# Survey of Smart Parking Systems
## 4 Vehicle Detection Techniques (VDT) Implemented in SPS
### Over-Roadway Sensors  
These sensors are placed above the road surface, i.e., unlike IRS, they work on vehicles and not below them. One of the most common technologies for identifying vehicles and parking slots in this category is closed circuit television (CCTV) [163; 205], which allows the occupation of a parking slot to be determined through captured images. Some of these implementations use algorithms of deep learning for image recognition to determine even the number plates of the vehicles; thus, the efficiency of the system is increased considerably [230].  
On the other hand, other types of ORS are infrared sensors. This technology is based on the combination of an emitter and a receiver, which are devices placed close together and usually form part of the same integrated circuit. By identifying an interference between the signals emitted, they detect an object in the environment [45]. Implementing this technology can be less expensive than that of other types of sensors. Many proposals and implementations with these circuits make use of Arduino modules, which are cheaper and easier to program [174; 175; 178].  
RFID readers are other widely used sensors [101; 126; 228]. This technology requires a reading device which is usually installed in each parking slot or at the entrance to them. The reading device emits a series of radio frequency waves to a chip that possesses information that will be sensed. These chips or RFID tags must be installed in each vehicle for the system to work [4], which is a disadvantage in this type of implementation, since, without this, the system cannot obtain information on the state ofthe parking slots. The chip picks up the waves emitted by the reader and transmits to it the data it stores. The advantage is that these chips are usually very precise in their detection function [163].  
The potential offered by the combined use of these different technologies and sensor standards offers high performance. The joint use of technologies such as RFID, WSN, and near-field communication (NFC) makes it possible to collect (in real time) the environmental parameters and information on the occupancy status of parking slots in a common space through a hybrid detection network made up of these technologies. This type of integration is used and analyzed in implementation environments for the Internet of things (IOT) [32; 227].  
Another new technology implemented in VDT, which can be classified as ORS, is the Bluetooth low-energy (BLE) beacon. These evolved discreetly but steadily since their first use in 2013, with the first version of Apple's iBeacon(r). The devices are based on micro-localization and are generally used in closed environments (due to their short range). In SPS, they are used to identify vehicles inside a closed car park, in order to guide them to a parking space based on their location. As these are short-range devices, a certain quantity must be available to cover the entire parking area to be able to detect vehicles from any point [79]. Identifying a vehicle can be done more precisely by using this Bluetooth technology through a driver's cell phone. Because the sensing is done on mobile devices and not on the vehicle itself, these devices can then also be classified within the VDT crowdsensing group (see Section 4.4).