## 6 Recommender System Scalability and Validation
### Scalability and Validation
Using a web client, the traffic operator can initiate a route request (source and destination) or block certain segments because of, e.g., roadworks, as shown in Figure 9. Multiple journey requests might be sent from different web clients (i.e., several traffic operators). Concerning the implementation the web visualization tool, the publicly available and free Open Street Map (OSM) was used to show the transport networks. The OSM is well maintained and continually updated by contributors from around the world. However, we used the Leaflet web framework to overlay traffic information on top of the OSM map and deliver the results to different devices including mobile interfaces. The communication between the multimodal recommender backend and the visualization web clients occurs using a full-duplex communication framework (e.g., Spring WebSocket). The provided web clients empower traffic operators to observe the traffic conditions of different modes of transport and test various scenarios to check the impact of their decisions. These clients are accessible using popular web browsers, e.g., Chrome, without the necessity to install any third-party software. The visualization interface comprises a total of five useful sections and features, as depicted in Figure 10a. The top section shows a pre-defined list of cities (for example, Sofia in Bulgaria, Coventry and Nottingham in the United Kingdom) to be simulated by the multiagent traffic management system. The second section enables the operator to submit a route request, containing some commuting preferences, such as the trip origin, trip destination, and type of route desired. The returned results yield the best route that satisfies these preferences, a mix of modes of transport, and estimated travel time and carbon emissions of the journey. The third section overlays the selected transport networks on top of the OSM map. The forth section allows the traffic operator to manipulate the traffic forecasts within a 30 min window and show various traffic attributes. The fifth and last section can be used to inspect specific route solutions. Furthermore, the traffic operators are supplemented with a k-shortest paths planner tool to simulate traffic conditions (e.g., number of vehicles, type, departure time) and observe its instant impact on route planning and guidance (Figure 10b). Figure 9: Multimodal recommender system to web clients communication. Figure 11 depicts the road segments, public transport segments, and public transport stops in the city of Sofia (Bulgaria). Different colors are used to depict the level of traffic congestion (heavy, medium, low) in the transport segments of the designated city. The regular traffic updates can be seen on the OSM map or verified through the log files. Moreover, the visualization tool provides a mechanism to test the quality of route recommendations. To this end, one can submit a commuting request that comprises a source and destination, departure time, and selected modes of transport. The submitted journey request is then passed to the route recommender agent for handling. Figure 11: (**a**) Road segments; (**b**) public transport segments; and (**c**) public transport stops (clockwise from top-left) overlaid on Sofia Open Street Map, where the segments represent agents. Figure 10: (**a**) A visualization web client for the traffic management center operators, with Open Street Map (OSM) set to Nottingham (United Kingdom). (**b**) K-shortest path planner tool. We tested the accuracy of the multimodal routes by generating several random route requests. On every occasion, the visual interface showed receipt of the request and drawing of the route solution on the OSM map for the fastest and eco-friendliest routes. For example, Figure 12 suggests two route plans for the same journey request in Nottingham (United Kingdom). It can be seen that the fastest route includes road segments, while the eco-friendliest route avoids the same segments that might induce higher CO\({}_{2}\) emissions.