Joint assessment of the impact of future railway services and design of feeder systems: towards an integrated approach

Urban railway services often constitute the backbone of the public transport offer of the territories in which they are deployed and shape the mobility patterns of travellers thanks to their high capacity. Although the attractiveness of a railway infrastructure can be largely attributed to its structure and frequency of trains, it is also dependent on the quality of feeder services that allow users to reach and leave the rail stations from and to further areas. Transportation models allow to estimate the impact that new services or policies can have on travellers. Traditional four step models consider mobility pattern on the level of large flows in a territory, making them suitable for study of rail services\cite{mcnally2007four}. However, they lack precision when it comes to understanding the much more disaggregated needs for local feeder services. The increasing popularity and recent developments of agent-based models, where each traveller and each vehicle is considered individually\cite{maggi2016understanding}, allows to build simulations with a level of details that is sufficient to study local feeder services in relationship to region-wide rail infrastructure. In this work, we propose an approach to assess the impact of future planned rail infrastructure in conjunction with the specification of feeder services. As a case study, we consider the Île-de-France region, which contains the city of Paris, where various new subway lines are being developed under the Grand Paris Express (GPE) project. The new lines will be delivered between 2026 and 2030 and will extend over a length of XX kilometers and include XX new stations. They are primarily intended to connect the suburban areas of the region without having to go through Paris. Whereas the future of operation of these future lines has been known since a few years ago, the feeder services, especially those linking new subway stations, remain to be specified. This can be explained by the fact that feeder services, whether they consist of fixed-route bus lines or other more dynamic services, are much more easily deployed than heavy rail infrastructures. Two studies on the impact of GPE on mobility in the Île-de-France region using agent-based models have been conducted in the past, relying on the MATSim simulation framework\cite{horni_multi-agent_2016} and the open Île-de-France simulation\cite{horl2021synthetic}. In first study, the assessment was conducted by assuming an on-demand service that can be used exclusively as a feeder\cite{chouaki2023towards}. In the second, feeder bus lines have been optimized using Mixed Integer Linear Programming\cite{chouaki_towards_2024}. In both cases, the investigation only focused on a reduced area concerned by the GPE project due to the computational complexity of the approaches. In this research, the integration of future GPE lines is systematized by a process that takes in the GTFS specification of the existing transportation offer in the Île-de-France region (available as open-data) and a specification of new railway services (station locations, lines and their on-peak and off-peak frequencies) and generates an updated GTFS that integrates the future offer. As for feeders, to go further in the impact assessment and in the scale of the analysis in comparison to the two previous approaches, we consider them here exclusively from the perspective of their operational implications on the users: the area around the rail station covered by the service, the average speed of the service and its frequency. Thus they are note integrated in the GTFS dataset but rather directly in the simulation model which is able to integrate them with the rest of the PT offer and propose alternatives that use feeder services if they allow to obtain better travel routes. Building on these functionalities, we perform two types of analysis comparing between three sets of simulation scenarios. As for the scenarios, we consider the \textit{baseline} simulation for the Île-de-France region, the \textit{GPE-NoFeeder} scenario where GPE lines are added without any new feeder service, and \textit{GPE-Feeder} scenarios where feeder services are added around all new stations resulting from the GPE project. As for the analyses, the first one consists in, without performing simulations with the considered public transport offers, a routing-based analysis where the best public transport route for the whole population's trips is computed under each scenario and compared. This allows to assess, on the population level, the share of public transport trips that are improved by the GPE lines and then by feeder services. The set of improved trips will then constitute potential levers for change in mobility behaviour. The second analysis directly looks at the modal shift by running full MATSim simulations, enabling mode choice for travellers and propagating the effect of better public transport alternatives on the overall trip chain. This ongoing study is meant to be fully replicable by relying on open-source software, open-data, and by sharing all the specific software components and simulation workflows in open-source.