Fixed traffic signals and vehicle actuated signals

Fixed and vehicle actuated signals 

• Fixed time signals are normally installed at the intersection of two major roads. 
• The signals will change even if no cars or pedestrians are present. 
• Fixed time signals are less efficient when traffic varies quite a bit through the day, but they are cost-effective because detectors are not required.
• Fixed-time signals are the rule in urban areas for reasons of regularity, network organization, predictability, and reducing unnecessary delay.

• Vehicle-Actuated Signals require actuation by a vehicle on one or more approaches in order for certain phases or traffic movements to be serviced. 
• They are equipped with detectors and the necessary control logic to respond to the demands placed on them. 
• Vehicle-actuated control uses information on current demands and operations, obtained from detectors within the intersection, to alter one or more aspects of the signal timing on a cycle-by-cycle basis.
• Timing of the signals is controlled by traffic demand.
• Variability allows the signal to allocate green time based on current demands and operations. 
• A proper clearance interval between the green & the red phases is also ensured.

The various advantages of actuated signals are stated below:

• They can reduce delay (if properly timed).
• They are adaptable to short-term fluctuations in traffic flow.
• Usually increase capacity (by continually reapportioning green time).
• Provide continuous operation under low volume conditions.
• Especially effective at multiple phase intersections.

    The main disadvantages are as following :

• If traffic demand pattern is very regular, the extra benefit of adding local actuation is minimal, perhaps non-existent.
• Installation cost is two to three times the cost of a pre-timed signal installation.
• Actuated controllers are much more complicated than pre-timed controllers, increasing maintenance costs.
• They require careful inspection & maintenance to ensure proper operation.

There are three basic types of actuated control, each using signal controllers that are somewhat different in their design:

1. Semi-Actuated Control
2. Full-Actuated Control
3. Volume-Density Control

The various types of detectors used for detection of vehicles are as following:

• Inductive loop detectors
• Magnetometer detectors
• Magnetic detectors
• Pressure-sensitive detectors
• Radar detectors
• Sonic detectors
• Microloop detectors etc.

In certain, less-trafficked areas, actuated signals (push buttons, loop detectors) may be appropriate; however, these must be programmed to minimize delay, which will increase compliance.

Fixed-time signals incur lower initial and ongoing maintenance costs than actuated signals.

Actuated signals in general are not preferable because of the maintenance requirements and upkeep of the detection on the street.

Drivers at unsignalized intersections benefit from a series of fixed-time signals, as they produce routine gaps in traffic that may be used to turn onto or cross the street. Fixed-time signals help make pedestrians an equal part of the traffic signal system by providing them with regular and consistent intervals at which to cross

Fixed-time signals incur lower initial and ongoing maintenance costs than actuated signals.

Actuated signals prioritize movement along the primary corridor and can present obstacles to cross traffic and pedestrians if timed to prioritize vehicle movements only.

Actuated signals should be timed to be as responsive to activation as possible, with delay kept to a minimum.

Many existing traffic signal controllers have the capacity to reduce delay, but remain in coordination rather than a free setting. Coordination, paired with long signal cycles, can result in delays of 80 seconds or more, reducing pedestrian compliance, increasing risk-taking behavior, and creating the impression that a push button is either non-responsive or malfunctioning.

At crossings where the signal is uncoordinated with adjacent traffic signals (free setting), designers can further reduce pedestrian delay by reducing the minimum green time. At coordinated signal locations, designers have multiple options to decrease delay, including increasing the permissive window, adjusting signal timing for responsiveness at certain times of day, and setting the signal to recall on the pedestrian phase.

In coordination with traffic signal timing, designers must consider spacing between traffic signals, looking at desirable crossing intervals to achieve a pedestrian-friendly environment.

 Fixed-time signals are recommended in all downtown areas, commercial centers, and urban areas in which pedestrians are anticipated or desired and speeds are intended to be low.

Use of semi- or fully-actuated signal operations should mainly be restricted to suburban arterials and rural roads. 

In areas with lower pedestrian traffic, actuation may be used along priority rapid transit corridors to increase the schedule reliability of transit service and avoid unnecessary delays.

 The responsiveness of an actuated signal should be prompt (as low as 5 seconds) based on the necessary transition time for approaching motorists to come safely to a stop. 

Wherever pedestrian movement crosses a high capacity transit line, major bicycle facility, or critical freight route, longer delays are acceptable.

For major bicycle routes, use upstream passive detection as opposed to push-button activation to minimize the time lag between detection and crossing.

Fully-actuated signal control may be used where vehicle and pedestrian volumes vary considerably throughout the day. 

Full-actuation can reduce the amount of delay by being responsive to ongoing shifts and patterns in the traffic system.

Semi-actuated control prioritizes the through movement of a major road and is not recommended on streets with frequent cross traffic or pedestrian demand from the minor approach unless a low cycle length is used (below 80 seconds). 

Any traffic signal with long delays for pedestrians may discourage crossings and become a barrier to travel, especially at busy intersections.

Actuated signals may be combined with a number of signalization treatments, including full signalization (of the major and minor approach) and pedestrian or half-signalization (stop sign on the minor approach).

Signalization is not always the best option for a given intersection. Stop or yield control may be preferable at intersecting local or residential streets.

Fixed-time signals are recommended in all downtown areas, central business areas, and urban areas in which pedestrians are anticipated or desired and speeds are intended to be low.

Use of semi- or fully-actuated signal operations should mainly be restricted to suburban arterials and rural roads. 

In suburban corridors, motorist compliance can be increased and delay reduced through use of actuation.

In areas with lower pedestrian traffic, actuation may be used along priority rapid transit corridors to increase the schedule reliability of transit service and avoid unnecessary delays.

The responsiveness of an actuated signal should be prompt (as low as 5 seconds) based on the necessary transition time for approaching motorists to come safely to a stop. 

In cases where the pedestrian movement crosses a high capacity transit line, major bicycle facility, or critical freight route, longer delays are acceptable.

For major bicycle routes, use upstream passive detection is preferred to push-button activation to minimize the time lag between detection and crossing.