road traffic monitoring responds to multiple needs, first of all the increase in traffic safety; the same highway code explicitly provides for its creation. it can be included in a wider user information system, taking shape of a necessary and important first phase.
There are now numerous techniques tested and technologies offered by the industry in this sector that can be used for the creation of a monitoring system. A first major distinction is that between:
- manual detection: require the continuous presence of a human operator
automatic detection: do not require the presence of a human operator
The manual detection appears as a versatile technique for the counting of vehicles, being able not only to detect the vehicle and recognize its type, but also to evaluate the maneuvers and the driver’s behavior. However, the human observer has natural limits that do not guarantee the correctness and accuracy of the information detected, first of all the non-instantaneous recording speed and tiredness, which determine a lower reliability with increasing detection time (yes believes that a surveyor is efficient for a maximum period of approximately 3 hours). Therefore they are generally used for short-term surveys. The main manual detection methods are:
- the fixed observer method;
the mobile observer method;
the stroke counter.
The need to continuously and accurately collect traffic data, over long periods and at all hours (both day and night), has led the technological research of the sector to produce increasingly refined automatic detection instruments and systems. In principle, an automatic system for measuring traffic parameters consists of four basic components: a detector (sometimes called a sensor), an interpreter, a recorder and a computer. The detector is the component sensitive to the quantity to be measured (vehicle passage, vehicle speed, etc.); for each information detected it generates a signal which is received and then deciphered by the interpreter; the coded signal is then transmitted to the recorder which, in fact, records the data necessary for processing; the latter can take place either in real time or at a later stage, depending on the type of system used.
We have already listed in the previous article the most popular automatic detection technologies; these also include pneumatic tubes and triboelectric cables.
Let’s see them in more detail.
Sensor consisting of a pneumatic tube positioned on the roadway and connected to a counter device placed on the roadside. When the wheels of a vehicle squeeze the pneumatic tube, a pressure wave is generated inside which generates an impulse in the meter.
Ease of installation and low cost are the main advantages of the sensor which, however, also has some disadvantages, including:
- counting inaccuracy in the presence of high flows
- the impossibility of obtaining the transit data of multi-axial vehicles
- danger of detachment of the pipe from the road pavement
- the risk of mechanical breakage of the tube by especially heavy vehicles
- the inability to count across multiple lanes
The operation of this sensor is based on the triboelectric effect, i.e. the electrification by rubbing of a dielectric material: when the wheels of a vehicle pass on the cable, the steel wires of the outer ring of the cable rub the surface of the dielectric material, electrifying it, and thus causing an accumulation of electric charge; this involves sending an electrical signal and then recording the passage of the vehicle axle.
Therefore, similarly to pneumatic tubes, the triboelectric cables count the vehicles in transit starting from the detection of the axles of the vehicles themselves. Compared to pneumatic tubes, however, triboelectric sensors are preferred because they are more robust and resistant, less visible and in any case not much more expensive.