# Survey of Smart Parking Systems
## 3 Types of SPS Reported in the Literature
### Parking Reservation Systems  
PRS use technology to reserve a parking space ahead of arrival, ensuring that the desired slot for the vehicle is held, and providing the system with the ability to indicate to the other drivers those places that were already reserved and are not available. PRS require data centers or servers to be set up, where all reservations and inquiry operations concerning available parking are processed and a communication system between the users and these servers is established. Users are able to reserve a parking slot and receive an answer from PRS through various communication media such as mobile phones or a web page [15; 16]. In addition, these systems may be able to provide real-time monitoring of current parking availability and forecast demand estimates based on historical data [17]. The types of reservations can be different in each system. There is the normal reservation (mentioned above) and others that, for example, combine real-time bookings where a driver can book a place while driving (for instance, a few minutes ahead), with timeshare bookings, or booking at any time in the future (for example, many days ahead) [18]. Another alternative is automatic reservations, where the user does not have to make the reservation but rather is assigned a parking spot based on the location of the final destination and parking availability [19]. This type of SPS can be implemented with varying degrees of difficulty, depending on the type of parking zone where it is being applied (e.g., covered private parking, public street parking, etc.). We consider that, in covered private parking, the reservations can be implemented more easily because there exists the ability to guarantee the availability of the reserved space by requesting verification from the vehicle or the driver who made the reservation. In contrast, in public street parking, actions such as user verification (driver or vehicle) to secure the reservation are more difficult to carry out and more costly to implement [20]. In this context, the system must be complete when it implemented, i.e., all drivers must use it so that it is easier for everyone to access the availability of parking slots. Alternatively, parking on the street (in the spaces reserved for this activity) may be subject to fines. In environments where there is no reservation check, the driver can choose to reserve or not a space for their vehicle [14]. One solution to this, although costly, is to implement a parking control in the system when the driver parks their vehicle in a reserved area, for example, using this alternative through radio frequency identification (RFID) readers that validate that the user who parked in that slot is the correct one [21; 22]. Another discussion within PRS is the kind of payment policies they implement. As one of the characteristics of PRS is being able to reserve parking slots, many papers incorporate the possibility of charging a fee for this. Private car parks mostly require payment, as do many parking places available on public streets in cities. In this context, PRS present different alternatives. For instance, there are options to collect an established fee to park the vehicle [23]. In addition, there are others that make this feature more flexible in order to improve the users' experience, for example, by offering a price-on-demand option. This gives users the ability to choose between the most convenient options (presumably located in more central parts of a city) or less expensive ones (usually found on the periphery) [20]. On the other hand, there are also dynamic pricing policies, and parking will be more expensive during the rush hour (more traffic and more parking places occupied) and cheaper otherwise [24]. This implies having the ability to identify when the rush hours are and set prices accordingly by taking into account parking space occupancy and historical traffic data or using real-time traffic data.