Assignment of walking trips to pedestrian network in the context of the 4-steps travel demand model

Abstract

In the transportation planning process, the Four Step Model (FSM) is used to define the needs and requirements of the transportation system within a city or a region. Despite its wide use, the model is focused on vehicular trips and fails to represent the demand of walking activity. The limited work that has been performed to address the misrepresentation of walking in the context of the FSM, still does not address the last step of the four step model, the assignment of the walking trips to the network. Stemming from this literature gap, and the need to enhance the role of walking activity within the transportation modeling, the main question of this thesis is to develop a method for the assignment of the walking trips to the street network. This research suggest a methodology to model large continuous space and walking trips on it. Thus, the focus is on modeling walking space taking into account the pedestrian scale for the model. To implement that, the size of the spatial analysis zones of the FSM is adjusted to walking scale. Next, continuous walking space is defined by the BGT spatial dataset that offers information for the land covers. In order to model continuous walking space two space discretization techniques are compared; first, Constrained Delaunay Triangulation (CDT) is applied on the walking area polygon, second, a regular quadrilateral grid is overlaid with the walking area. Walking activity is modeled on the network that is formed with the dual graph of each discretization method. In a subsequent step, the trip injection is performed with zone centroids and connectors which were created with the point clustering algorithm DBSCAN having as input points the graph nodes. Finally, A* algorithm is used for deterministic an all-or-nothing network assignment. The results of the applied methodology show that addressing walking within the FSM requires a finer grained approach due to the more localized scale of walking activity. Walking space needs to be defined as a continuous surface. In order to model such surface at the scale of the FSM, a discretization method is required to handle continuous space at large scale. Comparing the result of the two discretization techniques, polygon triangulation and regular grid, the triangulation dual graph results to the ’skeleton’ of the polygon while the regular grid dual graph offers a more dense and homogeneous polygon representation. Additionally, in the trip injection process, the nodes of the regular grid perform better than the triangulation graph nodes. Overall, throughout the whole implementation, the performance of using a regular grid, outweighs the polygon triangulation as discretization method.