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What is Inductive Loop Detector? 8 Important Points

Inductive Loop Detector

What is an inductive loop detector?

Inductive loop detectors are one kind of traffic sensors used to identify the different vehicle classes using the induced current from the loop of a wire which is fixed to the pavement. These are extensively used in highways as they possess very high accuracy and economical as well.

What are the different types of detectors?

There are various types of detectors available based on their functionality and usage. They are Pressure type, Push button type, Magnetic, Magnetometer, Inductive loop, radar type, sonic type, radiofrequency type and light emission detectors. Out of all these Inductive loop and magnetometer types are widely used for various purposes.

 How does the inductive loop detector work?

Inductive loops are very much useful in transmitting the signals by identifying the presence of vehicles passing through the loop. The entire process in which loop detectors work is depicted below.

Inductive Loop Mechanism

Inductive Loop Mechanism

 Inductive Loop Mechanism

There are mainly three components for loop detectors that govern the entire mechanism.

  • Inductive loop detectors consist of one or more turns of wire in a saw cut slot in the road surface at the area where vehicles will pass and need to be detected.
  • The ends of the loop are connected by a cable to an electronic amplifier which is usually placed in the controller cabinet.
  • When the vehicle moves over the loop and disturbs the loops magnetic field, this in turn sensed by the amplifier because the loop detector can detect either the presence or passage of the vehicle will introduce a new dimension to the traffic control.

Why traffic engineers use inductive loop detectors?

There are various types of detectors available, out of these inductive loop detectors are widely used by traffic engineers due to their and higher accuracy and a wide variety of applications.

The most common inductive loop detector applications fall into six categories and are described below.

Isolated intersection control:

Inductive loop detectors can be used for isolated intersections to regulate the flow of traffic without considering the operation of adjacent street signals.

Arterial Intersection control:

Inductive loop detectors used in open arterial networks ensures that the traffic progress along a roadway by the display of successive green signals for the flow of traffic.

Closed Network control:

In closed networks, Inductive loop detectors coordinate a network of intersection signals within a defined area such as the city’s Central Business District (CBD).

Area-wide system control:

In this type of system, all of the traffic signals within an area will be treated as a single system. Individual signals within this area may be controlled by using isolated intersection network concepts.

Priority vehicle system control:

Inductive loop detectors help in identifying certain vehicles such as ambulances, fire trucks and buses more continuously through busy intersections.

Pedestrian Signal control:

At busy intersections where pedestrians hardly get any time to cross the signals, inductive loop detectors can be used in pushbuttons to give the right of way for the pedestrians.

blog how traffic signal works

Type of Inductive Loops:

Inductive loops used for traffic signal operations are of three types and are described below.

Pre-timed signals:

These types of signals are having fixed cycle lengths irrespective of traffic flow along the arms.

Actuated signals:

Based on the actual traffic length, loop detectors will decide the cycle length for the signal.

Semi Actuated signals:

These types of signals are a mix of Actuated and pre-timed signals in which the mainstream traffic will have fixed cycle length and the cross street will have actuated signal for the incoming traffic.

How do you trigger an inductive loop?

Inductive loops can be triggered when a heavy metal object or vehicle passes through the loop placed over the pavement. When the vehicle moves over the loop or stops over the loop it will generate a vortex current in the loop and thus decreases their inductance. The reduced inductance activates the electronics element output relay, which sends a current to the controller to inform the presence of a vehicle. (2)

Cost of Inductive loop detector?

Inductive loop detectors are generally economical and hence they are widely used for various applications. Typical inductive loop detectors cost around Rs.1000/piece to Rs.10000/piece in India. Prices may vary from country to country based on the manufacturing and transportation charges etc.

  • National Electrical Manufacturers Association (NEMA) has developed a set of standards for inductive loop detectors.
  • Now all the loop detector amplifiers must conform to the NEMA standards.
  • There are two detector unit configurations defined by NEMA, first is the shelf-mounted detector available in both single and multi-channel models.
  • The second configuration is the card mounted detector unit, when a large no of detector amplifiers is needed the card rock mounted unit fits into a multiple card rack requiring less cabinet space.
  • NEMA has also developed some standards for output detector unit that it must be able to sustain a presence output for a minimum of 3 minutes before tuning out the vehicle.
  • There are two types of outputs, one is the electromechanical relay and solid-state output.
  • Solid-state outputs have no moving parts and are more reliable and more accurate in tracking the actual vehicle presence.

Alternatives to inductive loop detector?

There are some recent advances that can serve as alternatives for Loop detectors. They are described below.

Self-powered vehicle detector (SPVD):

This consists of a cylindrical in road sensor containing a transducer and a radio frequency emitter with antennas and a battery. The inroad sensor is powered by an internal battery and it is connected to a relay by a radio link.

Video imaging detection system (VIDS):

This contains a single video camera with processing hardware & software to provide data similar to a multiple loop detector.

Microwave/Radar detector:

In this type of detector, a microwave sensor is mounted on to the gantry/ pole so that when a vehicle passes through this a microwave beam is projected towards the vehicle and based on the reflected microwave the vehicles can be classified.

Ultrasonic detectors:

This is also similar to microwave detectors only but in these ultrasonic waves will be used to project onto the incoming traffic and the reflected waves can be analysed to differentiate the various vehicle classes.

Infrared detectors:

Infrared vehicle detectors will be of two types; one is passive and the other is active type. Passive type detectors simply measure the vehicles by their motion while active type detectors are based on the reflected light from vehicles.

All the specification and design-related concepts can be adopted from the Federal Highway detector Handbook.


I hope the blog provides you with adequate knowledge of Inductive Loop Detectors and their associated features. Please feel free to like, share and comment. Also, if you want to know more about Traffic Signal Design Process, we have developed an outstanding Traffic Signal Design Course.


P. Dinesh Kumar
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Please see our Pillar Post to know why we founded gcelab.com.



(1) https://www.researchgate.net/publication/287003681_Wireless_based_Smart_Parking_System_using_Zigbee

(2) https://www.fhwa.dot.gov/publications/research/operations/its/06108/02.cfm


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