Traffic control covers all measures aimed at distributing and controlling road traffic flows in time and space in order to avoid the onset of incidents or to reduce their impacts. Traffic control is carried out by network operators and controllers with reference to predetermined traffic management policies and plans. In most countries it is an activity done in coordination with the authorities in charge of traffic policing, often under their direct control. (See Traffic Management Plans)
It is possible to distinguish between:
Indirect control measures can be characterised as either preventive action or actions that are remedial or corrective. Indirect control methods are supported by travel information systems. (See Traveller Services)
Preventative action aims to warn drivers of current and forecast problems so they can make adjustments to their travel plans. It may include warning drivers of anticipated problems so that they can change their travel times, choose different routes or abandon their trip. This requires:
Data being generated by ITS field devices and floating vehicles (probes) are valuable resources that provide a foundation for the use of simulation models that can predict congestion and/or journey times. This enables logistics system managers and TCC operators to take action to avoid the onset of congestion—either by diversion or with advice on trip changes.
Remedial or corrective action is designed to limit the extent or impact of delays and congestion that occurs regularly on strategic routes, using measures to limit access and –at a regional level – to divert traffic onto less congested routes. Diversionary routes need to be planned in cooperation with the road authorities and operators of the alternative routes. Often, an alternative high-capacity route will not be available and other options would only be suitable for light traffic – not trucks and heavy goods vehicles. Some road authorities operate seasonal “holiday routes”. Others will only divert traffic when there is a road closure or an emergency (except at ramp meters, where the diversion is indirect).
Remedial action requires:
Closed-Circuit Television (CCTV) coverage of regular “hot spots” for accidents or congestion on the network are an essential part of modern Road Network Operations. Operators in the Traffic Control Centres (TCCs) will scan the CCTV images looking for signs of traffic disruption, such as a smoking vehicle that might break down, debris on the roadway, dangerous or excessive vehicular manoeuvres, or anything that might lead to a traffic incident and congestion. Ongoing developments using image recognition technique are enabling a degree of automation of incident detection. (See CCTV)
Many TCCs do not have sufficient operators to focus on camera scans, even using “camera tours” or “video tours” to monitor a sequence of cameras. More commonly they use system performance measures to alert operators to an incident.
The image below shows an innovative approach used by Florida DOT’s District 4’s TCC in Broward County (USA), in which the entire section of Interstates 95 and 75 can be monitored in its control area on wall projections. It displays the speed profile (average speed) of the traffic on the two interstates, so that operators can quickly identify hot spots. Taking proactive action before a suspected incident occurs is much like preventive maintenance – namely, “Fix the problem before it gets worse.”
Variable Message Signs are also referred to as Dynamic Message Signs and Changeable Message Signs. A Dynamic Message Sign (DMS) refers to any sign or graphics board where the message (text or pictogram) conveyed to the viewer can change. A DMS may be either a Variable Message Sign (VMS) or Changeable Message Signs (CMS) where:
A Portable Dynamic Message Sign is referred to as a PDMS
Roadside Dynamic Message Signs are the most common way of implementing network control strategies and communicating instructions to drivers. Those that are used on fast arterial roads, motorways and expressways are large constructions, mounted on gantries and positioned over the road. Some are mounted on masts at the side of the road. VMS/CMSs require a power supply and reliable communications between the control centre and the VMS installation.
VMSs provide the means of informing drivers of the need to be aware of approaching conditions. VMSs also have a key role in traffic management at the strategic level as part of a regional control tool. This is the case although VMSs are limited to the display of short messages which – even with pictogram enhancement – cannot convey the same amount of information as is possible via radio and other in-vehicle driver information systems. (See Use of VMS)
Categories of road and traffic situations for which messages can be displayed include:
Many VMSs are placed at strategic locations – for instance shortly before important motorway exits or motorway junctions. This is because at those points a traffic diversion is possible (depending on the message displayed). Sometimes these VMSs are also used at regular intervals along urban arterial roads and motorways, to provide a basic traffic management system – especially on roads that have no lane control systems.
The use of VMSs is, in most cases, coordinated from a Traffic Control Centre (TCC) where a control system will be used to control the display and to monitor the traffic. Where many VMSs are used, it is very important that the operator is able to have a clear overview of all messages displayed. The user interface should also help the operator in setting up, changing and cancelling the messages. (See Human Tasks and Errors)
Some messages will be automatically displayed, without intervention of an operator - for instance travel time indications based on automatically obtained traffic data. Other messages can be planned beforehand – for instance where there are road works or pre-announcements. In these cases the messages can be pre-programmed “off-line”, long before they are used. Some messages, such as those related to sudden incidents or weather circumstances, will need a quick reaction from a traffic operator.
In order to be ready for unpredictable situations, such as road closures in the case of accidents – it is advisable to have pre-prepared (or partly prepared) messages ready to display for all sorts of situations, supported by a control system that can handle these scenarios. In the case of foreseen events, the use of prepared messages can save a lot of operator time and possible confusion. (See Planning and Reporting).
A high level of sophistication is needed for maintaining modern, ITS-based traffic control systems. For example, control systems that require roadside information from detection equipment and/or camera images depend on the ability of the road network operator to install and maintain the systems. (See Vehicles and Roadways) Investment in the ITS infrastructure has to be matched with proper arrangements for equipment installation, communications and maintenance – and an appropriate budget. It may also be necessary to put in place essential administrative systems – for example a fully maintained, up-to-date database of vehicle registration plate numbers and the owner’s details, to be able to make full use of enforcement systems.
A problem experienced in some countries is theft of roadside equipment and related communication cables. Electrical wiring and electronic equipment has value and theft of equipment causes higher operating costs and unreliable incident information. The physical road condition can also impact on the detection systems that can be used. Inductive loops for instance, cannot be installed in pavements that are in a poor condition (potholes and substandard pavement materials). To some extent the move towards ‘non-wired’ systems will open the way for more secure infrastructure deployment.
The potential benefits of a traffic control system must be weighed against the potential cost of installing and maintaining those systems. Countries with economies in transition need to create (or procure under contract) the necessary organisational capability to carry out ITS equipment and software maintenance, to reap the benefits. Rather than rely on advanced systems which may be difficult to maintain and operate, the effective use of less sophisticated methods that use existing facilities may prove more effective. (See Developing ITS Capability and Priority Projects)