Traffic & Transportation

Ventura Traffic Citizen Information Brochures

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 signalís 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 canít 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