Combining Vehicle Routing Models and Microscopic Traffic Simulation to Model and Evaluating City Logistics Applications
J Barcelo, H Grzybowska, Universitat Politecnica de Catalunya, ES; S Pardo, TSS ? Transport Simulation Systems, ES
Taniguchi defines City Logistics as "the process of totally optimising the logistics and transport activities by private companies in urban areas while considering the traffic environment, traffic congestion and energy consumption within the framework of a market economy". The distribution of goods based on road services in urban areas contribute to traffic congestion, generates environmental impacts and in some cases incurs in high logistics costs. On the other hand the various stakeholders involved in the applications may have possibly conflicting objectives. Industrial firms, shippers, freight carriers, have individually established to meet consumer demands looking to maximize the company effectiveness and as a consequence from a social point of view the resulting logistics system is inefficient from the point of view of the social costs and environmental impacts. As a consequence the design and evaluation of City Logistics applications requires an integrated framework in which all components could work together. Therefore City Logistics models must be models that, further than including the main components of City Logistics applications, as vehicle routing and fleet management models, should be able of including also the dynamic aspects of the underlying road network, namely if ICT applications are taken into account. Some of the methodological proposals made so far are based on an integration of vehicle routing models and, dynamic traffic simulation models that emulate the actual traffic conditions providing at each time interval the estimates of the current travel times, queues, etc. on each link of the road network, that is, the information that will be used by the logistic model (i.e. a fleet management system identifying in real-time the positions of each vehicle in the fleet and its operational conditions - type of load, available capacity, etc. - to determine the optimal dynamic routing and scheduling of the vehicle.
This paper reports on the modelling framework developed and tested in the European Project MEROPE of the INTERREG IIIB Programme, and in the national project SADERYL, sponsored by the Spanish DGCYT. The modelling framework is supported by a Computer Decision Support System which core architecture consists of the following main components: A Data Base, to store all the data required by the implied models: locations of logistic centres and customers, capacities of warehouses and depots, transportation costs, operational costs, fleet data, etc.; a Data Base Management System, for the updating of the information stored in the data base; a Model Base, containing the family of models and algorithms to solve the related problems, discrete location, network location, street vehicle routing and scheduling, to account explicitly with the asymmetry of costs typical of urban applications etc.; a Model Base Management System, to update, modify, add or delete models from the Model Base; a Graphic User Interface, GUI, supporting the windows based dialogues to define and update data, select the model suited to the intended problem, apply the corresponding algorithm, visualize the problem and the results, etc.
Taking into account the nature of the problems addressed in City Logistic Application, and their underlying geographic reality, the framework in which the GUI has been embedded is that of a software platform with the main GIS functions required to support transport applications, which imports the digital map of the urban area, and generates automatically the graph its road network to generate the input for the Network Location and Vehicle Routing models.
The underlying dynamic simulation model (AIMSUN in this case) tacks individually the fleet vehicles, emulating in this way the monitoring of fleet vehicles in a real time fleet management system, gathering dynamic data (i.e. current position, previous position, current speed, previous speed, etc.) while following the vehicle, in a similar way as the data that in real life an equipped vehicle could provide. This is the information required by the "Dynamic Router and Scheduler" to determine which vehicle will be assigned the new service and which will be the new route for the selected vehicle.
Association for European Transport