CONTROLLER LOCATION

The signal controller may be attached to any convenient pole or, if a console cabinet is used, it may be placed wher-ever desired, provided that ,in either case the location cho-sen satisfies the following :-(a) A power supply can be

conveniently obtained.

(b) There will be an unob-structed view of all approaches to the intersecton in the event of manual operation.

When this condition cannot be satisfied and manual operation is fre-quently required, it may be desirable to install a special remote unit at a more favourable posi-tion with its switches in parallel with those of the controller proper.

(c) The cabinet does not unduly obstruct the pedestrian right of way.

(d) The cabinet will not be unduly exposed to acci-dental damage caused by passing traffic.

5.5 DETECTORS

Traffic detectors are a primary requisite of actuated signal controls as they sense vehicu-lar or pedestrian demand and relay these data to the local intersection controller or mas-ter controller so that the appropriate signal indications may be displayed. The selec-tion of the type, design, and installation of the various types of detectors is a func-tion of the operafunc-tional require-ments and physical layout of the area to be detectorised.

The functional characteristics of the most commonly used detectors are described below.

5.5.1 Types and Functions of Vehicular Detectors

The type of vehicle detection system used for an actuated signal control depends on the operational requirements of the intersection in terms of the type and use of data needed by the controller to operate efficieVtly. Most. new installa-tions use either inductive loop detectors, magnetic detectors, or magnetometers. The physi-cal design and construction of these commonly used

detectors is summarized below:

TRAFFIC SIGNAL CONTROLLERS AND DETECTORS

(a) Inductive Loop

Detectors : Loop detec-tors are by far the most commonly used today and are the standard form of detection in many agencies.

Essentially, this detec-tor installation consists of a loop which may be one or more turns of wire in a saw-cut slot in the road surface in the exact area where vehi-cles are to be detected.

The ends of this loop are connected by cable to an electronic amplifi-er usually located in the controller. A vehicle passing over, or resting in the loop, will unbal-ance a tuned circuit which is sensed by the amplifier.

(b) Magnetic Detectors : There are three types of magnetic detectors :-the standard magnetic detector, a directional magnetic detector, and the magnetometer. All three types consist of two components, an in-road sensor, and an amplifier unit. Although all 3taagnetic detectors operate on the basis of a change in the lines of flux from the earth's magnetic field, the magnetometer is a spe-cial type of magnetic detector.

The directional and nondirec-tional magnetic detector uti-lizes a coil of wire with a high-ly permeable core placed beneath the surface of the roadway. When a vehicle comes near or passes over the coil, the constant lines of flux passing through the coil are deflected causing a volt-age to by developed in the coil. A high--gain amplifier causes the voltage to operate a relay and transmit to the controller the message that a vehicle has been detected.

For these detectors to sense a change in the magnetic field, the vehicle must be in motion. Vehicles travelling less than 10 kmph are gener-ally not detected.

Consequently, magnetic detectors can provide the equivalent of passage or motion data, but not occupan-cy or presence data.

(c) Other Types of Detectors

Earlier detectors that have been use overseas include pressure pads, radar, and sonic detectors. Their use is now very limited.

The pressure detector requires a metal frame installed in the pavement to support and hold in place a pressure plate. The detector is activated by the weight of a vehicle causing a closure of contact plates sealed in the rubber pressure plate which

TRAFFIC SIGNAL CONTROLLERS AND DETECTORS

sends a signal to the con-troller. This detector is no more in use.

Radar detectors operate on the Doppler effect.

Microwaves are beamed toward the roadway by a transmitter. A vehicle passing through this beam reflects the microwaves back to the antenna denoting the motion of a vehicle.

The sonic detector also uses the Doppler principle. it trans-mits pulses of ultrasonic ener-gy toward the roadway

through a transducer. A vehi-cle passing through this reflects the energy at a differ-ent frequency back to the transducer which denotes a presence or passage of a vehicle. These detectors are of special value, when it is not possible or practical to install loops, magnetic, or magne-tometer detectors (e.g., on bridges or approaches with poor base materials).

The high-intensity light tor is a special purpose detec-tor system used for priority control for emergency and transit vehicles. It utilizes a high-intensity light emitted at a specific frequency from a transmitter mounted on the vehicle and a detector mount-ed on or near the traffic sig-nal. When the light from an emergency or priority vehicle is detected, the detector relays a signal to a phase

selector connected to the con-troller. However, this type of detector has never been used in this country.

5.5.2 Application of Vehicle Detectors

The application and design of the detection component of actuated traffic signal contol is explicitly related to controller operation which in turn is related to the physical and traffic characteristics of the location.

There are a number of ways in which detector application and design can be

approached. Detector location and configuration is depend-ent on

a) Type and capability of controller

b) Control mode

c) Traffic variable to be measured

d) Geometry of the inter-section and approach-es

e) Traffic flow characteris-tics (e.g., volume, speed, etc.)

TRAFFIC SIGNAL CONTROLLERS AND DETECTORS

Short loop detectors (up to 6 meter in length) constitutes the simplest and most widely used type of detector applica-tion. This short loop (small area) configuration is intended to detect a vehicle upstream of the stop' bar. When a vehi-cle passes over the detector, a "call" is placed and the con-troller responds as

pro-grammed.

Short loop detectors may take a number of forms and be located at varying distances upstream of the stop bar depending on the operational requirements. A common application is to space the detector loop about 30m upstream of the stop bar.

However this may vary in practise depending on the approaching vehicular speed.

Long loop detection can also be used and it is essentially a presence detection in that it registers the presence of a vehicle in the zone of detec-tion as long as the detector is occupied. This method is expensive but multiple small loops could be used to over-come this problem.

5.5.3 Detection of Small Vehicles A presence detector should be able to detect all licensed motor vehicles including a small motorcycle and hold its call until the display of a green to the phase. A hold time of 3

minutes is commonly speci-fied. A conventional detection loop configuration longer than 6 meter may not detect a small motorcycle.

5.6 LOCATION OF