# Portable Optical Fiber Bragg Grating Sensor for Monitoring Traffic Density
## 1 Introduction
### Fiber Optic and Optical Sensors  
An alternative option for monitoring basic traffic parameters are sensors employing optical fibers, which function on different principles. These sensors offer many significant advantages, such as reliability, accuracy, high sensitivity, electromagnetic and chemical resistance, electrical passivity and a broad temperature operating range. The greatest potential of optical fibers is in hybrid networks for data transmission, frequently in combination with its use as a sensor sensitive to certain measured parameters.  
Optical infrared sensor gates work on the principle of transmitting and receiving a laser or infrared beam from an emitter to a receiver [12; 13].  
Another type is a special fiber optic micro-motion sensor [14], which works on the principle of increasing optical fiber attenuation. The fibers are embedded in special protective covers and built into a roadway. This type of sensor is experimental in nature and not used in general practice.  
Other relatively frequently used types are WIM (Weigh-in-Motion) sensors. These are sensors built into roadways for monitoring the load from vehicle wheels and axles, vehicle speed, measuring the distance between axles, measuring vehicle width, etc. This type of sensor [15; 16; 17; 18] is often based on different types of fiber-optic interferometers. Due to their high sensitivity, interferometric sensors can also be placed non-destructively next to a road, as described by the authors of [19; 20; 21]. It monitors basic parameters, such as traffic density and vehicle speed.  
Another technology under discussion today is the DAS (Distributed Acoustic Sensor). DAS technology [22; 23] uses a single optical fiber to monitor multiple traffic variables, such as detecting a car, its direction, its speed, etc.  
[PERSON] et al. [24] described the use of fiber Bragg grating strain sensors to monitor the health of bridges as well as the means to monitor vehicle traffic patterns and critical data such as speed, weight, and classification of vehicle types.  
An invasive method for monitoring selected road traffic parameters which included detecting vehicles and vehicle speed and weight was conducted on the I-84 highway in Portland using five FBG (fiber Bragg grating) sensors installed into the asphalt pavement at a depth of 7 cm [25; 26]. Insufficient protection of the FBG sensors resulted in damage to the sensors. The authors successfully solved this problem by adding composite fiber protection to the FBG sensors.  
In the study by [PERSON] and [PERSON] [27], [PERSON] et al. [28], a three-dimensional Glass Fiber-Reinforced Polymer packaged Fiber Bragg Grating sensor (3D GFRP-FBG) is introduced for vehicle speed and wheelbase estimation. These sensors were evaluated in practice by installing them into a real roadway. The experimental results of this study achieved a measurement reliability of at least 95%. However, the sensor is relatively complex to manufacture and implement.  
[PERSON] et al. [29] presented on of the most recent studies. This paper presents an alternative WIM (Weigh-in-Motion) system to measure a vehicle's velocity, wheelbase and axial and gross weight based only on a single set of long-gauge FBG sensors. The results show that this method can achieve its function with good accuracy under different conditions.  
Fiber Bragg sensors are increasingly being used to monitor selected parameters in automotive traffic. Table 1 summarizes the selected types of fiber optic sensors and their parameters.  
This study followed up on the trend of expanding fiber optic sensors into the field of automotive traffic monitoring. This publication describes an innovative portable FBG sensor for traffic density monitoring, whose advantages are a high success rate in detecting cars and non-invasive installation onto surfaced areas. The sensor was designed and subsequently evaluated for detecting urban traffic, i.e., in the typical range of values of 10-80 kph. For higher speeds, follow-up research will be conducted.