Publications (167)262.98 Total impact

Article: A reliabilitybased optimization scheme for maintenance management in largescale bridge networks
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ABSTRACT: Incorporating network configurations in bridge management problems is computationally difficult. Because of the interdependencies among bridges in a network, they have to be analyzed together. Simulationbased numerical optimization techniques adopted in past research are limited to networks of moderate sizes. In this paper, a simple framework is developed to determine optimal maintenance plans for large networks with many bridges. The objective is to minimize disruption, specifically, the extra travel distance caused by potential bridge failures over a planning horizon and under a budget constraint. It is conjectured and then verified that the expected increase in vehiclemiles traveled due to failures can be approximated by the sum of expected increases due to individual failures. This allows the networklevel problem to be decomposed into singlebridge problems and tackled efficiently. The computational effort increases linearly with the number of bridges.Transportation Research Part C Emerging Technologies 02/2015; · 2.82 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: This paper extends Vickrey’s (1969) commute problem for commuters wishing to pass a bottleneck for both cars and transit that share finite road capacity. In addition to this more general framework considering two modes, the paper focuses on the evening rush, when commuters travel from work to home. Commuters choose which mode to use and when to travel in order to minimize the generalized cost of their own trips, including queueing delay and penalties for deviation from a preferred schedule of arrival and departure to and from work. The user equilibrium for the isolated morning and evening commutes are shown to be asymmetric because the schedule penalty in the morning is the difference between the departure and wished curves, and the schedule penalty in the evening is the difference between the arrival and wished curves. It is shown that the system optimum in the morning and evening peaks are symmetric because queueing delay is eliminated and the optimal arrival curves are the same as the departure curves. The paper then considers both the morning and evening peaks together for a single mode bottleneck (all cars) with identical travelers that share the same wished times. For a schedule penalty function of the morning departure and evening arrival times that is positive definite and has certain properties, a user equilibrium is shown to exist in which commuters travel in the same order in both peaks. The result is used to illustrate the user equilibrium for two cases: (i) commuters have decoupled schedule preferences in the morning and evening and (ii) commuters must work a fixed shift length but have flexibility when to start. Finally, a special case is considered with cars and transit: commuters have the same wished order in the morning and evening peaks. Commuters must use the same mode in both directions, and the complete user equilibrium solution reveals the number of commuters using cars and transit and the period in the middle of each rush when transit is used.Transportation Research Part B Methodological 11/2013; 57:286–299. · 2.94 Impact Factor 
Article: System optimum and pricing for the daylong commute with distributed demand, autos and transit
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ABSTRACT: The daylong system optimum (SO) commute for an urban area served by auto and transit is modeled as an auto bottleneck with a capacitated transit bypass. A public agency manages the system’s capacities optimally. Commuters are identical except for the times at which they wish to complete their morning trips and start their evening trips, which are given by an arbitrary joint distribution. They value unpunctuality – their lateness or earliness relative to their wish times – with a common penalty function. They must use the same mode for both trips. Commuters are assigned personalized mode and travel start times that collectively minimize society’s generalized cost for the whole day. This includes unpunctuality penalties, queuing delays, travel times and outofpocket costs for users, as well as travel supply costs and externalities for society.It is shown that in a SO solution there can be no queuing and that the set of SO solutions forms a convex set. Furthermore, if the schedule penalty that users suffer due to unpunctuality is separable into morning and evening components, then the set of commuters traveling by the same mode arrive at work and depart from work in the order of their wishes. These orders are in general different in the morning and the evening. It is also shown that there always is a SO solution in which users are at all times, and on both modes, either punctual or flowing at capacity. These problem properties are used to identify search methods, both, for SO solutions and for timedependent tolls and transit fares that preserve the solutions as Nash equilibriums. In every case studied, these prices exist. They must peak concurrently for the two modes in both periods.In special cases involving only one mode, only one period or concentrated demand the solution to the complete problem decomposes by period conditional on the number of transit users, and this facilitates the solution. In these cases the daylong SO cost is the sum of the SO costs for the two peaks considered separately. However, this is not true in general – the solution obtained by combining the two singleperiod solutions can be infeasible. When this happens, the optimum daylong cost will exceed the sum of the singleperiod costs. The discrepancy is about 40% of the total schedule penalty for an example representing a large city. Thus, to develop realistic policies the daylong problem must be addressed head on. An approximate method that yields closed form formulas for the case with uniformly distributed wishes is presented.Transportation Research Part B Methodological 09/2013; 55:98–117. · 2.94 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: This paper considers the optimization of public infrastructure systems, recognizing that these systems serve multiple user classes. Example application domains include: public transportation systems, electricity distribution grids, urban water distribution systems, and maintenance of pavement and bridge systems. Under the guidance of a policymaking body, the analyst chooses both the system design, including its layout and control, and the prices to be charged for the service. The goal of the optimization is to maximize society’s welfare recognizing that the system’s performance will in general depend on the system’s demand, and vice versa.The optimization problem is first formulated in its full complexity, where the prices, the demand and the system design are to be determined. It is then shown that if the user classes recognized in the analysis can be priced independently, and if the policy setting body specifies either the demand levels or the generalized prices experienced by each user group, then the resulting welfare maximization problem decomposes into three subproblems that can be solved sequentially: demand estimation, system design and pricing. It also turns out that the optimum design can always be obtained by minimizing the generalized cost of the system to society for the known fixed demand, as is conventionally done in practice. The resulting design is independent of how net user benefits are measured and of the pricing scheme.If the policymaking body does not specify cost or demand targets, and instead assesses benefits by means of consumer surplus then the optimum design is still the solution of a conventional design problem with fixed demand. In this case, however, the demand has to be obtained iteratively using a marginal cost pricing rule.The paper finally shows how government can structure payments to a forprofit agency so it will be induced to design and operate the system optimally for society.Transportation Research Part B Methodological 11/2012; 46(9):1288–1293. · 2.94 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: A model with few variables is said to be parsimonious. If it is also analytically tractable, physically realistic, and conceptually insightful, it is said to be effective. Effective parsimonious models have long been used in fields such as economics and applied physics to describe the aggregate behavior of systems as opposed to the behavior of their individual parts. In transportation, these models are particularly well suited to address big picture questions because they provide insights that might be lost when focusing on details. This paper presents an abbreviated history of effective parsimonious models in the transportation field, classified by subarea: regional and urban economics, traffic flow, queuing theory, network dynamics, town planning, public transportation, logistics, and infrastructure management. The paper also discusses the benefits of these models—fewer data requirements, reduced computational complexity, improved system representation, insightfulness—and ways of constructing them. Two examples, one from logistics and one from urban transportation, are used to illustrate these points. Finally, the paper discusses ways of expanding the application of effective parsimonious models in the transportation field.EURO Journal on Transportation and Logistics. 06/2012; 1(12).  [Show abstract] [Hide abstract]
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 sufficiency condition for an MFD's existence was violated. Ji et al. also found that freeway data from multiple regimes contribute to the scatter observed in data plots (10). The data in that study were not measured 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 considerably in time and space. (This is less stringent than the sufficiency 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 welldefined relations arise for freeway stretches of moderate physical length if the new sufficiency condition is satisfied. These relations can also be observed with data from ordinary loop detectors, and the relations are found to be reproducible across many days. All this is shown to be true even for freeway 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; 2260:815. · 0.44 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Through the use of a profitmaximizing continuum approximation model, this paper systematically analyzes the development and structure of informal transit systems as a function of the network, user, and modal characteristics. This study examines the evening commute problem along a linear corridor where passengers originate uniformly from a central business district and have destinations uniformly distributed along the corridor. Informal transit drivers who are profitmaximizing will be compared against the traditional case of coordinated, government service that aims to maximize the total welfare. Policies, such as fare regulation and vehicle licensing schemes, will be presented to help rationalize informal transit service using a governmentoperated service as the baseline.Research in Transportation Economics 01/2012; 39(1).  Transportation Research Record Journal of the Transportation Research Board 12/2011; 2249(1). · 0.44 Impact Factor
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ABSTRACT: As is well known, bus systems are naturally unstable. Without control, buses on a single line tend to bunch, reducing their punctuality in meeting a schedule. Although conventional schedulebased strategies that hold buses at control points can alleviate this problem these methods require too much slack, which slows buses. This delays onboard passengers and increases operating costs.It is shown that dynamic holding strategies based on headways alone cannot help buses adhere to a schedule. Therefore, a family of dynamic holding strategies that use bus arrival deviations from a virtual schedule at the control points is proposed. The virtual schedule is introduced whether the system is run with a published schedule or not. It is shown that with this approach, buses can both closely adhere to a published schedule and maintain regular headways without too much slack.A oneparameter version of the method can be optimized in closed form. This simple method is shown to be nearoptimal. To put it in practice, the only data needed in real time are the arrival times of the current bus and the preceding bus at the control point relative to the virtual schedule. The simple method was found to require about 40% less slack than the conventional schedulebased method. When used only to regulate headways it outperforms headwaybased methods.Transportation Research Part B Methodological 12/2011; 45(10):18311845. · 2.94 Impact Factor  Transportation Research Record Journal of the Transportation Research Board 12/2011; 2260(1):815. · 0.44 Impact Factor

Article: Design and implementation of efficient transit networks: Procedure, case study and validity test
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ABSTRACT: This paper presents and tests a method to design highperformance transit networks. The method produces conceptual plans for geometric idealizations of a particular city that are later adapted to the real conditions. These conceptual plans are generalizations of the hybrid network concept proposed in Daganzo (2010). The best plan for a specific application is chosen via optimization. The objective function is composed of analytic formulae for a concept's agency cost and user level of service. These formulae include as parameters key demandside attributes of the city, assumed to be rectangular, and supplyside attributes of the transit technology. They also include as decision variables the system's line and stop spacings, the degree to which it focuses passenger trips on the city center, and the service headway. These decision variables are sufficient to define an idealized geometric layout of the system and an operating plan. This layoutoperating plan is then used as a design target when developing the real, detailed master plan. Ultimately, the latter is simulated to obtain more accurate cost and level of service estimates. This process has been applied to design a high performance bus (HPB) network for Barcelona (Spain). The idealized solution for Barcelona includes 182Â km of oneway infrastructure, uses 250 vehicles and costs 42,489Â [euro]/h to build and run. These figures only amount to about one third of the agency resources and cost currently used to provide bus service. A detailed design that resembles this target and conforms to the peculiarities of the city is also presented and simulated. The agency cost and user level of service metrics of the simulated system differ from those of the idealized model by less than 10%. Although the designed and simulated HPB systems provide suboptimal spatial coverage because Barcelona lacks suitable streets, the level of service is good. Simulations suggest that if the proposed system was implemented sidebyside with the current one, it would capture most of the demand.Transportation Research Part A Policy and Practice 11/2011; 45(9):935950. · 2.73 Impact Factor 
Article: Managing evacuation networks
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ABSTRACT: This paper proposes a nonanticipative, adaptive, decentralized strategy for managing evacuation networks. The strategy is nonanticipative because it does not rely on demand forecasts, adaptive because it uses realtime traffic information, and decentralized because all the information is available locally. It can be used with a failed communication network.The strategy pertains to networks in which no links backtrack in the direction of increased risk. For these types of networks, no other strategy exists that can evacuate more people in any given time, or finish the evacuation in less time. The strategy is also shown to be socially fair, in the sense that the time needed to evacuate all the people exceeding any risk level is, both, the least possible, and the same as if lessatrisk individuals did not participate in the evacuation. The strategy can be proven optimal even when backflows happen due to driver gaming.Highlights► Considered are evacuation networks whose links point in the direction of decreasing risk. ► These networks can be optimally managed in realtime without demand information. ► The proposed strategy is adaptive and decentralized: it only uses locally available realtime data. ► The strategy is fair: all people within a given risk level are evacuated in the least possible time. ► The strategy works well even if people are allowed to choose their point of entry in realtime.Transportation Research Part B Methodological 11/2011; 45(9):14241432. · 2.94 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: A separate turn phase is often used on the approach leg to an 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 midblock presignal. If drivers behave deterministically, the capacity that can be achieved is the same as if there were no left turns. However, if the reorganization is too drastic, it may be counterintuitive to drivers. This can be remedied by reorganizing traffic on just some of the available 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 presignal 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 Part B Methodological 06/2011; 45(5):769781. · 2.94 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: A simple model of traffic flow is used to analyze the spatiotemporal distribution of flow and density on closedloop homogeneous freeways with many ramps, which produce inflows and allow outflows. As we would expect, if the onramp demand is spaceindependent then this distribution tends toward uniformity in space if the freeway is either: (i) uncongested; or (ii) congested with queues on its onramps and enough inflow to cause the average freeway density to increase with time. In all other cases, however, including any recovery phase of a rush hour where the freeway's average density declines, the distribution of flow and density quickly becomes uneven. This happens even under conditions of perfect symmetry, where the percentage of vehicles exiting at every off ramp is the same. The flowdensity deviations from the average are shown to grow exponentially in time and propagate backwards in space with a fixed wave speed. A consequence of this type of instability is that, during recovery, gaps of uncongested traffic will quickly appear in the unevenly congested stream, reducing average flow. This extends the duration of recovery and invariably creates clockwise hysteresis loops on scatterplots of average system flow vs. density during any rush hour that oversaturates the freeway. All these effects are quantified with formulas and verified with simulations. Some have been observed in real networks. In a more practical vein, it is also shown that the negative effects of instability diminish (i.e., freeway flows increase) if (a) some drivers choose to exit the freeway prematurely when it is too congested and/or (b) freeway access is regulated in a certain trafficresponsive way. These two findings could be used to improve the algorithms behind VMS displays for driver guidance (finding a), and onramp metering rates (finding b).Transportation Research Part B Methodological 06/2011; 45(5):782788. · 2.94 Impact Factor 
Article: Clockwise hysteresis loops in the Macroscopic Fundamental Diagram: An effect of network instability
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ABSTRACT: A recent study reported that the Macroscopic Fundamental Diagram of a medium size city exhibited a clockwise hysteresis loop on a day in which a major disturbance caused many drivers to use unfamiliar routes. It is shown below that, even in a perfectly symmetric network with uniform demand, clockwise loops are to be expected when there are disturbances, especially if the disturbances cause a significant fraction of the drivers to not change routes adaptively. It is also shown that when drivers are not adaptive networks are inherently more unstable as they recover from congestion than as they are loaded. In other words, during recovery congestion tends more strongly toward unevenness because very congested areas clear more slowly than less congested areas. Since it is known that uneven congestion distributions reduce network flows, it follows that lower network flows should arise during recovery, resulting in clockwise loops. Fortunately, the presence of a sufficient number of drivers that choose routes adaptively to avoid congested areas helps to even out congestion during recovery, increasing flow. Thus, clockwise loops are less likely to occur when driver adaptivity is high.Transportation Research Part B Methodological 05/2011; 45(4):643655. · 2.94 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: This chapter summarizes recent findings on the bullwhip effect in decentralized multiechelon supply chains based on a systemcontrol approach. The influence of the supply chain operation (e.g., ordering policy and lead time) is separated from that of the customer demand. Robust results that hold for any customer demand are derived for both deterministically and stochastically operated chains. We demonstrate the importance of robust analysis. It is shown that instability is an inherent property of the system, e.g., of the ordering policies used by the suppliers, but it is independent of customer demand. We first present analytical stability conditions for deterministically operated chains. The demand can be arbitrary and random. These chains are modeled and their stability is evaluated in the frequency domain. We unify some techniques used in the literature, and present analytical results with or without the knowledge of customer demand. We also allow additional randomness to arise from unpredictably varying factors in the operating environment, such as supplier behavior and transportation lead times. We then develop linear matrix inequality stability conditions to predict the bullwhip effect and bound its magnitude. Examples are shown for both types of chains. We also show the effect of advance demand information on the bullwhip effect.01/2011: pages 537564;  [Show abstract] [Hide abstract]
ABSTRACT: Schedulebased or headwaybased control schemes to reduce bus bunching are not resilient because they cannot prevent buses from losing ground to the buses they follow when disruptions increase the gaps separating them beyond a critical value. (Following buses are then overwhelmed with passengers and cannot process their work quick enough to catch up.) This critical gap problem can be avoided, however, if buses at the leading end of such gaps are given information to cooperate with the ones behind by slowing down.This paper builds on this idea. It proposes an adaptive control scheme that adjusts a bus cruising speed in realtime based on both, its front and rear spacings much as if successive bus pairs were connected by springs. The scheme is shown to yield regular headways with faster bus travel than existing control methods. Its simple and decentralized logic automatically compensates for traffic disruptions and inaccurate bus driver actions. Its hardware and data requirements are minimal.Transportation Research Part B Methodological 01/2011; · 2.94 Impact Factor 
Article: Macroscopic relations of urban traffic variables: Bifurcations, multivaluedness and instability
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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 networkspecific 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 realworld 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 01/2011; 45(1):278288. · 2.94 Impact Factor 
Conference Paper: Morning Commute with Competing Modes and Distributed Demand: User Equilibrium, System Optimum, and Pricing
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ABSTRACT: The morning commute problem for a single bottleneck, introduced in Vickrey (1969), is extended to model mode choice in an urban area with timedependent demand. This extension recognizes that street space is shared by cars and public transit. It is assumed that transit is operated independently of traffic conditions, and that when it is operated it consumes a fixed amount of space.As a first step, a single fixedcapacity bottleneck that can serve both cars and transit is studied. Commuters choose which mode to use and when to travel in order to minimize the generalized cost of their own trip. The transit agency chooses the headway and when to operate. Transit operations reduce the bottleneck’s capacity for cars by a fixed amount. The following results are shown for this type of bottleneck:1.If the transit agency charges a fixed fare and operates at a given headway, and only when there is demand, then there is a unique user equilibrium.2.If the transit agency chooses its headway and time of operation for the common good, and users choose when to travel for the common good, then there is a system optimum solution with less cost and no queuing.3.Timedependent tolls and fares that achieve this system optimum are given.Finally, it is also shown that Results 2 and 3 apply to urban networks that serve a demand which is distributed in time and space, and which may include a population of captive transit riders. It is found that in many cases, additional transit service should be provided during a specified period in the rush.Kuhmo Nectar conference on transportation economics; 01/2011  [Show abstract] [Hide abstract]
ABSTRACT: This paper analyzes urban multimodal transportation systems in an aggregated way. To describe the aggregate behavior of traffic in cities, use is made of an idea that is now receiving some attention: the macroscopic fundamental diagram (MFD). We demonstrate through simulation how the MFD can be used to monitor and control a real network, in this case a portion of San Francisco, using readily available input data. We then show how different modes interact on the same network and discuss how these interactions might be incorporated into an MFD for multimodal networks. The work unveils two main results: first, it confirms recent results showing that restricting access to a city's congested areas can improve mobility for all travelers, including those who endure the restrictions; and second, that dedicating street space to collective transport modes can improve accessibility for all modes, even those from which space is taken away.Transportation Planning and Technology 12/2010; · 0.43 Impact Factor
Publication Stats
6k  Citations  
262.98  Total Impact Points  
Top Journals
Institutions

1970–2013

University of California, Berkeley
 • Institute of Transportation Studies
 • Department of Civil and Environmental Engineering
Berkeley, California, United States


2007

Universidad de Navarra
 IESE Business School
Iruña, Navarre, Spain
