Papers by Hedayat Zokaei-Aashtiani
Improving synchronization in a bus network has a profound effect on reducing waiting times and in... more Improving synchronization in a bus network has a profound effect on reducing waiting times and increasing satisfaction of system’s users. By doing so, the scheduler create a more attractive transit system that generates the opportunity for increasing the number of riders. In this research, the problem of creating timetables with maximum synchronization is modeled as a mixed integer programming (MIP) problem. The objective of this model is to minimize the total waiting times of passengers at transfer nodes. For this purpose, the model takes advantage of uneven headways. Some of features that distinguish this research are more realistic objective function, and considering adjacent runs before and after the planning horizon. These features make the research more practical. The problem is solved using CPLEX as an exact method. Besides, approximate solutions can be achieved by setting up different time limits on this software. As numerical example, these methods are used for solving the ...
Several years ago, we proposed and studied a linearization and decomposition algorithm for comput... more Several years ago, we proposed and studied a linearization and decomposition algorithm for computing general urban traffic equilibria. The procedure applies to models that include link interactions, multiple commodities (e.g., modes of transit, or user classes), and with demand relationships general enough to model interactions between origin-destination pairs, such as travelers at an origin choosing trips from among several possible destinations. As reported in this paper, our computational experience with this algorithm has been very promising. It suggests that linearization and decomposition methods enable general problems to be solved with computational effort comparable to that of specialized techniques used for solving subclasses of equilibrium problems that have equivalent optimization formulations.
The applied Aashtiani’s Complementarity formulation to solve a large-scale static traffic equilib... more The applied Aashtiani’s Complementarity formulation to solve a large-scale static traffic equilibrium problem. This formulation requires very weak assumptions and is capable of computing a very general form of traffic equilibrium including multi-modal, multi-class user, multi-variate demand functions, and multi-variate link performance functions equilibria. The traffic equilibrium problem for a large metropolitan area has been formulated as a nonlinear Complementarity problem and solved via decomposition and linearization techniques. A dynamic memory data structure has been introduced to accommodate the data manipulation needs for this path-based formulation. The results of this approach have been compared to that of the conventional Beckman’s formulation under various scenarios. It is shown that the Complementarity formulation not only is capable of handling real-life problems, but also outperforms the conventional Frank-Wolf algorithm. The benefits of this approach become even mor...
ABSTRACT "Most of the results of this report were presented at the National ORSA/TIMS me... more ABSTRACT "Most of the results of this report were presented at the National ORSA/TIMS meeting, Chicago, May 1975." "7-113-77." Cover title. Bibliography: p. 35-38. Supported in part by the U.S. Deaprtment of Transportation, Transportation Advanced Research Program (TARP) contract no. DOT-TSC-1058 Supported in part by the Office of Naval Research under contract. N00014-75-C-0556 by H. A. [i.e. Z] Aashtiani and T. L. Magnanti.
Scientia Iranica
In transportation literature, User Equilibrium (UE) has been widely studied since early 1950's, m... more In transportation literature, User Equilibrium (UE) has been widely studied since early 1950's, many studies of which define equilibrium flow of traffic for uncapacitated networks based on Wardrop's first principle, implying also a Nash Equilibrium (NE). Although, in general, the two equilibria (UE and NE) are not explicitly the same, they are shown to be equivalent under special conditions, for uncapacitated UE, when volume-delay functions are separable, continuous, nondecreasing and non-negative. A good deal of research is devoted to explain UE in capacitated networks based on Wardrop's first principle and the concept of generalized costs. However, UE for capacitated networks, even under the defined special conditions, is not equivalent to NE. This paper extends Wardrop's first principle to explain UE in capacitated networks, which, under the same special conditions of uncapacitated networks, would represent an NE as well. Moreover, a complementarity equilibrium model is proposed for UE, based on an extension of Wardrop's principle.
Transportmetrica A: Transport Science
Transportation Research Part A: General
Supervised by Thomas L. Magnanti. Thesis (M.S.)--Massachusetts Institute of Technology, Sloan Sch... more Supervised by Thomas L. Magnanti. Thesis (M.S.)--Massachusetts Institute of Technology, Sloan School of Management, February 1976. Includes bibliographical references (leaves 64-67).
SIAM Journal on Algebraic and Discrete …, 1979
... ADDRESS = Cambridge, MA. TYPE = BOOK. AUTHOR = Ben-Ayed, O. and Boyce, DE and Blair, CE. TITL... more ... ADDRESS = Cambridge, MA. TYPE = BOOK. AUTHOR = Ben-Ayed, O. and Boyce, DE and Blair, CE. TITLE = A general bilevel linear programming formulation of the. ... PUBLISHER = Springer-Verlag. EDITOR = A. Bensoussan and JL Lions. ADDRESS = New York. ...
Journal of Transportation Engineering, Dec 17, 2011
Transportation Planning and Technology, Jul 1, 2011
This paper develops a model for estimating unsignalized intersection delays which can be applied ... more This paper develops a model for estimating unsignalized intersection delays which can be applied to traffic assignment (TA) models. Current unsignalized intersection delay models have been developed mostly for operational purposes, and demand detailed geometric data and complicated procedures to estimate delay. These difficulties result in unsignalized intersection delays being ignored or assumed as a constant in TA models.Video and
Http Dx Doi Org 10 1080 0308106042000316367, Feb 1, 2007
ABSTRACT Braess' phenomenon, also known as Braess' paradox, is a phenomenon that ... more ABSTRACT Braess' phenomenon, also known as Braess' paradox, is a phenomenon that has received considerable attention in transportation engineering and planning, as well as in other fields. It has an important implication in the area of investment in transportation networks, namely that adding a new link in a network may increase the cost to the users of that network. In this paper we show this phenomenon in a new environment. Unlike traditional examples, which involve the physical addition of a link to a network (a 0/1 integer decision variable), an example is presented where the decision variable is continuous in nature. Moreover, this example conveys two new messages. First, it is shown that some logical criterion of assigning common resources among users of a network proportional to the number of users competing for common resources may not serve the purpose of efficiency in the sense of minimizing total user travel cost (time). Second, it is demonstrated that the very interdependence of variables and decisions may lead to multi‐equilibrium solutions to the equilibrium flow problem, some of which may not satisfy the once taken for granted characteristic that if the flow pattern satisfies equilibrium conditions, then no user may be better off by unilaterally changing paths.
Transportation Research Record Journal of the Transportation Research Board, 2002
The transit assignment process applied as part of the development of the Tehran transportation mo... more The transit assignment process applied as part of the development of the Tehran transportation model is described. The process includes development of various models for dwell time as a function of transit volume. Dwell time is the time a transit vehicle spends at a stop to allow passengers to alight and board. This method was implemented by using EMME/2 transportation planning software. The calculation of dwell time is necessary in modeling transit assignment because an accurate estimation of dwell time will lead to more precise transit assignment results. The area analyzed in the model comprises various transportation analysis zones in the city of Tehran. The model output was shown to be statistically significant. Calculations were found to be valid when compared with observed data.
This study develops a simple method for estimating delay at intersections with and without signal... more This study develops a simple method for estimating delay at intersections with and without signals. A knowledge of the magnitude of intersection delay can assist planners to optimize traffic flow at intersections. The technique described is applied by using EMME/2 transportation planning software. Calculation of intersection delay is also deemed necessary when modeling traffic flow. An accurate estimation of intersection delay will lead to more precise traffic assignment results.
Computer-Aided Civil and Infrastructure Engineering, 2015
SIAM Journal on Algebraic Discrete Methods, 1981
Network equilibrium models arise in applied contexts as varied as urban transportation, energy di... more Network equilibrium models arise in applied contexts as varied as urban transportation, energy distribution, spatially separated economic markets, electrical networks, and water resource planning.
Transportation Research Part B: Methodological, 2015
"This article has appeared in Proceedings, IEEE 192 Large-Scale Systems Symposium"--P.1... more "This article has appeared in Proceedings, IEEE 192 Large-Scale Systems Symposium"--P.1. Includes bibliographical references (leaves 11-12). Supported in part by Dept. of Transportation DOT-TSC-1058. Supported in part by National Science Foundation 79-26225-ECS. by Hedayat Z. Aashtiani and Thomas L. Magnanti.
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Papers by Hedayat Zokaei-Aashtiani