TRAFFIC – DETECTORS

Traffic loops are the original automobile detection device, and more technically known as in-ground induction loops.  They operate in the same way as a metal detector, only upside down.  A metal detector is basically a solenoid, a simple device with a mild current running through a loop of wire.  As long as the loop is in motion, any nearby metal will interfere with the electro-magnetic field the current is creating, and will register a slight drop in the amperage.   Ferrous metals (iron and steel) are the most magnetic of the metals, and are therefore most likely to affect the current and register a signal on the ammeter.

Loop wires are imbedded in to the asphalt, but at a very shallow depth, usually no more than a few inches, and detect the motion of the passing of large metal objects, automobiles.  Just like the metal detector, the wire loop needs to be as close to the metal object as possible, so the loops are placed in a thin shallow cut in the asphalt, either in a circular or rectangular shape, and a sealant placed on top.  Typically, they need to be placed no more than about 6 inches under the surface of the asphalt, and preferably less, just enough to protect them from passing traffic, while still being able to detect that traffic.

The frequency used for induction loops can vary greatly.  The manufacturers of the equipment have offered various frequencies so the technicians placing the loops can alter the transmitter inside the cabinet to a frequency that will work best for that particular area and type of traffic that is most common.  However, the highest frequency available has been 200 kHz, and many technicians have gotten into the habit of automatically setting the frequency to 200 kHz, even when a lower frequency might have worked better.

VIDEO DETECTION CAMERAS

Detection cameras are attached to the top of the signal post, usually positioned about half way out across the extended arm of the signal post.  Most detection cameras are similar to a motion sensor, but much more precise.  The camera is set to focus on a number of zones, one for each lane of traffic coming into the intersection.  Whether there are 2, 3, 4, or more lanes of traffic, each lane is a separate zone, but one camera can detect the separate motions in each lane.  In this way, it only takes one camera to watch multiple lanes of traffic, as long as they are all in the same direction from the camera.  Therefore, it only requires 4 cameras to monitor all of the traffic coming in to an intersection, even if each street has multiple lanes.  Detection cameras have been rapidly replacing loop wires, although there are plenty of both types across the country.

Few municipalities ever tear out the old loop system when installing detection cameras, not unless the entire intersection is being rebuilt.  So, even though an intersection may have detection cameras, there will often be abandoned loops and loop boxes still present.

Another new vehicle detection device is a puck.  These are only found in some areas at the present time, but they are an interesting device.  They are called pucks because they are about the size of a hockey puck, round and about 4 inches in diameter.  Each puck contains a very small radio transmitter, and a small battery that lasts for about 5 years.  Each puck sends out a distinct frequency to a nearby receiver, typically in a traffic cabinet.  They require no wiring of any kind, and can be imbedded in to the asphalt or concrete with a small cut in the road.

A few notes here.  Once placed, pucks tend to look similar to a geophysical test hole, a small, round cut in the asphalt.  However, pucks are no more than 6 inches in diameter, and geophysical test holes are usually 10 or 12 inches in diameter.  Also, not all pucks are isolated devices.  Some pucks may be hardwired to a cabinet.

EMERGENCY VEHICLE DETECTORS

Besides the traffic systems that detect all vehicle traffic, there are also detection devices for specific types of vehicles.  These are emergency vehicle detection devices, technically known as Emergency Vehicle Preemption (EVP).  When activated they preempt the normal sequence and turn all lights red except for the direction in which the vehicle is coming that activated the device.

Any emergency vehicle approaching the intersection will be detected by the traffic system, override its’ current timing, and switch to green for the oncoming fire truck or police car.  This can be done in several ways.  Many of these systems are built to detect low broadcast radio waves from transmitters in the emergency vehicles.  Other systems can detect the flashing red strobe lights of the vehicle, while others are designed to detect the sound of a siren.  There are a number of different manufacturers producing these devices, some with an activator placed inside the emergency vehicle itself such as the radio wave activators, others with the activation device on the signals poles that detect the sound of the sirens.  Municipalities have a number of choices to choose from, and many cities may use more than one type of device.

TRAFFIC COUNTERS

Traffic counters are used as both a temporary or permanent feature, meant to calculate the amount of traffic on a given road.  This can be on a residential street that is under consideration for widening, or on an Interstate where it may be used to count traffic for highway funding purposes.

Temporary traffic counters often use above ground wiring, but the permanent counters use buried loop wires out to the street, or they can use a video detection camera, similar to an intersection camera, to detect each passing automobile.

Any of the traffic counters can be a stand-alone feature, usually relying on power from a small solar panel, or even built to rely on battery power.  Those that are battery-operated will have a metal box attachment where the battery is stored, usually a 6 volt lantern battery.  If the counter is not powered by battery or solar, then it must have a feed from a power secondary, and if it is on or near an Interstate, then it may even feed from an ATMS cabinet.

INTERCONNECTS

Interconnects are not a detection device, but it has been placed here for convenience.

Many people use the term “those lights are timed” when they are referring to a series of traffic lights that turn green one after another in a particular direction.  All traffic lights are of course timed to be able to switch from red-yellow-green, but signals that turn green with a set delay between them are cascading lights.

Cascading traffic lights (also known as controlled lights) can be found on major streets where once the traffic is started at intersection one, the interconnect ensures that intersection two will be on green once the traffic reaches that area, at least as long as the vehicles are moving at the proper speed, usually the maximum speed limit of that road.

Interconnects are always telephone service wire, not fiber optic cable.  The operation of cascading lights is a simple process and only requires a two-conductor line that runs from one cabinet to the next.  Fiber optic cable is often running parallel with traffic systems on state roads, but this is part of the ATMS system, not the cascading lights.  The ATMS fiber cable will connect into their own traffic cabinets along the route, while the interconnect cable will connect into the regular traffic cabinet.

Since interconnects are actually telephone services, they may be labeled on traffic prints with the acronyms of BSW, CRW, or RSW.  Many interconnects will pass through loop wire pull boxes to the next intersection, or may have pull boxes of their own if the next intersection is far down the road.