What You Need To Know About Traffic Signal Systems
Each year, the city receives many inquiries concerning the operation of traffic signals within the City. The public's understanding of the function of traffic signals can improve driving habits by reducing speeding and associated traffic accidents. The more drivers know about the operation of traffic signals the less they will be frustrated when waiting for a signal to change.
Why are traffic signals needed?
As traffic volumes increase beyond the capability
of lesser controls such as a four-way stop, it may be necessary to install
a traffic signal. Before installing a traffic signal at an intersection,
established minimum criteria must be satisfied: A review includes: The
amount of vehicular and pedestrian traffic; the need to provide interruption
to the major flow for side street vehicles and pedestrians; special conditions
such as hills and curves; the accident history of the intersection and
the proximity of schools.
Are traffic signals the answer to solving traffic problems?
Advantages of Traffic Signals
Signals offer maximum control at intersections.
They relay messages of both what to do and what not to do. The primary
function of any traffic signal is to assign right-of-way to conflicting
movements of traffic at an intersection. This is done by permitting conflicting
streams of traffic to share the same intersection by means of time separation.
By alternately assigning right of way to various traffic
movements, signals provide for the orderly movement of conflicting flows.
They may interrupt extremely heavy flows to permit the crossing of minor
movements that could not otherwise move safely through an intersection.
When properly timed, a traffic signal increases the traffic handling capacity
of an intersection, and when installed under conditions that justify its
use, a signal is a valuable device for improving the safety and efficiency
of both pedestrian and vehicular traffic. In particular, signals may reduce
certain types of accidents, most notably right-angle (broadside) collisions.
Disadvantages of Traffic Signals
While many people realize that traffic signals
can reduce the number of right-angle collisions at an intersection, few
realize that signals can also cause a significant increase in rear-end
collisions. Normally, traffic engineers are willing to accept an increase
in rear-end collisions for a decrease in the more severe right-angle accidents;
however, when there is no right-angle accident problem at an intersection
and a signal is not needed for traffic control, there is no safety benefit
and the installation of traffic signals can actually cause a deterioration
in the overall safety at the intersection. Traffic signals are not a cure-all
for traffic problems. The primary goal of the traffic engineer is to attain
the safest and most efficient overall traffic flow possible. In addition
to an increase in accident frequency, unjustified traffic signals can
also cause excessive delay, disobedience of signals, and diversion of
traffic to residential streets.
Traffic signal equipment
Traffic signals are more costly than is commonly
realized, even though they represent
a sound public investment when justified. A modern signal can cost up
to $150,000.
This money pays for:
- A Traffic Signal Controller
- Signal Heads
- Vehicle Detectors
- Signal Poles and Supports
The Controller is the signal’s brain. It consists of electrical or computer controls that operate the selection and timing of traffic movements in accordance with the varying demands of traffic as registered with the controller unit by detectors.
Signal Faces are part of a signal head provided for controlling traffic in a single direction and consisting of one or more signal sections. These usually include solid red, yellow, and green lights and sometimes red, yellow and green turn arrow lights as well. The Signal Head can contain one or more signal faces.
Detectors are devices for indicating the passage or presence of vehicles. In Ventura these consist of wire loops placed in the pavement at intersections. They are activated by the change of electrical inductance caused by a vehicle passing over or standing over the wire loop.
Special signal functions
Traffic Signal Preemption
The transfer of signal control to a special
signal operation is called preemption. There are three common types of
preemption, based on reason for preemption: Railroad, Emergency Vehicle,
and Transit Vehicle Preemption.
Railroad Preemption
Railroad preemption is initiated when a train
passes over advance detectors located on the tracks ahead of the railroad
crossing. The purpose of preemption is to clear tracks of traffic stopped
on them by traffic signals.
Emergency Vehicle Preemption
Emergency vehicle preemption can be used for
any authorized emergency vehicle, but normally only for fire engines.
The purpose is to obtain a green light for the emergency vehicle as soon
as possible or to hold an existing green light. To obtain a green light,
existing green lights, including pedestrian intervals, are abbreviated.
After the yellow change interval, a green light is given to the approach
to be used by the emergency vehicle.
There are three means of signal preemption from emergency vehicles: mobile
radio, siren sensor, and modulated strobe light. Light modulated output
of a vehicle mounted emitter is received by a detector at a signal.
Flashing Red
According to the California Vehicle Code, when
a red lens is illuminated with rapid intermittent red flashes, a driver
shall stop before entering the crosswalk on the near side of the intersection.
The driver may proceed subject to the rules applicable to making a stop
at a 4-way stop controlled intersection.
Flashing Yellow
When a yellow lens is illuminated with rapid
intermittent yellow flashes, a driver may proceed through the intersection
or past the signal only with caution.
Dark Signals
When a traffic signal has gone dark due to
power failure it is considered to function the same way as a 4-way stop
controlled intersection and a driver must stop before entering the intersection.
Signal timing
Traffic signals assign the right of way to
various traffic movements for different time intervals depending on traffic
flow levels. Pre-timed signals have preset time intervals for different
times of the day including the morning, noon, and evening peak travel
periods. The City has a few pre-timed signals remaining in the downtown
area.
Traffic actuated signals use detectors located in the pavement on the approaches to traffic signals to monitor and assign the right of way on the basis of changing traffic demand. These signals attempt to assign most of the available green time to the heaviest traffic movements.
Coordination of traffic signals
The greatest benefits to the public for each dollar
spent on traffic operations improvements come from the coordination of
adjacent traffic signals to provide smooth movement of the traffic through
groups of signals on an arterial street. The coordination of traffic signals
to facilitate smooth traffic flow (progressed movement) along a street
is a proven technique. The quality of flow along a street is basically
a function of the spacing of the signals along the street, the prevailing
speed of traffic on the street, and the traffic signal cycle length. The
amount of traffic and the proportion of the green time given to the progressed
movements are also important.
Traffic Signal coordination goals
Many drivers ask why they have to wait so long
for a signal to change. Many of these drivers are waiting to enter a major
arterial street from a side street. This is even more frustrating when
no traffic can be seen on the arterial. To allow the coordination of the
arterial, the side street must wait until the main traffic movement on
the arterial has gone through the intersection. It is possible that the
arterial traffic can’t be seen immediately, but will soon be passing through
the intersection. Major coordinated arterial in the City include Victoria
Avenue, Telephone Road , Telegraph Road, Main Street, Thompson Boulevard,
Harbor Boulevard and Kimball Road.
The goal of coordination is to get the greatest number of vehicles through the system with the fewest stops in a comfortable manner. It would be ideal if every vehicle entering the system could proceed through the system without stopping. This is not possible, even in well-spaced, well designed systems. Therefore in traffic coordination the majority rules and the busiest traffic movements are given precedence over the smaller traffic movements. The master cycle length in the City for signal coordination is 110 seconds. Therefore if you missed the green light on the side street approach to a major street intersection it is possible you may have to wait up to 110 seconds to receiver another green light.
Future signal coordination
goals
City Traffic engineers and technicians are constantly upgrading the
City's signal system by monitoring traffic volumes on streets and turning
movements at busy intersections. This information is then used to help
traffic flow easier with less delay. The city is planning to implement
Traffic Response Signal timing where a computer will sense traffic levels
and adjust signal timing to best suit traffic demand. Once the system
is in place, it is anticipated that delays to side street traffic will
also be minimized during times when arterial street traffic is given priority
in order to coordinate the signals