## 2 Research Status  
In the middle of the nineteenth century, the first mechanized parking system was based on the vertical Ferris wheel, which effectively solved the problem of stacking vehicles in limited space. Since then, the same system has been used extensively in the United States, Asia and Europe [23].  
In 2006, [PERSON] et al. designed a routing system. This system can forecast traffic conditions to some extent, but the vehicles are all driven by people, so the accurate driving time for each vehicle cannot be calculated.  
In 2011, [PERSON] et al. designed a model of the large parking lots guidance information system. It mainly uses the ant colony optimization to design the paths for parking space guidance. Ant colony optimization is a good way to find the shortest path from the start point to the end. However, the algorithm only considers the distance of the route. There is a great possibility that vehicles will line up in the same short way and wait for a long time [24].  
[PERSON] et al. presented a new system for intelligent navigation inside a parking lot in 2015 [25]. After modeling the parking lot with a time-varying graph, the proposed system applies a time-varying shortest path algorithm and dynamically tunes arc transit times based on planned vehicle routes as well as traffic flow sensor data. The proposed system showed an improvement of about 40% in travel time when compared with a system that only applies the conventional Dijkstra algorithm. But the differences in driving skills of human drivers bring a lot of errors to the system. It will not be a good choice for a driverless parking lot.  
In early 2015, the high-tech Internet company Alpark began to develop intelligent parking systems [26; 27]. Under the intelligent parking lot control system developed by the company, the driver only needs to drive into the parking lot. The parking lot can automatically identify the license plate,guide the car to the empty parking space and plan the route. The technology solves the parking problem to some extent [28; 29; 30; 31]. It is a relatively complete model of the intelligent parking guidance system. However, since the vehicle is driven by the driver, the intelligent parking system cannot control the vehicles' speed and the actual driving route. It makes the guiding route planning of the following vehicles difficult.  
On 14 September 2018, the automatic valet parking technology jointly developed by Daimler and Bosch was exhibited in Beijing [32]. In the underground parking lot of the Mercedes-Benz passenger car China R&D center, two Mercedes-Benz E63s were used for valet parking operation at the same time. The two-lane dual-vehicle synchronous valet parking process was successfully realized. After the vehicle enters the parking lot, vehicle control is handed over to the parking lot [33; 34]. As shown in Figure 1, single-line light detection and ranging systems (Lidars) are placed in the parking lot. They are mainly used to sense the accessible areas and obstacles in the parking lot [35], so as to control the vehicle for emergency braking or winding in time.  
The appearance of this technology can make the vehicles in the parking lot completely controlled by the base station. It reduces the occurrence of unexpected situations. However, in terms of route planning, the system cannot adjust the route according to the real-time road conditions in the parking lot, and it is prone to occur congestion when facing large traffic.  
Therefore, it is meaningful to avoid congested roads in the process of route planning. In the new technology background of the combination of the V2X environment and driverless technology, we did research on how to design the route to reach each parking space with the least time based on the real-time working conditions in the parking lot.