ITS and other measures can be applied to control traffic to deal with recurring congestion across urban areas and on regional networks of motorways, freeways, expressways and other arterial roads. The general objective of these measures is to make better use of available capacity through traffic flow improvement strategies, or in many cases by adding extra traffic capacity.
Systems to manage traffic are diverse and can be considered at two levels:
Traffic Control Centres (TCCs) responsible for regional networks have the job of coordinating traffic management on the most heavily trafficked routes. (See Traffic Control Centres) Traffic management at this tactical level involves the implementation of localised schemes, such as ramp metering, automatic incident detection and use of CCTV, supported by the widespread use of VMS. The control systems themselves can be grouped into four categories:
Aside from budgetary considerations, the choice of measures depends largely on the local operating context which covers a spectrum of different operating environments – from high-speed multiple-lane motorways that experience recurring congestion to all-purpose rural 2-lane roads with only seasonal traffic problems.
For a particular section of road the operating environment is determined by a combination of three factors:
The European EasyWay project has developed a classification system for operating environments on the Trans-European Road Network that reflects the level of service expected by road-users, the frequency of recurring congestion and traffic incidents and the feasibility of possible ITS solutions to deal with these problems. It can be used to select the parts of the road network where improvement using traffic management would be beneficial – and for prioritising where to implement different measures. For core (network-wide) ITS services the operating environment classification can be used to prioritise implementation if this has to be done in phases.
Traffic signal control on all-purpose single and dual carriageway arterial roads is frequently used at isolated intersections, often using a demand-responsive control strategy. The installation of traffic signals on any high-speed road demands great attention to traffic safety. Adopting motorway style control and signalling on all-purpose arterial roads is primarily an investment issue: when the need is perceived to be significant the investment budget will be found and the supply base will respond.
As traffic volumes continue to increase, the need to consider areas of the regional network that require special treatment in terms of traffic management will also increase. Sometimes the only tactic during a traffic incident is to keep traffic queuing on the highway to avoid causing much wider disruption. One example is where regional inter-urban routes enter a conurbation, requiring the motorway control systems to be closely integrated with the urban traffic control system. (See Integrated Strategies)
Strategic traffic management is concerned with advising drivers of major incidents and problems so they can adapt journey plans accordingly. Strategic measures are of particular benefit to long-distance traffic and make full use of traveller services – in particular dynamic (traffic responsive) journey planning and en-route information. (See Traveller Services) The network controller’s objective is to balance the flows of traffic on the different routes to the same destination area. A more even distribution of traffic helps reduce congestion and, in the event of a major incident, the ability to respond quickly by diverting traffic away from the affected area. Even a small reduction in demand resulting from advance information can have a dramatic effect on recovery times if it can prevent saturation and flow breakdown.
Advances in technology have made the concept of regional and strategic traffic control more realistic. Strategic traffic management systems are installed to monitor traffic across a regional network of strategic roads where alternative routes exist. These measures provide warnings of serious disruption to traffic with advice and directions on the recommended routes for long-distance traffic.
Network traffic assignment models (such as Motorway CONTRAM in the UK) are sometimes used to decide when to initiate a strategic diversion route. Traffic and travel information systems (VMS, advisory systems and other en-route driver information) are used to advise drivers when there is congestion and delays. (See En-Route Information)
These regional control systems require the interchange of information between local tactical control centres and the strategic regional centres. In Europe several system suppliers offer modular systems that combine urban traffic control and highway traffic functionality to enable local and regional network control.
The trend towards more integrated traffic management will drive greater functional integration of traffic monitoring, traffic information systems and traffic control. Provided there is sufficient spare capacity on the regional network to allow effective strategic re-routeing, network control should significantly reduce delays, increase safety and reduce pollution.