Inductive Loop Detection

The Backbone of Intelligent Transport Systems

Ever seen rectangular, diamond or chevron black lines in the asphalt surface of a road and wondered what they are?

Image of inductive vehicle detection loops on a UK motorway, MIDAS system

Inductive loop vehicle detection remains one of the most resilient and ubiquitous technologies within Intelligent Transport Systems (ITS). Despite the rapid evolution of "above-ground" sensors (such as radar, LiDAR and AI-driven video analytics), inductive loops continue to be the benchmark for reliability, accuracy and longevity in traffic management.

Understanding Inductive vs. Induction

A common misnomer in the industry is referring to these sensors as "induction loops". While the linguistic difference is subtle, the technical distinction is significant:

Induction Loops: Typically used for wireless power transfer (e.g. charging electric toothbrushes).

Inductive Loops: Passive sensors designed specifically to detect the presence of metal (vehicles) via electromagnetic change.

Contrary to popular urban myths, standard inductive loops at traffic signals cannot be used to charge EVs! Since the 1960s, their primary role has been providing high-quality data for traffic signal control, vehicle counting, incident detection and access control.

Standards and Design Configurations

Inductive loops are not "one size fits all". Their geometry is dictated by their specific application, whether for a high-speed motorway or a complex urban junction.

In the UK, the size and shape of detector loops in highway applications are defined in a series of national and local-authority specifications and requirements.

Some common applications in traffic systems:

Chevron - These are typically used by vehicle activated traffic signals

Rectangular - Presence detection, vehicle counting and classification

Diamond - Mainly used for MOVA, and advanced vehicle activation mode for traffic signals

Butterfly - A more sensitive implementation which is good for detecting cycles

The Installation Lifecycle

Precision during installation is the primary factor for a loop to achieve a long lifespan. The process involves:

Slot Cutting: A specialist saw cuts a narrow channel into the carriageway in the shape of the required loop. A "cut-back" slot is also cut to connect the loop to the roadside equipment.

Cleaning: Slots must be entirely free of debris and moisture. Any trapped water can lead to "loop float", insulation failure or sealing failure.

Winding: A specialist insulated wire (commonly Hypalon or an equivalent high-temp cross-linked polyolefin) is wound into the slot. Typically, 3 to 4 "turns" are required; the number of turns inversely correlates with the loop's size to maintain the required inductance (L).

The Twist: The "loop tails" (the wires leading back to the cabinet) must be twisted, usually a minimum of 5 turns per metre. This cancels out environmental noise and ensures the tails themselves don't act as detectors.

Sealing: The slot is reinstated with an epoxy resin or a hot-pour bituminous sealant to protect the cable from traffic loading and weather.

Image of inductive vehicle detection loop slot cutting and loop sealing

Operation and Detection Logic

When a detector pack energises the loop, it creates an inductive field. As a metallic vehicle enters this field, it acts as a "shorted turn," decreasing the loop's inductance. The detector card senses the consequential frequency shift and triggers a "demand" to the traffic controller.

Image of inductive vehicle detector loop packs mounted within a roadside cabinet

This "under-ground" approach offers a distinct advantage: environmental immunity. Unlike cameras or radar, inductive loops are unaffected by heavy rain, thick fog, snow or direct glare from the sun.

Testing and Commissioning

To meet National Highways and local authority requirements, loops must undergo rigorous electrical testing before and after sealing:

Inductance (L): Typically measured in microhenries.

Insulation Resistance: Ensuring the cable jacket hasn't been nicked during installation (ideally >100M?).

Resistance (R): Checking for continuity and "dry" joints.

For MOVA (Microprocessor Optimised Vehicle Actuation) systems, the From Stop Line (FSL) distance is critical. If a loop is installed even half a metre out of position, it can degrade the efficiency of the entire junction's timing algorithm.

Vulnerabilities and Maintenance

While robust, loops are not invincible. Their primary "enemies" are:

Utility Works: The leading cause of loop failure is third-party "street works" cutting through the wires.

Carriageway Deterioration: Cracking, "rutting" or potholes can snap the internal copper core.

Poor Installation: Sharp corners in the slot-cut (rather than 45° "cross-cuts") can stress the cable, leading to premature failure.

Image of a damaged inductive vehicle detection loop

The Verdict

Inductive loops remain the "gold standard" for a reason. Their ability to provide 99.9% detection accuracy in all weather conditions makes them the essential foundation of any modern Intelligent Transport System. While we move toward a world of connected and autonomous vehicles (CAVs), the humble copper loop in the road remains our most reliable source of "ground truth" data.

Image of a System D, chevron shaped, inductive vehicle detection loop

To find out lots more about Inductive Loop Vehicle Detection, try watching our video about the subject on the @ITSNow YouTube Channel -
See the video

Our books, Traffic Signals and Traffic Control also provide much more information on this subject, see our Publications page -
Publications