cf. city flows is a comparative visualization environment of urban bike mobility designed to help citizens casually analyze three bike-sharing systems in the context of a public exhibition space.

Exhibition setup with three screens, and a controller column in center front.

Visitors observing bike-sharing in New York, Berlin, and London.

High-res screens showing small-multiple views.

Three high-resolution screens show the space of flows of New York City, Berlin, and London through visualizing the bike-sharing systems of these global cities. By showing the flow of multiple cities side by side, we can compare their extent and dynamics. Tracing urban movements accentuates different urban structures, and contrasts grid-plan cities like New York with historically grown cities such as Berlin. It also enables us to observe and dwell on similarities and differences in various bike-sharing systems. With our visualizations we want to understand the pulse of urban mobility and create portraits of a city defined by its transient dynamics. Cycling is increasingly recognized as critical component of future urban mobility. Riding a bike is largely independent of other traffic and unaffected by road congestion, and has health and environmental benefits. In the last few years, many cities around the world installed bike-sharing systems in order to stimulate casual use of bicycles by locals and visitors alike. We used data from these systems to visualize various aspects of bike-sharing mobility. cf. city flows is an installation that combines multiple visualizations of bike-sharing journeys in three cities. It applies established mapping and visualization techniques within a highly aestheticized framework in order to animate visitors to engage with urban mobility.

Visualization Design cf. city flows has three viewing modes, all visualizing trips of rented bikes, but focusing on different levels of spatial and temporal granularity of cycling mobility: The citywide view aggregates all trajectories of bike-sharing trips for a given day and animates the trails for trips at a given time.

aggregates all trajectories of bike-sharing trips for a given day and animates the trails for trips at a given time. In the station view only the bike trips to and from a selected station are shown, allowing the distinction between incoming and outgoing.

only the bike trips to and from a selected station are shown, allowing the distinction between incoming and outgoing. A small-multiple view visualizes spatiotemporal patterns for three selected stations each in an exploded view that separates incoming from outgoing and morning from afternoon/evening trips.

Visitors can switch between these viewing modes resulting in smooth transitioning into the next scene. Each view shows an animation moving through the day and highlights bike trips of the current time with a fading trail for better visibility. The views of all three screens are temporally and spatially coordinated, showing the same time of day and having the same map scale. Lacking actual GPS tracks, the trip trajectories are rendered as smooth paths of the calculated optimal bike routes (see Methodology for details). As different trajectories still share some segments of the street network, aligning the bike movements with actual roads makes it possible to compare the accumulated use of urban infrastructure.

Citywide view showing bike trajectories.

Citywide view Trajectories of all bike trips within a given time frame are animated on top of the background map. Underneath, all bike trips of the day are displayed as thinner, lighter paths in order to give a subtle impression of the whole space of flows and to allow comparing the current activity with the overall bike-sharing network.

Trails are used to visualize the bike trips underway at the currently selected time. The trails are rendered in what we call a firefly style with the current position shown as the brightest and the previous ones fading, resembling how a moving light source appears in a long exposure photo.

Station view showing bike trips to and from a selected station.

Station view Trips to and from a selected station are displayed on the map. For each city, a station with high activity throughout the day is highlighted and its name displayed. By selecting a single station we can distinguish the directionality of bike trips via color coding and closely observe changing ratios between incoming and outgoing trips.

Thanks to the color mapping and the fewer shown trips, this view reveals more specific spatiotemporal patterns. In London, for instance, many trips start at Waterloo Station in the morning, which hints to the commuters arriving by public transit at the train station and cycling to their workplaces.

Small-multiple view showing urban fingerprints.

Small-multiple view A detailed perspective on the activity of three bike stations per city is presented by featuring an arrangement of small multiples. The bike trips for a selected bike station are separated into four glyphs by directionality and temporality and set in a two-by-two grid. All trips to that station are displayed in the upper row and all trips from that station in the lower row. All trips in the morning are aggregated in the left column and all trips in the evening in the right column.

For each city, this view shows urban fingerprints for three stations with different bike-sharing properties. They can reveal spatiotemporal mobility patterns, such as commuters arriving to an office district in the morning and leaving the area in the evening (see station "Old Slip & Front St" in Lower Manhattan). In comparison with the small multiples on the right, more trips begin in the morning and end in the evening, which might indicate a residential or leisure area (see "E 14 St & Avenue B" close to Stuyvesant Town, a large residential complex).

Interactive dashboard The interaction with the visualization is provided via a tablet embedded in a plinth at about one meter distance to the display wall. A dashboard on the tablet shows further details about the bike-sharing systems such as the total number of stations, bikes, and trips as well as morning and evening trips per city. The temporal fluctuation of trips over the day is displayed as a horizon chart. Below the city details, users can switch between the views by tapping on one of the circular buttons. The buttons only allow for discrete steps in order to follow the deliberate sequence between the three views. At the bottom of the dashboard, an interactive time slider shows the current time, and allows visitors to select any time of day directly. Each temporal selection is reflected in all screens instantaneously.

Bike journeys in New York, Berlin, and London Watch all viewing modes for each of the cities.