Showing papers by "Rolf H. Möhring published in 2005"

System-Optimal Routing of Traffic Flows with User Constraints in Networks with Congestion

Abstract: The design of route guidance systems faces a well-known dilemma. The approach that theoretically yields the system-optimal traffic pattern may discriminate against some users in favor of others. Proposed alternate models, however, do not directly address the system perspective and may result in inferior performance. We propose a novel model and corresponding algorithms to resolve this dilemma. We present computational results on real-world instances and compare the new approach with the well-established traffic assignment model. The essence of this study is that system-optimal routing of traffic flow with explicit integration of user constraints leads to a better performance than the user equilibrium, while simultaneously guaranteeing superior fairness compared to the pure system optimum.

Partitioning graphs to speed up dijkstra's algorithm

Abstract: In this paper, we consider Dijkstra's algorithm for the point-to-point shortest path problem in large and sparse graphs with a given layout. In [1], a method has been presented that uses a partitioning of the graph to perform a preprocessing which allows to speed-up Dijkstra's algorithm considerably. We present an experimental study that evaluates which partitioning methods are suited for this approach. In particular, we examine partitioning algorithms from computational geometry and compare their impact on the speed-up of the shortest-path algorithm. Using a suited partitioning algorithm speed-up factors of 500 and more were achieved. Furthermore, we present an extension of this speed-up technique to multiple levels of partitionings. With this multi-level variant, the same speed-up factors can be achieved with smaller space requirements. It can therefore be seen as a compression of the precomputed data that conserves the correctness of the computed shortest paths.

Acceleration of shortest path and constrained shortest path computation

Abstract: We study acceleration methods for point-to-point shortest path and constrained shortest path computations in directed graphs, in particular in road and railroad networks. Our acceleration methods are allowed to use a preprocessing of the network data to create auxiliary information which is then used to speed-up shortest path queries. We focus on two methods based on Dijkstra's algorithm for shortest path computations and two methods based on a generalized version of Dijkstra for constrained shortest paths. The methods are compared with other acceleration techniques, most of them published only recently. We also look at appropriate combinations of different methods to find further improvements. For shortest path computations we investigate hierarchical multiway-separator and arc-flag approaches. The hierarchical multiway-separator approach divides the graph into regions along a multiway-separator and gathers information to improve the search for shortest paths that stretch over several regions. A new multiway-separator heuristic is presented which improves the hierarchical separator approach. The arc-flag approach divides the graph into regions and gathers information on whether an arc is on a shortest path into a given region. Both methods yield significant speed-ups of the plain Dijkstra's algorithm. The arc flag method combined with an appropriate partition and a bi-directed search achieves an average speed-up of up to 1,400 on large networks. This combination narrows down the search space of Dijkstra's algorithm to almost the size of the corresponding shortest path for long distance shortest path queries. For the constrained shortest path problem we show that goal-directed and bi-directed acceleration methods can be used both individually and in combination. The goal-directed search achieves the best speed-up factor of 110 for the constrained problem.

Conflict-free Real-time AGV Routing

Abstract: We present an algorithm for the problem of routing Automated Guided Vehicles (AGVs) in an automated logistic system. The algorithm avoids collisions, deadlocks and livelocks already at the time of route computation (conflict-free routing). After a preprocessing step the real-time computation for each request consists of the determination of a shortest path with time-windows and a following readjustment of these time-windows. Both is done in polynomial-time. Using goal-oriented search we get computation times which are appropriate for real-time routing. Additionally, in comparison to a static routing approach, used in Container Terminal Altenwerder (CTA) at Hamburg Harbour, our algorithm had an explicit advantage.

Minimizing Total Delay in Fixed-Time Controlled Traffic Networks

Abstract: We present two different approaches to minimize total delay in signalized fixed-time controlled inner city traffic networks. Firstly, we develop a time discrete model where all calculations are done pathwise and vehicles move on “time trajectories” on their routes. Secondly, an idea by Gartner, Little, and Gabbay (GLG) is extended to a continuous, linkwise operating model using “Link Performance Functions” to determine delays. Both models are formulated as mixed-integer linear programs and are compared and evaluated by PTV AG’s simulation tool VISSIM 3.70.

05031 Summary -- Algorithms for Optimization with Incomplete Information

Abstract: This paper summarizes the objectives and structure of a seminar with the same title, held from January 16th to 21th 2005 at Schloss Dagstuhl, Germany

ATMOS 2005 Abstracts Collection - Selected Papers from the 5th Workshop on Algorithmic Methods and Models for Optimization of Railways.

Abstract: This issue contains six papers that were presented in preliminary form at the 5th Workshop on Algorithmic Methods and Models for Optimization of Railways (ATMOS 2005), held at Palma de Mallorca, Spain, October 7, 2005 in conjunction with ALGO 2005. These papers are representative of several areas of research within the scope of ATMOS: rolling stock circulation and engine assignment, station location, line planning, railway traffic scheduling and dispatching, transfer optimization within network design, and fast traffic information systems.

ATMOS Preface - Algorithmic Methods and Models for Optimization of Railways.

Abstract: This issue contains six papers that were presented in preliminary form at the 5th Workshop on Algorithmic Methods and Models for Optimization of Railways (ATMOS 2005), held at Palma de Mallorca, Spain, October 7, 2005 in conjunction with ALGO 2005. These papers are representative of several areas of research within the scope of ATMOS: rolling stock circulation and engine assignment, station location, line planning, railway traffic scheduling and dispatching, transfer optimization within network design, and fast traffic information systems.