Digital computers are used to control traffic signals along arterials and in
networks in many cities throughout the world.
Here the basic issues and concepts invovled in computer control of surface
street traffic are discussed.
With the current emphasis on ITS, computer control of systems is now classified
as Advanced Transportation Management Systems (ATMS) and the control centers
themselves as Transportation Management Centers (TMCs).
Originally, it was assumed that the power of the
digital computer could be used to control many traffic signals from one
location, allowing the development of control plans.
The basic concept can be summarized thus: the computer sends out signals along
one or more arterials.
There is no feedback of information from detectors in the field, and the
traffic-signal plans are not responsive to actual traffic conditions.
Earlier,the plans for such a system are developed based on the engineers usage
of data from field studies to generate plans either by hand, or by
computer,using packages available at the time.
The computer solutions were then run on another machine, or in off hours on the
control computer when it was not being used for control of the traffic signals.
Though this ``off-line'' system of control plans gives an image of a
deficient system, there are many advantages of this ``limited'' system.
These include:
Ability to update signals from a Central Location:
The ability to retime signals from a central location without having to send
people along an entire arterial to retime the signals individually at each
intersection saves lot of time.
Ability to have multiple plans and special plans:
In many localities a three-dial controller is quite sufficient: if traffic is
generally regular, three basic plans (A.M. peak, P.M. peak, off-peak) can meet
the needs.
The computer opens the possiblity to have an N-dial controller, with special
plans stored for certain days.
With appropriate plans stored for each such event, the plans can be called up
by time of day, or by operator intervention.
Information on equipment failures:
The early systems simply took control of electromechanical controllers, driving
the cam-shaft from the central computer and receiving a confirmation signal.
Failure to receive this signal meant trouble.
The information provided by the control computer allowed such failures to be
detected and repair crews dispatched.
Performance data on contractor or service personnel:
With a failure detected and notification made, the system can log the arrival
of the crew and/or the time at which the intersection is returned to active
service.
Figure 1:
Computer control system with detector information used
The ability of a computer to receive great amount of data and process it is made
use of by detectors in the field for sending information back to the central
location.
If the information is not being used in an ``online'' setting and hence
still does not influence the current plan selection.
Typically, the computer is being used as the tool for the collection of
permanent or long-term count data.
Fig. 1 shows a computer control system that actually uses the
traffic data to aid in plan selection.
This can be done in one of three principal ways:
1. Use library - Monitor deviations from expected pattern: This concept uses a
time-of-day approach, looking up in a library both the expected traffic pattern
and the preselected plan matched to the pattern.
The actual traffic pattern can be compared to the expected, and if a deviation
occurs, the computer can then look through its library for a closer match and
use the appropriate plan.
2. Use library - Match plan to pattern: This is a variation on the first
concept, with the observed pattern being matched to the most appropriate
prestored pattern and the coresponding plan veing used.
3. Develop plan on-line: This concept depends on the ability to do the
necessary computations within a deadline either as a background task or on a
companion computer dedicated to such a computations.
This approach presumes an advantage to tailoring the control plan to specific
traffic data.
It is necessary to note that the time between plan updates is constrained by
the speed with which the on-line plan computations can be done.
The desire to have more frequent updates implicitly assumes that the real
traffic situation can be known precisely enough to differentiate between
consecutive update periods.
