. These are shown to be near-optimal for a variety of numerical examples. Results also indicate that optimal AB-type designs outperform optimized all-stop service in a variety of cases. The AB-type service is found to be especially competitive when travel demands are high, trip origins are unevenly distributed along a corridor, and patrons have relatively high values of time. In these cases, AB-type service is found to reduce system costs by as much as 8%.
Transportation Research Part A: Policy and Practice, 1999
Separate turn phases are often used on the approach legs to intersections with heavy left turns. ... more Separate turn phases are often used on the approach legs to intersections with heavy left turns. This wastes capacity on the approach because some of its lanes cannot discharge during its green phases. The paper shows that the problem can be eliminated by reorganizing traffic on all the lanes upstream of an intersection using a mid-block pre-signal. If drivers behave deterministically, the capacity that can be achieved is the same as if there were no left turns. However, the reorganization is so drastic that it may be counterintuitive to drivers. This can be remedied by reorganizing traffic on fewer lanes. It is shown that such partial reorganization still increases capacity significantly, even if drivers behave randomly and only one lane is reorganized. The paper shows how to optimize the design of a pre-signal system for a generic intersection. It also identifies both, the potential benefits of the proposed system for a broad class of intersections, and the domain of application where the benefits are most significant.
Transportation Research Record: Journal of the Transportation Research Board, 2012
Transportation Research Part A: Policy and Practice, 2010
Real data show that reserving a lane for carpools on congested freeways induces a smoothing effec... more 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 arises because disruptive vehicle lane changing diminishes in the presence of a carpool lane. The effect is reproducible across days and freeway sites: it was observed, without exception, in all cases tested. Queueing analysis shows that the effect greatly reduces the times spent by people and vehicles in queues. By ignoring the smoothing effect at one of the sites we analyzed, for example, one would predict that its carpool lane increased both the people-hours and the vehicle-hours traveled by well over 300%; when in reality the carpool lane and its attendant smoothing reduced both measures. The effect is so significant, in fact, 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 1200 vph. It follows that strategies designed to induce smoothing by other means also hold promise for managing congestion, both for freeways that have carpool lanes and those that do not. Possible strategies of this kind are discussed.
Operations Research, 2003
With 12,500 members from nearly 90 countries, INFORMS is the largest international association of... more With 12,500 members from nearly 90 countries, INFORMS is the largest international association of operations research (O.R.) and analytics professionals and students. INFORMS provides unique networking and learning opportunities for individual professionals, and organizations of all types and sizes, to better understand and use O.R. and analytics tools and methods to transform strategic visions and achieve better outcomes. For more information on INFORMS, its publications, membership, or meetings visit
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Journal of Transportation Engineering, 1994
Operating conditions at stop‐controlled approaches to highway intersections depend, to a large ex... more Operating conditions at stop‐controlled approaches to highway intersections depend, to a large extent, on the gap‐acceptance behavior of motorists. Several researchers have previously proposed and demonstrated the application of discrete‐choice modeling techniques for predicting gap‐acceptance probabilities. The work described in this paper has used logit modeling to predict the probability that a randomly chosen motorist will accept a given gap in the conflicting traffic stream based upon characteristics of the gap. This gap‐acceptance function is then used in a stochastic queuing model to predict vehicle delay. The specific methodology presented in this paper is applicable to right‐turning traffic at a T‐intersection. Evaluation of the methodology using an empirical data set suggests that the proposed analysis approach offers significant potential. Issues related to extending the applicability and generality of the proposed methodology are briefly discussed in the conclusions of this paper.
all vehicles would change little. The HOV lane would allow HOVs to bypass most of the congestion ... more all vehicles would change little. The HOV lane would allow HOVs to bypass most of the congestion without significantly increasing total vehicle-hours of travel. This means that even in problematic cases, HOV lanes can usually be preserved and allowed to perform their intended societal function: reducing people-hours of travel without significantly increasing vehicle-hours of travel by favoring HOVs where freeway queues arise. To avoid increased vehicle delays, however, and perhaps even to increase bottleneck capacities, some HOV-lane installations should be modestly altered near bottlenecks. This report also describes field studies necessary to determine where and how such alterations should be deployed.
Transportation Research Part A: Policy and Practice, 2012
The paper explores how the coordination of vehicle schedules in a public transit system affects g... more The paper explores how the coordination of vehicle schedules in a public transit system affects generalized costs. We consider an idealized system that delivers its users to a common destination by requiring each to transfer from a feeder-to a trunk-line vehicle. Continuum models are used first to analyze cases in which the trunk-line vehicle schedule is given exogenously. We find that when feeder vehicles are dispatched in coordination with this exogenous trunk-line schedule, the reduction in user cost often outweighs the added cost to the feeder operation. In cases when the frequencies of trunk and feeder services can be established jointly, the models show that coordination can be Pareto improving, meaning that operator and user costs both diminish. Conditions that give rise to these cost savings are specified. Practical implications are discussed.
Institute of Transportation Studies, 1997
It is shown that all the phase transitions in and out of freely flowing traffic reported earlier ... more It is shown that all the phase transitions in and out of freely flowing traffic reported earlier for a German site could be caused by bottlenecks, as are all the transitions observed at two other sites examined here. Furthermore, all the evidence indicates that bottlenecks cause these transitions in a predictable way, and no evidence is found that stoppages (jams) appear spontaneously in free flow traffic for no apparent reason. The most salient phenomena observed at all locations are explained in terms of a simple theory specific to traffic. 1. CAUSES This report provides additional empirical observations and an alternative explanation of some traffic data from a German highway [1, 2, 3]. A few studies of North American freeways have also identified related traffic flow properties and the origin of traffic disturbances [4, 5]. Reassuringly, the observations on both continents are consistent with each other. However, it will also be shown that neither the German nor the North American data support the conclusion in [1]: that free flowing traffic will spontaneously break down randomly, without obvious reasons, and then remain in that state due to traffic's tendency to self-maintain congestion. Rather, all the evidence indicates that traffic "breaks down" (queues form) at locations of freeway inhomogeneities (bottlenecks) due to reproducible exogenous reasons, and that, following breakdown, the bottleneck flow behaves in a predictable way. Furthermore, there is no evidence that queues self-maintain away from a bottleneck [6]. To the contrary, all indications are that highways behave like crowds of people going through a series of queues at well defined locations; e.g., fans who wait in line to buy football tickets and then queue through one or more gates to enter the stadium. Some definitions derived from this analogy are introduced below; see e.g., [8] for additional explanations. Free flow (or unqueued) conditions refer to that state of traffic where small disturbances can only flow forward in space (e.g., as in supersonic gas flow). People walking away from the ticket window, for example, are in free flow because if one of them momentarily slows down just a little, this does not affect the queue upstream of the server. The free flow status of highway traffic can be determined by correlating the curves of cumulative flow at successive detectors with a time lag, as described in
... observed during the rush, but that these relatively low flows were short-lived relative to th... more ... observed during the rush, but that these relatively low flows were short-lived relative to the rush. In the center and median lanes (Figures 6 and 7), these so-called flow collapses (Cassidy and Windover, 1995) prevailed for just under 20 minutes before being replaced by higher ...
University of California Transportation Center, 2008
Though there have been numerous studies of freeway weaving sections (i.e., segments in which an o... more Though there have been numerous studies of freeway weaving sections (i.e., segments in which an on-ramp is followed by an off-ramp), there remains a significant lack of empirical and theoretical understanding of the traffic behavior that causes weaving sections to become bottlenecks with varying discharge flows. The present research entails empirical analysis and theoretical modeling of what triggered the bottleneck activations and discharge flow changes in two freeway weaving sections. Both sites were recurrent bottlenecks during the rush, and investigations revealed that changes in the spatial patterns of vehicular lane-changes, especially among Freeway-to-Ramp (F-R) maneuvers, caused variations in bottleneck discharge flow. When the F-R maneuvers were concentrated near a weaving section's on-ramp, they became more disruptive, resulting in bottleneck activations with diminished discharge flows. Findings further indicated that the spatial distributions of these lane changes, in turn, were dictated by the traffic conditions in the auxiliary lane (i.e., the lane connecting the off-ramp to the upstream on-ramp). Reductions in on-ramp flows increased the attractiveness of the auxiliary lane, thus motivating F-R drivers to perform their maneuvers nearer the onramp. Conversely, increases in on-ramp flows motivated F-R drivers to perform their maneuvers over a wider stretch of the weaving section. Based on these empirical findings, the study formulated a theory for mandatory lane changing (i.e., lane changes required of a desired Origin-Destination pattern); and used this theory to enhance an existing microsimulation model of car-following and lane ii changing. With this new theory, the driver's decision to attempt a lane change is determined by the vehicle's distance from the downstream end of the weaving section's diverge area, the number of lanes to be crossed in reaching the desired destination, and the difference in densities between the driver's target lane and her current one. The model reproduces the observed mechanisms of bottleneck activation and discharge flow changes in weaving sections. These empirical findings, together with the outcomes of simulation, point to two key features of driver behavior in weaving sections: i) traffic conditions (especially densities) in an auxiliary lane influence drivers' decisions regarding where to perform mandatory lane changes; and ii) the spatial distributions of lane changes determine weave bottleneck discharge flows.
California Partners For Advanced Transit and Highways, 1998
The primary goal of this project is to design a signal controller algorithm to capitalize on the ... more The primary goal of this project is to design a signal controller algorithm to capitalize on the extended information provided by wide-area detection at isolated intersections. The title of the work emphasizes machine vision, or video image processing, because that technology promises to be the first viable wide-area detector; however, the results of this work could be applied to any wide-area detector capable of detecting vehicles over the entire length of the so called "dilemma zone" on each approach to the intersection. Where the dilemma zone is defined as the area between the legal stopping distance (i.e., the location upstream of which all drivers are legally required come to a stop in response to the yellow phase), and the minimum stopping distance, beyond which point, most drivers pass through the intersection in response to the yellow. The scope of this work is restricted to isolated intersections because control issues are different for isolated intersections and arterials. Sizable benefits from vehicle actuated (VA) control can be realized at isolated intersections. Machine vision tools are rapidly approaching the stage where they can be used for widearea vehicle detection at intersections. To bypass the ongoing development cycle, this project developed a control algorithm assuming the detectors are fully functional. Using computer simulation, we evaluated different control strategies and compared the performance of the proposed wide-area detection system with conventional signal controllers. The results show that wide-area VA control can yield significant improvements over conventional VA control strategies. Once wide-area detectors become available, the control algorithm can serve as the foundation for a prototype system.
Uc Berkeley Center For Future Urban Transport a Volvo Center of Excellence, 2007
Simulations and field experiments in previous works suggest that a freeway's general purpose lane... more 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 principles that can be used to plan city-wide systems of both, high occupancy vehicle lanes on freeways and dedicated bus lanes on surface streets.
Institute of Transportation Studies, 2002
Traffic data near the junction of a single-lane on-ramp (with a ramp meter) and a three-lane free... more Traffic data near the junction of a single-lane on-ramp (with a ramp meter) and a three-lane freeway were measured for six weekdays during the rush and studied. On each of these days, the merge became a bottleneck with queue discharge rates that were substantially lower than the flows that had passed the merge prior to the bottleneck's activation. On some days, these earlier high flows persisted for many minutes. The bottleneck always occurred when inflows from the on-ramp surged in the presence of high flows arriving from the freeway. Often, the on-ramp surges persisted for no longer than a minute or two and a wide range of these surges was observed. The data show strong correlation between the magnitudes of the on-ramp surges and the merge area's outflows that were measured during the final minutes before the bottleneck activations. These short-run outflows were markedly higher on days when surges from the on-ramp were low. This implies on-ramp metering can be an effective means of postponing this bottleneck's activation, thereby prolonging higher outflows from the merge. Further study of the data indicates that vehicles often
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Papers by Michael Cassidy