# Smart Recommendations for Renting Bikes in Bike-Sharing Systems
## 1 Introduction  
Due to the growing need for reducing air pollution and, at the same time, offering mobility services of satisfactory quality to citizens, new means of urban mobility are being explored in many large cities around the world. Bike-sharing systems (BSSs) are just one of the many solutions that have been proposed and implemented in recent years. In some cities, they have grown to considerable size. For example, the system Velib ([[http://www.velib-metropole.fr/](http://www.velib-metropole.fr/)]([http://www.velib-metropole.fr/](http://www.velib-metropole.fr/))) in Paris has \(\sim\)15,000 bikes, the Santander Cycles project in London (tfl.gov.uk/modes/cycling/santander-cycles) has \(\sim\)11,000 bikes and cities in China such as Hangzhou or Wuhan employ as many as 78,000 and 90,000 bikes, respectively [1]. Some of those systems are so called free-floating systems (e.g., _BikeShare_ in Seattle or DB _Callabike_ in Munich) that allow citizens to pick up and return a bike at any location within a certain area. Others are station-based--that is, they rely on a set of rental stations with fixed locations. Station-based BSSs have the advantage of providing better control to avoid unauthorized access. When electric bikes are deployed in the BSS--as, for example, in the BiciMAD system in Madrid (Spain)--stations usually serve as charging spots for the bicycles' batteries as well. Effectiveness and user satisfaction within station-based BSSs strongly depend on the availability of bikes (or slots for returning rented bikes) at the places where users search for them or demand them. Therefore, proper management policies need to be implemented not only for _strategic_ decisions (e.g., regarding the positioning and dimensioning of rental stations, the selection of adequate bicycle models, etc. ), but also at the _operational_ level. In particular, BSSs are usually faced with agglomeration or unbalancing problems: available bikes are concentrated at certain areas (and at certain times of the day), whereas in other areas few or no bikes are available. The effect is that users may experience a poor quality of service, being unable to find a bike (or a return slot) at a given station or area. Solutions such as reserving bikes (or slots) at some stations palliate this problem, but proper bike balancing mechanisms are needed in order to tackle the problem at its core. In many systems, trucks are used to address such balancing problems, by moving bicycles from (almost) full to (almost) empty stations. Truck movements are scheduled either _statically_ (usually at night) or _dynamically_ (during BSS operation), with the aim of finding a near-optimal match between the expected demand and the available resources. However, truck movements are expensive, and raise environmental concerns. Today, BSSs are often conceived as cyber-physical systems [2,3]. Bikes may be endowed with several battery-powered sensors that measure important information such as position or battery level (in case of electric bicycles). Sensors at stations may capture rental attempts that were frustrated--e.g., due to mechanical failures of slot clamps--and, of course, gather key information regarding the operating state of the system, such as the numbers of available and occupied slots. This information is then transmitted to a software application at the BSS control centre, and helps operators to make timely management decisions in an online manner. Part of this information is often shared with the BSS users, who may use smartphone apps that inform them about the current state of stations in real time. Users can then use this information to make more informed decisions as to where to rent or to return a bike. However, due to the dynamic nature of the BSS environment, it often transpires that, when a user reaches a certain station, initially available bikes have already been taken by other users. Similarly, information on available parking slots may already be outdated when a user arrives to a station.