6.1 Inductive Loop Detection Fundamentals and Configurations
Key Takeaways
- A vehicle over an inductive loop induces eddy currents in its conductive mass, which lowers the loop's inductance and raises the resonant frequency the detector electronics unit is tracking.
- Loop inductance can be approximated as L = K x feet of sawcut, where K is 0.5 for one turn, 1.5 for two turns, 3.0 for three turns, and 5.0 for four turns, in microhenries per foot.
- The practical turns-versus-area guideline is 6 turns for 6 to 12 square feet, 5 turns for 12 to 20, 4 turns for 20 to 60, 3 turns for 60 to 240, and 2 turns above 240 square feet.
- Lead-in cable adds roughly 21 microhenries per 100 feet of #14 AWG cable, and that series inductance dilutes the loop's signal — system efficiency is loop inductance divided by total inductance.
- A 6 by 6 foot loop suits a 12-foot lane; narrower lanes commonly use 5 by 5 foot loops to avoid splashover, the false detection of vehicles in the adjacent lane.
6.1 Inductive Loop Detection Fundamentals and Configurations
A detector call is the input that drives everything an actuated controller does. When a phase will not serve, when a left turn drops a car, when a side street holds green at 3 a.m., the fault is in the detection system far more often than in the controller. A Level II field technician who understands the physics can measure a loop and know whether it is good before touching the cabinet.
How a Loop Detects a Vehicle
An inductive-loop detection system has three parts:
- The loop — one or more turns of insulated wire installed in a sawcut in the pavement, or a preformed loop placed before paving.
- The lead-in — the twisted pair from the loop to the nearest pull box, spliced there to a shielded home-run cable that runs to the cabinet.
- The electronics unit — the detector card in the cabinet's input file or detector rack.
The electronics unit drives the loop as part of a resonant circuit, typically operating between 20 and 60 kHz. When a vehicle's conductive mass enters the loop's magnetic field, the field induces eddy currents in the metal. Those eddy currents create an opposing magnetic field, and the net effect is a decrease in the loop's inductance and a corresponding increase in the resonant frequency. The electronics unit watches for that change; when it exceeds the sensitivity threshold, the unit outputs a call.
The key consequence for a technician: a loop detects a change in inductance, not a weight, not a magnet, and not a presence in any absolute sense. A loop cannot tell a technician what is above it — only that the inductance moved. That is why a corroded splice, a water-filled pull box, or an adjacent loop on the same frequency can all produce false calls that look exactly like a vehicle.
Loop Inductance Arithmetic
A field technician verifies loop construction with a direct-reading inductance meter at the pull box. The approximation that makes this practical:
| Number of Turns | K (µH per foot of sawcut) |
|---|---|
| 1 | 0.5 |
| 2 | 1.5 |
| 3 | 3.0 |
| 4 | 5.0 |
Worked example. A 6 ft × 6 ft loop has a 24-foot perimeter. Wound with three turns:
Measured at the pull box, a healthy 3-turn 6×6 loop reads in the low 70s of microhenries — laboratory measurement of exactly this loop at 20 kHz gives about 74 µH. If the meter reads roughly 48 µH, the loop has two turns, not three, and the contractor's certification is wrong. If it reads 120 µH, there are four turns or the sawcut is longer than the plan shows.
Turns Versus Loop Area
Too few turns and the loop's inductance is small compared with the lead-in, which destroys sensitivity. Too many and the loop's inductance can exceed the electronics unit's input range. The practical guideline:
| Loop Area (length × width) | Number of Turns |
|---|---|
| 6 – 12 sq ft | 6 |
| 12 – 20 sq ft | 5 |
| 20 – 60 sq ft | 4 |
| 60 – 240 sq ft | 3 |
| 240 sq ft and up | 2 |
When loops are connected in series, the turns per loop may be reduced. Conversely, when a single loop is used with a long lead-in of 500 feet or more, add one or more turns to raise the loop's share of the total inductance.
Lead-In Dilution and System Efficiency
The lead-in cable is an inductor in series with the loop, and it contributes no sensitivity at all — it only dilutes the signal. Lead-in inductance runs about 0.20 to 0.22 µH per foot, or roughly 21 µH per 100 feet of #14 AWG cable.
Worked example. Four 3-turn 6×6 loops (72 µH each) wired in series with a 300-foot lead-in (about 66 µH):
The same four loops wired in parallel give 72 ÷ 4 = 18 µH:
That is the entire argument for series wiring. Parallel connection is easy to make in a pull box and quietly destroys the detection system's sensitivity.
Loop Shapes and Where They Are Used
| Configuration | Typical Use | Characteristics |
|---|---|---|
| 6 × 6 ft square | Standard stop-bar detection on a 12-foot lane | The workhorse. Detects passenger cars reliably; may miss a motorcycle centered on a diagonal. |
| 5 × 5 ft square | Narrow lanes, closely spaced lanes | Smaller field reduces splashover into the adjacent lane. |
| 6 × 40 ft (or longer) rectangle | Long presence detection, left-turn bay storage | Detects a queue anywhere in the zone; often 2 turns due to area. |
| Quadrupole (figure-eight) | Bicycle and motorcycle detection; lanes with splashover problems | Concentrates the field along the center line of the lane, sharply reducing lateral spill. |
| Diagonal / chevron | Bicycle lanes | Presents the wire at an angle to a narrow bicycle frame, improving coupling. |
| Series-connected pairs | Advance and stop-bar on one channel | Keeps inductance high; both zones report on the same detector input. |
Splashover and Crosstalk — Two Different Problems
These two failures produce the same symptom (calls that nobody made) and have completely different fixes.
- Splashover is a magnetic problem: the loop's field extends into the adjacent lane and detects a vehicle that is not in the intended lane. Fixes are physical or configuration-based — reduce sensitivity, use a smaller loop, or replace with a quadrupole.
- Crosstalk is an electrical problem: two loops operating at nearly the same frequency couple through their lead-ins or through the pavement, and each sees the other's excitation. The fix is to set adjacent detector channels to different frequency selections. Most NEMA detector cards offer two to four selectable frequencies specifically for this.
A technician who tries to solve crosstalk by lowering sensitivity ends up with a loop that no longer detects small vehicles and still crosstalks.
Detection Zone Placement
Where the loop sits determines what the timing can do.
- Stop-bar presence detection — loop at the stop line, presence mode, short passage time. Holds a call as long as a vehicle waits.
- Setback / advance detection — loop placed upstream by the distance a vehicle travels in the desired passage time. On high-speed approaches, advance loops are placed to cover the dilemma zone, roughly the region 2.5 to 5.5 seconds of travel time from the stop line, so the controller can avoid terminating green with a vehicle in that band.
- Count / system detection — placed mid-block, well clear of stop-bar queue spillback, so it measures free-flowing volume and occupancy for traffic-responsive operation.
- Queue / spillback detection — placed at the back of a storage bay to detect when a turn bay has overflowed.
A technician measures a 6 by 6 foot loop at the pull box with a direct-reading inductance meter and obtains 36 microhenries. The plan calls for three turns. What does the measurement indicate?
Four identical 72-microhenry loops must be combined onto a single detector channel with a 300-foot lead-in contributing about 66 microhenries. Which connection produces the better detection sensitivity, and why?
Two adjacent through-lane loops each register calls when a vehicle occupies only one of them. Frequency settings on both detector channels are identical. What is the most likely problem and its correct fix?
Which loop configuration is specifically used to improve bicycle and motorcycle detection while reducing lateral field spill into adjacent lanes?