## 1 Introduction  
Nowadays, a lot of cities have started to look for intelligent transportation systems. The main principle of intelligent transportation systems is to assign the appropriate traffic signal time or manage the parking lots intelligently according to the information collected by vehicle detectors [1]. At present, there are various vehicle detection methods, such as inductance loop [2; 3; 4], geomagnetic sensor [5], microwave radar, infrared, video, etc. The video vehicle detector has the advantages of high detection accuracy, stability and reliability and widely used in the world. It not only can be used to monitor road conditions, capture speeds, count vehicles, classify vehicle also plays an important role in the reduction of the number of road accidents [6]. However, the cost of video vehicle detectors is still relatively high compared with other vehicle detectors and many traffic information collection scenarios do not require image data. The inductance loop vehicle detector is widely used because of its low cost, strong practicability and high accuracy. However, it is very difficult to install and maintain the inductance loop vehicle detectors on the roads, the surface structure of the roads will be seriously damaged and the aesthetics of the roads or parking spaces will be seriously affected by the installed inductance loop vehicle detectors. Due to the power supply problem, it is difficult to install the inductance loop vehicle detectors in some scenarios such as the roadside parking space monitoring application. Generally, the battery-powered geomagnetic sensor vehicle detectors are available 5-10 cm in height and 5-15 cm in diameter. And the cost of those detector is about $35 in China and the working life of those is up to 3 years. Therefore, the geomagnetic sensor vehicle detector has the advantage of low power consumption, low cost and easy deployment. The geomagnetic sensor vehicle detectors have been widely used for collecting traffic data and gradually replaced the inductance loop vehicle detectors in a variety of occasions in recent years. The earth is a natural magnet with a surface magnetic field of about 500-600 mGs. The magnetic field remains relatively stable in a certain area. When a ferromagnetic substance such as vehicle having a magnetic permeability is present in the area, the magnetic field of the region is disturbed, resulting in the magnetic strength becoming larger or smaller [7]. Geomagnetic sensor is a kind of chip that can convert magnetic signal into electrical signal. Therefore, vehicles can be detected by analyzing the change in the magnetic field around the geomagnetic sensor. However, there is a geomagnetic signal blind zone between the front and rear axle of a vehicle [8]. The blind zone is very obvious in the high-chassis large vehicles such as trucks, buses, SUVs (Sport Utility Vehicles), etc., which leads to multiple-detection problems in detecting those vehicles [9] when running at low speed on roads. Simultaneously, the stopping position of a vehicle is not standard when waiting for the traffic light to change or parking on a parking lot, the geomagnetic signal blind zone problem will cause the vehicle to be undetected by the vehicle detector with the traditional single magnetism-based method. The vehicle actuated signal control strategy has been widely adopted in traffic control system and requires the real-time information about the presence of vehicles from vehicle detectors installed on roads. The inaccurate vehicle detection results will cause the traffic signal controller to assign wrong signal timings when the traffic signal controller equipped with the vehicle actuated signal control strategy [10]. What is more, the coordinated adaptive traffic control system such as Sydney Coordinated Adaptive Traffic System (SCATS), which requires number of spaces and total space time during green of each cycle to implement maximum throughput, minimum stops and minimum delay strategies [11].