Traffic Impact Attenuators

TRAFFIC IMPACT ATTENUATORS

• Traffic Impact Attenuators or Crash Impact Attenuators are impact absorbing devices installed in highly hazardous locations so as to reduce kinetic energy of the irregularly moving vehicle and to stop or redirect the vehicle away from the main hazard with least damage to life and property

 

• A safety crash barrier is a system engineered to prevent fatal injuries by absorbing shock energy and converting it into rotational energy. It functions by virtue of absorbing energy from the impact and deflecting it into the barrier. This energy is converted into rotational energy into the barrier.

 

• The attenuator acts as a protection cushion in the event of an accident. Impact attenuators are designed to absorb the colliding vehicles kinetic energy and bring it to stop safely

• By safely dissipating the vehicle's kinetic energy, impact attenuators help prevent fatal injuries.

 

•  Impact attenuators can be categorized by the method used to dissipate kinetic energy:
• Momentum transfer. Many early models used successive rows of sand- or water-filled barrels or modules. Momentum is transferred to the sand or water, reducing the speed of the impacting vehicle.
• Material deformation. Many newer attenuators use crushable materials (like various kinds of foam) that create a crumple zone, absorbing energy. Others flatten a corrugated steel guard rail section, or split a steel box beam.
• Friction. Some attenuators work by forcing a steel cable or strap through an angled slot or tube, converting kinetic energy into heat.
  

Delineators

TRAFFIC DELINEATORS

• Traffic delineators are devices which are used for guiding traffic through potentially hazardous road conditions or when there are confusing construction situations in effect at a given location. 
• Delineators provide highly visible boundaries which help to identify changes in traffic flow that alert drivers to upcoming road conditions like curves, on and off ramps, and other road changes up ahead.
• Traffic delineators include everything from cones to barrels, however the term delineator is used on those traffic control devices which are tall (height) and have a comparatively short diameter.
• One of the most important requirements of a delineator is that it should be visible over a long distance under normal conditions.
• Delineators should be able to withstand impact and hence they are made of rugged plastic
• They are often coated with retro-reflective sheet for increased visibility at night.
• There are different types of delineators. But the most common ones are:
• Open-top delineators
• Flat-panel delineators and
• Delineators with handles
• All the above listed delineators may or may not have reflective sheeting on them.
• Traffic delineators are generally used for channeling motor vehicle traffic through specific locations where it is necessary to guide motorists along a pathway that is different from the normal flow of traffic
• Traffic delineators are more effective as they are highly visible and extremely effective at establishing traffic flow around hazardous locations. They can also be effectively used for temporarily establishing a traffic pattern
• They are frequently used at construction sites or accident sites. Around work zones delineators are used to direct traffic safely around the work area in order to protect the drivers and construction workers.

Area traffic controls

 AREA TRAFFIC CONTROLS

An Area Traffic Control (ATC) System involves coordination of traffic signals over a complete network of signals covering an area that might be considered homogeneous from the point of view of traffic operation. Such a system is computer aided as the problem is extremely complex because of crossing of several routes at common intersections.

• ATC systems are intelligent, real-time, dynamic traffic control systems which are designed to effectively respond to rapid variations in dynamic traffic conditions. 

• It is an advanced process to control the traffic. It is a traffic responsive system that use data from vehicle detectors and optimize traffic signal time in real time. 

• The timing plan of traffic controllers changes automatically. The technique employs digital computers for achieving the desired objective.

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.

Traffic signals affecting pedestrian safety

 TRAFFIC SIGNALS AFFECTING PEDESTRIAN SAFETY

It has been observed that most pedestrian accidents in built-up areas occur at intersections. The "Traffic Conflicts Technique" is used to examine risk to pedestrians at intersections. 

• It has been observed that reduction in speed reduces the pedestrian's risk
• A "zebra crossing" located less than 2m from the intersection increases the pedestrian safety
• The collected data must distinguish between accidents occurring during pedestrians walking against red light and green light
• An exclusive pedestrian signal phase was found to be extremely safety-beneficial. However, such traffic signals meant exclusively for pedestrians are possible only in small towns with a very low population
• Pedestrians crossing a road running with heavy traffic is influenced heavily by size of the town and traffic volume
• Short waiting times and police enforcement are the most efficient measures to reduce frequency of red-walking

Factors affecting design of traffic signals

FACTORS AFFECTING DESIGN OF TRAFFIC SIGNALS

The IRC method for designing traffic signs implies the following factors affect the design of traffic signals

• Cycle: A signal cycle is one complete rotation through all of the indications provided
• Cycle length: Cycle length is the time in seconds that it takes a signal to complete one full cycle of indications. It indicates the time interval between the starting of of green for one approach till the next time the green starts. It is denoted by C. 
• Interval: Thus it indicates the change from one stage to another. There are two types of intervals - 
• change interval and 
• clearance interval. 
• Change interval is also called the yellow time indicates the interval between the green and red signal indications for an approach. 
• Clearance interval is also called all red is included after each yellow interval indicating a period during which all signal faces show red and is used for clearing off the vehicles in the intersection.
• Green interval: It is the green indication for a particular movement or set of movements and is denoted by Gi. This is the actual duration the green light of a traffic signal is lit.
• Red interval: It is the red indication for a particular movement or set of movements and is denoted by Ri. This is the actual duration the red light of a traffic signal is lit.
• Phase: A phase is the green interval plus the change and clearance intervals that follow it. Thus, during green interval, non conflicting movements are assigned into each phase. It allows a set of movements to flow and safely halt the flow before the phase of another set of movements start.
• Lost time: It indicates the time during which the intersection is not effectively utilized for any movement. For example, when the signal for an approach turns from red to green, the driver of the vehicle which is in the front of the queue, will take some time to perceive the signal (usually called as reaction time) and some time will be lost here before he moves.

Illumination and location of traffic signals

ILLUMINATION AND LOCATION OF TRAFFIC SIGNALS

A traffic signal is used as an instructing device that indicates the road user to act according to the displayed sign. Following the traffic signal ensures road safety and to make things simple to understand, these signals have been using a universal colour code.

• Red is generally the sign of danger or warning and on traffic lights, it signals potential hazard ahead and hence, is an indicator for the motorists to stop. 
• A flashing red light indicates the same as the stop light, which is, basically, top stop. Once you have stopped, you can proceed ahead after making sure there’s no obstacle in your path.
• A yellow traffic light is a warning signal that lets you know that the red signal is about to be displayed. Hence, when you see the yellow light, you should start slowing down to come to a stop in anticipation of red light. 
• A flashing yellow light is basically a warning signal to make the road users alert and slow down if needed.
• The green light signals safety and the word GO.
•  A traffic signal setup includes controller, traffic lights and detection. The controller works as the ‘brain’ of the entire setup and has the information that is required to make sure the lights work as per the required sequences. Traffic signals can run under a variety of different modes which can be dependent on location and time of day.
• Traffic Signal power supplies supply power to traffic signal cabinets
• The traffic signal power supply is an electrical device in the control cabinet that converts AC to correct DC voltages for various devices in the traffic signal cabinet. The nominal voltage of the power supply is 24VDC
• One of the effective options used by traffic management teams is the use of LED or light-emitting diode lamps to be able to consume less power during operations. This will also make it easier to provide backup power, such as the use of uninterruptible power supply (UPS).
• The first illuminated traffic signal was installed in London, England, in 1868.
  
• The first electric traffic signal was installed in Cleveland, Ohio, in 1914.
  
• The first signal to use the familiar green, yellow, and red lights was installed in New York City in 1918. It was operated manually from an elevated observation post in the middle of the street
  
• A modern traffic signal system consists of three basic subsystems: the signal lights in their housing, the supporting arms or poles, and the electric controller. The signal lights and housing are known as the signal light stack. 
• A single stack usually consists of three lights: a green light on the bottom to indicate the traffic may proceed, a yellow light in the middle to warn traffic to slow and prepare to stop, and a red light on the top to indicate the traffic must stop.
  
• Each light has a fresnel lens which may be surrounded or hooded by a visor to make it easier to see the light in bright sunlight. 
• A fresnel lens consists of a series of concentric angled ridges on the outer surface of the lens which bend the light to focus it in a parallel beam. 
• The light stack may have a dark-colored backing plate to make the signals more distinguishable by blocking out surrounding lights from buildings and signs.
  
• The electric controller is usually mounted in a weather-proof box on one of the corners of the intersection. 
• More elaborate traffic signals may also have electromagnetic sensors buried in the roadway to detect the flow of traffic at various points.
  
• The lens for each light is made of tinted glass or plastic.
• The bulb, known as the lamp, is designed for long life. 
• The bulb is partially. surrounded by a polished metal reflector to direct the light forward. 
• The hood or visor is made from aluminum or molded plastic.
• The supporting arms or poles are usually made of galvanized steel for strength and corrosion-resistance. They may also be made of fiberglass. 
• The controller is housed in a steel or aluminum enclosure. 
• The electrical components within the controller consist of switches, relays, and timers
• The wiring between the components is copper with a heavy neoprene rubber or plastic insulation.
• Each signal lens shall be illuminated independently and should be clearly visible from a distance of at least 400 m under normal atmospheric conditions.

Location of signals

• The Indian Road Congress (IRC) defines a road traffic signal as any power operated by which traffic is regulated, warned or directed to take some specific action.
• The primary consideration in placement of signals is visibility
• Drivers approaching a signalised intersection should be given a clear indication of their right-of-way assignment
• The lateral and vertical angles of sight towards a signal face determined by typical driver eye position, vehicle design and the vertical longitudinal and lateral position of the signal face are the critical elements to be considered while determining the location of signals
• The geometry of each intersection to be signalised along with the vertical grades and horizontal curves  should be considered in signal face placement.