Triggering Traffic Signals

Triggering Traffic Signals

Introduction

Many traffic signals in the United States are triggered by various types of sensors in the ground. There are varying designs and types of these sensors, but almost universally they have difficulty sensing anything smaller than a car. Scooterists, bicyclists and motorcyclists often find themselves waiting through several cycles of a light without getting a green.

Fortunately, there are steps you can take to help trigger the lights. And, in some states, it's even legal to run them in some circumstances.

How Triggers Work

Sensors that detect vehicles in order to maximize the efficiency of traffic lights are called "demand-actuated" triggers. These take a number of forms including lasers, video cameras and various rollover sensors in the street. The most common ones are inductive loop sensors. Without getting into too much of the science behind this, inductive loops are essentially metal coils (wrapped around a metal core) under the street with inductance meters that detect changes in the loops' electromagnetic fields. Positioning a mass of conductive (usually metal) material over the loops or passing a similar mass over the loops alters the inductance. This change is measured by the meter and used to trigger a change in the light.

This is essentially the same technology as most metal detectors, just widened. Note that what the inductive loops detect is metal, not electromagnetic fields. This is important for the discussion of magnets, below.

If you want to read up on the science behind these, see the Links section below.

Triggering the Sensors

Positioning

The easiest way to trigger sensors is to adjust the position of your scooter on the sensors. You'll have to check out the cutouts in the road and try to determine which type of sensor you're dealing with.

The most common are Dipole and Quadropole Loops. These will usually look like circles or rectangles. A Quadropole Loop will usually be bisected by another cut or look like a side-by-side pair of cuts.

Dipole Loops:



Quadropole Loop:


Best positions:

To maximize the likelihood that an inductive loop sensor will detect your scooter, it is important to position yourself over the most sensitive portion of the loop. There are several common shapes of inductive loop sensors, each with a different "sweet spot" for bicycles and powered two wheelers as shown in Figure 1. The two most common shapes are the dipole loop (Figure 1(a)) and the quadrupole loop (Figure 1(b)). For either of these two loop patterns, position both wheels directly over the sawcut for the wire, choosing either side for the dipole loop and using the center sawcut for the quadrupole loop. (The center sawcut of the quadrupole has twice as many wires in it as the outer sawcuts and is a more sensitive location.) If the signal does not detect you, you may wish to try leaning the bike over toward the center of the dipole loop, or to either side for the quadrupole loop. Newer traffic signal installations usually feature quadrupole loops, which are better at detecting bicycles than the older dipole loops. The third type of sensor loop, the diagonal quadrupole (Figure 1(c)), is designed to provide better detection of small vehicles such as scooters positioned anywhere over the sensor.