ACTUATED CONTROLLERS

A traffic-actuated controller operates with variable vehicu-lar and pedestrian timing and phasing intervals which depend on traffic volumes or the presence of pedestrians.

The flows are determined by vehicular detectors placed in the roadway or by pedestrian actuation of push buttons. The basic applications of actuated control include semi-actuated, full-actuated, and volume den-sity.

5.3.1 SEMI-ACTUATED CONTROLLERS

These devices provide the mean for traffic actuation on one or more, but not all of the intersection approaches. It is applicable primarily to an intersection of a heavy - vol-ume, urban or suburban traffic arterial with a relatively lightly travelled minor road. The essential operating features of the controller are :

a) Detectors are on minor approaches only.

b) Major, road receives a minimum green period in each cycle.

c) Major road receives green indefinitely after minimum period, until interrupted by the minor phase detector, actuation.

d) Minor phase receives green after actuation provided major phase has completed mini-mum green period.

e) Minor phase receives minimum initial green period.

f) Minor phase green is extended by additional actuations until preset maximum limit is reached or a gap in actuations greater than the unit extensions occur.

g) Additional actuation will be remembered if max-imum has been

reached on minor phase and will return green after major phase interval.

h) Yellow change and all-red clearance intervals are preset for each phase.

This kind of control is excel-lent for use where a light side-road volume cannot safely cross major flows without sig-nalisation. If sideroad flows are sporadic, the regular inter-ruption of the major flow with pre-timed control cannot be justified. Where both road vol-umes fluctuate widely, semi-actuated control should not be used, since there are no detectors on one or more legs.

TRAFFIC SIGNAL CONTROLLERS AND DETECTORS

5.3.2 Full - Actuated Controllers This provides for actuation by vehicles on all legs of the intersection. It is applicable primarily to an isolated inter-section of roads that carry approximately equal traffic vol-umes, but. where distribution between approaches varies and fluctuates. It then becomes necessary to take into consideration the

demands on all approaches.

The essential operating fea-tures of the controller are : a) Detectors on all

approaches

b) Each phase has preset initial interval to provide starting time for stand-ing vehicles.

c) Green interval is extended by a preset unit extension for each actuation after the initial interval expires, provid-ed a gap greater than the unit extension does not occur.

d) Green extension is lim-ited by a preset. maxi-mum limit (some equip-ment can provide two maximums per phase).

e) Yellow change and all-red clearance intervals are preset for each phase.

f) Each phase has a recall switch

- when both recall switches are off the green will remain on one phase when no demand is indi-cated on the other phase.

- when one recall switch is on, the green will revert to that phase at every opportuni-ty.

- when both recall switches are on, the controller will cycle on a fixed-time basis in the absence of demand on either phase (ore initial interval and one vehicle interval on each phase).

Because of their actuated nature, full-actuated con-trollers cannot be coordinated with other signals without los-ing the flexibility for which they were designed. Demand patterns for which they are applicable, as well as the inability to coordinate make the requirements of isolated locations (about 2 km

between adjacent signals) a fairly strong one.

TRAFFIC SIGNAL CONTROLLERS AND DETECTORS

5.3.3 Volume - Density Controllers This class of device offers additional responsiveness in signalisation for isolated inter-sections. Green time is allot-ted on the basis of volumes on approach legs. Unlike sim-ple actuated signal, the vol-ume-density signals does not merely react in a predeter-mined fashion to an actuation, but is able to record and retain information regarding volume, queue length, and delay times. In addition, a phase will lose the green by any one of three mechanism.

a) There are no vehicles producing any further demand on the approach.

b) The maximum green phase is reached.

c) The time gap between vehicles on the

approach exceeds the maximum standard.

The last mechanism is the

"density" function of the sig-nal. At the beginning of a green phase, the maximum time gap might be, for exam-ple, 5 seconds. As the green phase continues, the maxi-mum time gap decreases. The phase is lost when the maxi-mum gap is exceeded, or when the maximum length of phase is reached, whichever comes first. This type of con-trol provides the greatest

flexibility in traffic-actuated controllers, in that it is capable of taking into consideration the number of vehicles waiting on an approach, as well as the volume on the approach with the green indication. Its use is primarily applicable to an isolated intersection with wide traffic fluctuations

between roads. The essential operating features of the con-troller are :

a) Detectors on all approaches

b) Each volume density phase has an initial green time that, can be varied by :

- added initial, - computed initial,

or

- extensible initial, c) Passage time is the

extended green time created by each addi-tional actuation after the initial green time has elapsed. This time is set as that required to travel from the detector to the stop line.

d) Passage time is

reduced to a minimum gap after a preset time.

TRAFFIC SIGNAL CONTROLLERS AND DETECTORS

e) Maximum green or extension limits are preset for each phase.

f) Yellow change and all-red clearance intervals are preset for each phase.