Road Network Operations
& Intelligent Transport Systems
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Traffic Control

A number of traffic control strategies can be implemented in Road Network Operations in order to improve traffic flow, prevent congestion and enhance throughput. ITS software – supported by a wealth of real-time data enabling accurate estimates to be made of the status of the road network – is used to develop optimal management and control strategies that support network policy objectives. These will vary from one location to another, but commonly include maximising traffic throughput, minimising delays and congestion, maintaining road safety for all road-users – including safe crossings for pedestrians and cyclists – environmental targets (to reduce noise levels and/or air pollution) and bus/tram signal priority for some locations.

Control methods include:

Urban Traffic Control (UTC), with:

  • optimal/adaptive signal control (where signal timing is adjusted in real-time to accommodate detected changes in traffic patterns);
  • public transport (transit) signal priority or emergency vehicle signal pre-emption;
  • arterial traffic signals.

Motorway control systems, including:

  • ramp metering (to control the rate of traffic entering a motorway or other high capacity limited-access road)
  • variable speed limits to optimise traffic flow and prevent breakdown.

Field controllers are needed to implement these strategies. They are the “brains” of the local system, and provide the means for accessing, monitoring and controlling field equipment (such as a ramp meter, a traffic signal, or a vehicle detector).

Computer software is needed to provide these functions. Some of the functions that an ITS system software may be required to provide include urban traffic control, traffic control on arterial roads and motorway control systems.

Urban Traffic Control

Urban Traffic Control refers to a package of technologies aimed at managing and controlling traffic flowing over urban networks – to minimise delay, maximise efficiency, improve safety and reduce emissions and fuel consumption. A large part of urban traffic control involves software to optimising signal plans at intersections to achieve these objectives. This requires extensive sensor networks to collect real-time traffic information – for example, loop detectors, closed-circuit TV cameras and video image processing, or non-intrusive traffic detectors. Based on the collected information, intelligent algorithms aim to optimise the signal plans. Different levels of control and sophistication are seen in urban traffic control systems (See Urban Traffic Control).

Traffic Control on ARTERIAL Roads

Several types of field controllers are available which respond to traffic demands to facilitate turning movements and allow time for cross-traffic. In the USA the Type 170 Controller was developed in the early 1970s by the California Department of Transportation. Its successor – Type 2070 – was introduced in 1992. More recent examples are the NEMA signal controllers and the Advanced Traffic Controller (ATC – 2005) – the most advanced controller in the USA.

Traffic signal controllers work on the basis of a timing cycle that is broken into “phases” – the order in which each traffic stream is given green time, whilst other traffic is held at red. A simple cross-road intersection may have just two phases: North/South, and East/West. A busy four-way intersection, with large volumes of turning traffic, might have up to eight phases – one phase for each of the four traffic directions and a phase for each of the turning movements.

In the United Kingdom, a relatively new controller called Microprocessor Optimised Vehicle Actuation (MOVA) was developed to overcome some of the limitations associated with traditional Vehicle Actuation (VA) control. A unique feature of MOVA is that it has two modes of operation – one for congested traffic conditions and one for uncongested or free-flow conditions. For free-flow conditions, the aim of MOVA is to deal with any queues that have accumulated during the red phase. An algorithm assesses the traffic loads from different intersection approaches and determines whether extending the green time is beneficial. If it is, the green phase is extended to let traffic through. This continues until the controller moves to a different phase. During congestion, MOVA’s operational objective changes to maximise the capacity or throughput for the intersection as a whole.

Motorway Control Systems

Motorway control systems focus on better management of motorway segments to enhance capacity and increase throughput. Over the years, several Decision Support Systems (DSS) have been proposed and developed to help this process. These DSS can provide recommendations to traffic operators on possible traffic control strategies – such as dynamic route guidance, ramp metering, changeable speed limits and optimal signal timing.

Automated motorway control systems (sometimes referred to as Active Traffic Management or ATM systems), use different concepts to achieve their goal – such as speed harmonisation, temporary shoulder usage, dynamic routing and signing, junction control and ramp metering (See Highway Traffic).

Active Traffic Management has been widely implemented in Europe, and is becoming a tool for managing congestion (both recurring and non-recurring), in the USA as well. The main technological components of ATM are similar to the UTC systems and include extensive sensing and monitoring systems, communications, controllers, and intelligent algorithms.

The benefits of Active Traffic Management systems include:

  • increases in average throughput for congested periods;
  • increases in overall capacity of 3% to 22%;
  • decreases in primary incidents of 3% to 30% – and in secondary incidents of 40% to 50%;
  • overall harmonisation of speeds during congested periods;
  • decreased headways and more uniform driver behaviour;
  • increase in journey time reliability;
  • the ability to delay the onset of flow breakdown with stop-start conditions.
Reference sources

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