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Oscillating electrical signal is applied to the loop.
The metal content of a moving vehicle chassis changes the electrical properties
of circuit.
Changes are detected at a roadside unit, triggering a vehicle event.
A single loop system collects flow and occupancy.
The speed can be calculated by the assumptions that are made for the mean length
of vehicles.
Two-loop systems collect flow, occupancy, vehicle length, and speed.
Advantages
- It is a very cheap technology. Almost every dynamic traffic control system
in this world uses IDL data.
Disadvantages
- Loops are damaged by utility and street maintenance activities or
penetration of water.
- IDLs with low sensitivity fail to detect vehicles with speed below a
certain threshold, and miscount vehicles with complex or unusual chassis
configurations, or vehicles with relatively low metal content (e.g.
motorcycles).
- IDL data supplied to traffic control systems have a very low sample rate.
- Not suitable for mounting on metallic bridge decks.
- Some radio interference occurs between loops in close proximity with each
other.
A typical single loop system is shown in Fig. 1.
The system consists of three components: a detector oscillator, a lead-in cable
and a loop embedded in the pavement.
The size and shape of loops largely depend on the specific application.
The most common loop size is 1.83 m by 1.83 m and shape is hexagonal as single
turn or two or three turns as shown in Fig. 1.
When a vehicle stops or passes over the loop, the inductance of the loop is
decreased.The decreased inductance then increases the oscillation frequency and causes the
electronics unit to send a pulse to controller, indicating the presence or
passage of a vehicle.
Single loop detectors output predicts occupancy and traffic count data within
specific time intervals like 20 sec, 30 sec.
Figure 1:
Schematic diagram of single loop detectors, source
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