Loop Detector vs. Exit Probe vs. Radar Sensor

An inductive loop detector is usually the strongest choice for lane-specific vehicle presence, an exit probe is intended mainly to trigger free-exit opening from a moving vehicle, and a properly selected radar sensor provides above-ground motion or presence detection without cutting pavement. The correct device depends on gate function, site construction, traffic pattern, and required detection logic.

The Three Technologies Do Different Jobs

Loop detectors, exit probes, and radar sensors can all send a relay signal to a gate operator, but they do not sense vehicles in the same way. The important question is not simply, “Will it detect a car?” It is whether the device must detect movement, maintain presence while a vehicle is stopped, ignore cross traffic, control free exit, or hold a gate safely open over a defined lane.

When an Inductive Loop Detector Is the Better Choice

An inductive loop consists of loop wire or a preformed loop connected to a detector module. The detector tunes to the loop’s inductance and responds when a metallic vehicle changes the electromagnetic field. Depending on the detector settings, the output can provide continuous presence, pulse-on-entry, pulse-on-exit, direction logic, delay, or other control functions.

This technology is commonly selected when the gate system needs a defined detection zone in one lane. Typical functions include free exit, shadow-loop control, vehicle presence at an access device, barrier activation, and reversing or interrupt logic. A properly configured presence loop can continue holding its relay while a vehicle remains over the loop.

The installation is more demanding than the electronics alone suggest. Loop size, number of turns, lead-in twisting, inductance, wire insulation, saw-cut depth, corner treatment, sealant, rebar, adjacent loops, and pavement condition all affect reliability. Cracked asphalt, moving concrete sections, water intrusion, or damaged insulation can cause intermittent faults even when simple continuity appears normal.

When an Exit Probe Is the Better Choice

An exit probe is normally buried beside or beneath the driveway and connected to a controller. It senses a magnetic-field disturbance caused by a moving ferrous vehicle. Because detection depends on vehicle mass, speed, distance, and sensitivity, a slow or lightweight vehicle may need to pass closer to the probe than a larger, faster vehicle.

The main application is free exit: an approaching vehicle triggers the operator’s open-only input without a keypad, remote, or credential. This can be practical on long private driveways where cutting a full loop into the pavement is undesirable.

Many exit probes are motion detectors rather than presence detectors. Once the vehicle stops, the probe may no longer maintain an active output. That makes a motion-only probe unsuitable as a reversing loop, shadow loop, or vehicle hold-open device. Confirm the exact product manual because magnetoresistive loop-replacement sensors may offer presence detection even though they are sometimes grouped loosely with probes.

When a Radar Sensor Is the Better Choice

Radar sensors mount above ground and project an adjustable detection field. Some models detect motion only; others use distance measurement and software logic to detect both approaching and stationary vehicles. This distinction must be verified before using radar for anything beyond an activation command.

Radar is useful where decorative pavers, post-tensioned concrete, bridge decks, damaged pavement, buried utilities, or continuous traffic make loop installation difficult. Directional settings, cross-traffic filtering, human cancellation, sensitivity, detection depth, and relay timing can often be adjusted for the lane.

The tradeoff is that the detection area is created by mounting geometry rather than buried wire. Sensor height, tilt, lateral aiming, vehicle approach angle, nearby moving gates, vegetation, pedestrians, and reflective metal surfaces can change the result. A radar sensor that works well for straight traffic may require different positioning at a curved driveway or wide gate throat.

Do Not Confuse Vehicle Detection With Entrapment Protection

A vehicle detector can control an open, reverse, shadow, or hold-open input, but it is not automatically an approved monitored entrapment-protection device. Required photo eyes and sensing edges must remain connected and compatible with the operator’s monitored safety inputs. An exit probe should never be treated as protection for a stationary vehicle or person in the gate path.

Technician’s Corner

South Florida Water Intrusion Changes the Failure Pattern

Heavy rain can enter damaged loop-wire insulation, low-quality splices, conduit, or poorly sealed saw cuts. The loop may test correctly when dry and become unstable after a storm. Use direct-burial materials, watertight splices, correct sealant, and surge protection suited to the equipment.

Rebar and Pavement Movement Matter

Closely spaced steel reinforcement can reduce loop sensitivity or complicate probe placement. Cracks and thermal movement can stretch loop wire. Before cutting, identify reinforcing steel, drainage, utilities, and pavement joints rather than selecting the detector first and forcing the site to fit it.

Radar Must Not Watch the Moving Gate

A radar field aimed through a swing leaf, sliding gate panel, barrier arm, moving sign, or wind-blown vegetation may create nuisance activations. Configure the field around the traffic lane and test every gate position, not only the fully open position.

A Dry Contact Does Not Make Devices Interchangeable

All three technologies may provide NO, NC, and COM relay contacts, but that does not mean they perform the same function. Confirm supply voltage, relay mode, pulse versus presence output, fail-safe behavior, detector socket or harness, and the operator terminal being controlled.

Before You Choose a Vehicle Detector

• Define the function: free exit, access-device presence, shadow, reverse, barrier activation, or traffic counting.
• Decide whether the output must remain active while the vehicle is stopped.
• Measure the lane, vehicle path, mounting options, and distance from the gate.
• Check pavement type, rebar, utilities, drainage, cracks, and future resurfacing plans.
• Confirm sensor power, relay type, pulse or presence mode, wire length, enclosure rating, and surge protection.
• Verify compatibility with the operator input and keep required monitored safety devices in place.

Related Technical Categories

• Inductive Loop Detectors
• Loop Wire and Preformed Loops
• Exit Probes and Magnetic Vehicle Detectors
• Radar and Microwave Vehicle Sensors
• Gate Safety Sensors and Photo Eyes
• Vehicle Detection Accessories

Replacement-part advisory: Verify the detector model, sensing technology, motion or presence capability, power voltage, relay output, socket or harness, lead length, mounting requirements, operator input function, and product generation before selecting replacement vehicle-detection hardware.