Urban Traffic Control

Urban traffic control (UTC) systems are a specialist form of traffic management which integrate and co-ordinate traffic signal control over a wide area in order to control traffic flows on the road network. Integration and co-ordination between adjacent traffic signals involves designing a plan based on the occurrence and duration of individual signal aspects and the time offsets between them and introducing a system to link the signals together electronically. A traffic responsive signal control system is a means of adjusting the traffic signal settings (cycles, green splits and offsets), which optimise a given objective function, such as minimising travel time or stops, in real-time based upon estimates of traffic conditions. There are many different UTC systems in operation around the world, but they can provide the basis for an extended control system, generally termed Urban Traffic Management and Control (UTMC).

UTC systems can be used to obtain better traffic performance from a road network by reducing delays to vehicles and the number of times they have to stop. UTC systems also can be used to balance capacity in a network, to attract or deter traffic from particular routes or areas, to give priority to specific categories of vehicles such as public transport or to arrange for queuing to take place in suitable parts of the network.

Demand impacts usually reduce travel time, but reduced travel times and good network performance may increase road capacity. This may cause a shift in demand towards car use. UTC systems may not make a positive contribution to all policy objectives.

Terminology

Urban traffic control (UTC) systems are a specialist form of traffic management which integrate and co-ordinate traffic signal control over a wide area in order to control traffic flows on the road network. Integration and co-ordination between adjacent traffic signals involves designing a plan based on the occurrence and duration of individual signal aspects and the time offsets between them and introducing a system to link the signals together electronically. A traffic responsive signal control system is a means of adjusting the traffic signal settings (cycles, green splits and offsets), which optimise a given objective function, such as minimising travel time or stops, in real-time based upon estimates of traffic conditions.

An idealised time-distance diagram showing signal co-ordination with a fixed time plan
(http://www.roads.dft.gov.uk/roadnetwork/ditm/tal/traffic/07_99/)

UTC systems can provide the basis for an extended control system, generally termed Urban Traffic Management and Control (UTMC) (Routledge et al, 1996). They include incorporating emergency service vehicles and priority for public transport such as bus priorities, and their integration with information systems such as variable message signs, real-time driver information systems and route guidance and parking guidance and information system.

Types of UTC systems

There are many different UTC systems in operation around the world, but they can be distinguished into two basic types based on different control strategies. These are fixed time systems and traffic responsive systems. The following provides an overview of the capabilities of some fixed time and adaptive UTC systems (Wood, 1993; IHT, 1997):

• Fixed time systems
  Many UTC systems are variants of this type of system. The method used to calculate the timings defines the objective that the system will seek to minimise. This is often used to minimise network vehicle delay. The designer has considerable control over his objectives and can optimise different parts of the network in different ways to give different objectives. Although the timings can be biased against the main traffic movements, traffic can be restrained by adjusting the splits at critical junctions. Also, the cycle time can be kept low to reduce total capacity and extra stages can be introduced at junctions to reduce the green time available for the main traffic movements.
  Fixed time systems cannot respond dynamically because they use pre-calculated timing plans. Therefore, the systems do not respond automatically to incidents, such as accidents, which cause loss of capacity in the network. In addition they will not respond to changed demand when a route guidance system is working. Most importantly, they do not respond to changes in traffic patterns over time. Fixed time plans are good at implementing fixed strategies, such as limiting traffic capacity as certain times of day.
• Plan selection systems
  Plan selection systems use fixed time plans, but select which plan to use by information from detectors strategically placed in the network, rather than by time of day. However, this system has not been shown to be better than simple time-of-day implementation of fixed time plans. If necessary, they can be forced to run a specific plan for a special event. Some decisions made by plan selection logic prove to be inappropriate. The extra delay caused by choosing the wrong plan cancels the benefit of changing plan at the right time when the logic makes a good decision. Therefore, plan selection systems almost have the same advantages and disadvantages as fixed time systems.
• Plan generation systems
  Plan generation systems generate their own fixed time plans from detector data and implement them. Compared to fixed time operation, the system is much less under the control of the traffic engineer because it is not as easy to define exactly how the system should operate. In principle the system could respond to unexpected incidents, although in practice it may not be allowed to make a large enough change to the existing plan to respond effectively.
• Local adaptation
  Some systems use a measure of local adaptation at the controllers to modify the action of centrally imposed fixed time plans. The basic operation is that an appropriate fixed time plan is run and the local controllers can omit or terminate early the side road stages depending on the local demand for the stage in the current cycle. Local adaptation then increases the main road green in some cycles, which should lead to better progressions on the main roads. Local adaptation is usually combined with plan selection or plan generation.
• Traffic responsive centralised systems
  Traffic responsive systems are fully dynamic. The system is based on a central computer and communication to individual traffic controllers. The advantages of responsive systems are that they should respond to traffic demand, day-to-day variation, unexpected incidents and trends over time. A responsive system adjusts its control to react to changing inputs from its traffic detectors. In principle, similar techniques can be used to respond to other inputs from real-time driver information systems and route guidance. A centralised system has the advantage that all the relevant information, from the detectors and other sources is available in the same place.
• Traffic responsive systems with distributed processing
  The features and advantages of distributed responsive systems are basically the same as those of centralised responsive systems. A major difference is in the communication systems used. A centralised responsive system has continuous communication between each controller and the central computer. A distributed responsive system has a router module and each controller is connected to the neighbouring controllers. Messages can be passed between any two controllers, routing the message by intermediate controllers where necessary. A distributed responsive system should be able to work with a route guidance system, but the interaction will be more complicated than for a centralised responsive system.

Wood (1993) reviewed existing UTC systems in the world. The following table shows the name and types in major UTC systems around the world.

*Acronym
TRANSYT Traffic Network Study Tool
SCATS Sydney Co-ordinated Adaptive Traffic System
SCOOT Split, Cycle and Offset Optimisation Technique
UTMS Universal Traffic Management System
OPAC Optimisation Policies for Adaptive Control
UTOPIA Urban Traffic Optimisation by Integrated Automation
SPOT System for Priority and Optimisation of Traffic

The programs of UTC systems defined three generations of traffic control (Venglar and Urbanik, 1995). The first generation were the fixed time systems, which consisted mainly of timing plans that were developed from off-line analysis programs and stored in computer memory. TRANSYT was of this generation and has been developed in UK since the 1960s. TRANSYT is used in many countries around the world and is the effective world standard method of calculating fixed time signal plans.

The first-and-a-half generation is something of a hybrid, which automated the timing plan development process and detected when precalculated timing plans should be modified. Plan selection systems, plan generation systems and local adaptation are included in this generation. The best known system of this type is SCATS in Australia . Since 1983 SCATS has been installed around the world and is currently used in over 70 urban centres in 15 countries with a wide user base of systems.

Second generation systems involved the real-time production and implementation of plans through on-line techniques based on surveillance data gathered from vehicle detectors. Timing plans are implemented periodically, of the order of once every five or ten minutes. SCOOT, developed in UK , is of this generation. SCOOT has been introduced is now used in over 170 towns and cities in the UK and overseas.

The third generation implements and evaluates a fully traffic-responsive, online control system. Signal timing parameters must be changed continuously in response to real-time measures of traffic variables. Third generation systems are being developed today with distributed intelligence such as OPAC, PRODYN, UTMS and UTOPIA.

Technology

IHT (1997) reviewed the main technology of UTC systems. Traffic signals in a UTC area are usually controlled by a central computer, which sends electronic instructions by telephone-type cables to each junction signal controller. Signal controllers receive traffic information from vehicle detectors installed near the stop line on side roads. Inductive loops, microwave detectors and video-processing detection systems can be used as presence detectors. Inductive loops are also used to measure traffic volumes, detector-occupancy and speed in traffic responsive systems occasionally. Local co-ordination may also be achieved either by linking controllers by dedicated cable or by cableless links between microprocessor-based controllers.

Transmission of the data from the central computer is by means of in-station transmission units (ITU). These receive the signals from the computer and transmit the information to the various junctions, by means of time-division or by frequency-shift multiplexing, which accommodates several channels on each data-line by the use of data-concentrators at either end of the line.

At each of the junctions controlled by the central system, an out-station transmission unit (OTU) is installed in the controller. This receives the signal from the data-line and interfaces with the local control equipment. The OTUs use transmission protocols which are specified by the manufacture of the in-station transmission system and similar units have to be used in any subsequent expansion of the system.

UTC systems generally include a number of graphic display facilities, which offer a fuller understanding of the traffic situation in the control area. The facilities include diagrams, giving information about queue build-up and dispersal, displays of individual junction operation and time distance diagrams, to assist in analysing traffic flow and journey times.

A roving terminal is a portable terminal which communicates with the UTC systems via a cellular radio link. This provides access to the system from any location, making direct comparisons between the actual traffic and the situation being modelled.

Some intelligent transport systems technologies such as Automatic Incident Detection (AID), CCTV Surveillance are used for the extension of UTC systems.

The flow of information in SCOOT based UTC system

Why introduce urban traffic control systems?

UTC systems can be used to obtain better traffic performance from a road network by reducing delays to vehicles and the number of times they have to stop. UTC systems also can be used to balance capacity in a network, to attract or deter traffic from particular routes or areas, to give priority to specific categories of vehicles such as public transport or to arrange for queuing to take place in suitable parts of the network.

The other potential benefits which can be obtained from the installation of UTC systems include (IHT, 1997):

• improved facilities for pedestrians and cyclists;
  
• allocation of priority to emergency vehicles responding to incidents and reducing vehicle attendance times, using special signal-timing plans to favour key routes from fire and ambulance stations;
  
• implementation of diversion schemes to deal with emergencies or special events and other control strategies such as tidal flow schemes;
  
• improved utilisation of car parks and a reduction in the amount of circulating traffic by providing car park information systems;
  
• improved fault monitoring and maintenance of equipment, leading to a reduction in the delays and potential safety hazards caused by faulty equipment; and
  
• interaction with other network management systems such as a route guidance system.

Demand impacts

UTC systems generally aim to produce the minimum total queue-length on the network or the minimum total vehicle hours for a given amount of travel, but reducing travel times and increasing capacity over a significant area may cause a shift in demand towards car use. However, UTC systems may also have the potential to reduce or limit congestion by analysing the congestion and determining the critical part of the network that causes a particular problem. As most systems also improve travel times for buses to the same degree, or possibly further by giving priority to buses, the overall effect on demand would seem to be neutral.

Short and long run demand responses

It is unlikely that there will be significant change in demand response over time. However, increasing the supply through reduced travel times may induce re-routing within the network and so erode possible benefits in the signalised area in the longer term.

* This is likely to increase shift in the long run if reliability of public transport becomes better than before.

Supply impacts

There will physically be no increase in the supply of road space, but reduced travel times and good network performance may in practice increase road capacity. When UTC systems accompany the introduction of physical restrictions such as bus priorities and light rail systems, the supply impacts will be greater by adjusting the traffic signal setting between car use and public transport.

Financing requirements

UTC systems require some technological equipment such as central computer, signal controllers and vehicle detectors in any type of system. In addition, traffic responsive systems usually use inductive loop detectors, with the expense of installing and maintaining. For example, TRANSYT costs £10,000- £15,000 per junction and £20,000 - £25,000 for SCOOT (Source?). When UTC systems are implemented for specific objectives (such as traffic restraint) on selected roads in the network, the design of customised systems are required, and are usually expensive.

Expected impact on key policy objectives

UTC systems have potential to contribute to a number of key objectives through reduction in congestion, but the scale of contribution is dependent on the specific traffic management objectives.

Expected impact on problems

UTC systems may increase car use, but may also reduce congestion. Hence they have the potential to contribute to the alleviation of a number of key problems..

Expected winners and losers

If reducing delays to vehicles leads to reduction of congestion the benefits will accrue to all road users. However, winners and losers will depend on the traffic management objectives through UTC systems.

Barriers to implementation

SCOOT, UK

Context

The Transport Research Laboratory (TRL) in collaboration with UK traffic systems suppliers developed the SCOOT (Split Cycle Offset Optimisation Technique) urban traffic control system. SCOOT is now co-owned by Peek Traffic Ltd, TRL Ltd and Siemens Traffic Controls Ltd. Early systems were tested in the late 1970’s in Glasgow. The development of SCOOT for general use was carried out in Coventry with the first commercial system being installed in Maidstone in 1980. SCOOT is now used in over 170 towns and cities in the UK and overseas.

SCOOT is a fully adaptive traffic control system which uses data from vehicle detectors and optimises traffic signal settings to reduce vehicle delays and stops. There are a number of basic philosophies which lead to the development of SCOOT. One of these was to provide a fast response to changes in traffic conditions to enable SCOOT to respond to variations in traffic demand on a cycle-by-cycle basis. SCOOT responds rapidly to changes in traffic, but not so rapidly that it is unstable; it avoids large fluctuations in control behaviour as a results of temporary changes in traffic patterns. (SCOOT website)

SCOOT not only reduces delay and congestion but also contains other traffic management facilities. For example, in 1995 a new facility was introduced to integrate active priority to buses [link to bus priority] with the common SCOOT UTC system. The system is designed to allow buses to be detected either by selective vehicle detectors or by an automatic vehicle location (AVL) system. (SCOOT website)

Further information regarding the details (such as the development, how it works, installing, facilities etc.) can also be found in the SCOOT website.

Impacts on demand

SCOOT is in use worldwide and has been shown to give significant benefits over the best fixed time operation. The effectiveness of the SCOOT strategy has been assessed by major trials in five cities (Wood, 1993; SCOOT website). The results from the trials are summarised in the table below.

* Vehicle - actuated

Comparisons of the benefits of SCOOT, against good fixed time plans, showed reductions in delays to vehicles of average 27% at Foleshill Road in Coventry - a radial network in Coventry with long link lengths. In Worcester the use of SCOOT rather than fixed time UTC showed considerable saving which was estimated to be 83,000 vehicle hours or £ 357,000 per annum at 1985 prices. The replacement of isolated signal control in Worcester by SCOOT was also estimated to save 180,000 vehicle hours per annum or £ 750,000 per annum. In Southampton, economic benefit, excluding accident and fire damage savings, amounted to approximately £140,000 per annum at 1984 prices for the Portswood/St. Denys area alone.

Research by Bell (1986) suggests that SCOOT is likely to achieve an extra 3% reduction in delay for every year that a fixed-time plan "ages". Further, the effects of incidents have been excluded from many of the survey results to ensure statistical validity. Since SCOOT is designed to adapt automatically to compensate for ageing and incident effects, it is reasonable to expect that, in many practical situations, SCOOT will achieve savings in delay of 20% or more.

In 1993 a SCOOT demonstration project in Toronto showed an average reduction in journey time of 8% and vehicle delays of 17% over the existing fixed time plans. During weekday evenings and Saturdays, vehicle delays were reduced by 21% and 34%. In unusual conditions following a baseball game, delays were reduced by 61%, demonstrating SCOOT's ability to react to unusual events. (Siemens Automotive, 1995)

In Sao Paulo in 1997 a survey showed that SCOOT reduced vehicle delays by an average of 20% in one area tested and 38% in another over the existing TRANSYT fixed time plans. It was estimated that financial benefits to Sao Paulo as a result of these delay reductions would amount to approximately $1.5 US million per year. (Mazzamatti et al, 1998)

Impacts on supply

Field trials of bus priority using SCOOT survey were carried out in areas of Camden Town and Edgeware Road in London in 1996. The Camden network consisted of 11 nodes and 28 links. The Edgeware Road site was a linear network consisting of 8 nodes and 2 pelican crossings. The bus routes were surveyed for the periods 7:00 - 12:00 and 14:00 - 19:00. The results show that greater benefits can be obtained where there is lower saturation level. (Bretherton et al, 1996)

[Preferably include Toronto results under case study] [*Surely reduced steps/starts reduces accidents? Any evidence of this?]

UTOPIA/SPOT, Italy

Context

UTOPIA (Urban Traffic Optimisation by Integrated Automation) / SPOT (System for Priority and Optimisation of Traffic) is designed and developed by FIAT Research Centre, ITAL TEL and MIZAR Automazione in Turin, Italy. The objective of the system is to improve both private and public transport efficiency. The system has been fully operational since 1985 on a network of about forty signalised junctions in the central area of Turin. The area also contains a tram line and control of the trams is integrated within UTOPIA/SPOT (Wood, 1993). UTOPIA/SPOT is now used in several cities in Italy and also in the Netherlands, USA, Norway, Finland and Denmark.

The system uses a hierarchical-decentralised control strategy, involving intelligent local controllers to communicate with other signal controllers as well as with a central computer. Central to the philosophy of the UTOPIA/SPOT system is the provision of priority to selected public transport vehicles at signalised junctions and improvements in mobility for private vehicles, subject to any delays necessary to accommodate priority vehicles (Wood, 1993). The French PRODYN system and the German MOTION system have some similarities to SPOT, but have not been used outside their counties (Kronborg and Davidsson, 2000).

Impacts on demand

The improvements attributed to UTOPIA in Turin have been calculated a previous traffic responsive control strategy rather than against a fixed time system. Benefits of implementing UTOPIA were shown to give an increase in private traffic speed of 9.5% in 1985 and 15.9% in 1986, following system tuning. In peak times the speed increases were 35%. Public transport vehicles, which were given absolute priority, showed a speed increase of 19.9% in 1985 (Wood, 1993).

SPOT was introduced in Scandinavia in the early 1990's (Kronborg and Davidsson, 2000). In Oslo, Norway, SPOT started to be operated in four intersections with high priority to public transport in 1996. Only traffic parallel with the tram routes was evaluated and had good results (15% reduction in travel time).

Impacts on supply

UTOPIA/SPOT has been explicitly designed with public transport vehicle priority in mind (Wood, 1993). Buses and LRT vehicles are given absolutely priority at junctions, subject to the accuracy in forecasting their arrival time. In Turin LRT are given higher priority than buses because they have more passengers but extra priority can be assigned on a vehicle by vehicle basis if required.

Gaps and weaknesses

Many papers or reports on UTC systems evaluated only the impact on efficiency such as reduction in journey time, delay and stops compared with previous types of system. However, reducing travel times can increase road capacity, and increasing capacity over a significant area may cause a shift in demand towards car use and increase car traffic volume. The potential for the benefits of UTC systems to be eroded by induced traffic needs to be borne in mind. Relatively little information is available on environmental or safety benefits.

Appropriate contexts

UTC systems are typically implemented in dense urban road networks area over which traffic conditions are monitored by loop detectors and controlled by traffic signals. However the system is also suitable for small networks. It is particularly effective where traffic flows are unpredictable e.g. random changes in traffic patterns such as often occur in popular tourist areas.

When junctions are some distance apart (more than about 1 km) isolated junction control using a system such as MOVA may be more appropriate. Other site-specific factors would also influence the decision on method of control. MOVA (Microprocessor Vehicle Actuation) is a modern microprocessor technology for isolated intersections to optimise signal timings.

Adverse side-effects

If congestion is reduced sufficiently to allow increased speed, risk of accidents will increase. There are no specific safety features unique to most UTC systems. However, longer inter-greens for pedestrians will be effective for safety, so that pedestrian facilities can be required. In principle information from pedestrian detectors could be included in the optimisation process for traffic responsive systems.

References

Abbott, P.G., Hartley, S., Hickman, A.J., Layfield, R.E., Mccrae, I.S., Nelson, P.M., Phillips, S.M. and Wilson, J.L (1995) The environmental assessment of traffic management schemes: A literature review. TRL Report 174. Crowthorne. TRL.

Bell, M. C. and Bretherton, R. D. (1986) Ageing of fixed time traffic signal plans. Proceedings of IEE 2nd International Conference of Road Traffic Control.

Bretherton, R. D., Hounsell, N. B. and Radia, B. (1996) Public transport priority in SCOOT. Proceedings of the 3rd World Congress on Intelligent Transport systems.

IHT (Institution of Highway and Transportation) (1997) Transport in the urban environment. Chapter 41 Co-ordinated signal systems.

Kronborg, P. and Davidsson, F. (2000) Improvements for s Scandinavian SPOT urban traffic signal control system. TFK - Transport research institute. Stockholm.

Mazzamatti, M.V., Netto, D.V.V.F., Vilanova, L.M. and Ming, S.H. (1998) Benefits gained by responsive and adaptive systems in São Paulo. IEE Road Transport Information and Control. Conference Publication No 454.

Routledge, I., Kemp, S. and Radia, B. (1996) UTMC: The way forward for urban traffic control. Traffic Engineering and Control 37(11) 618-623.

Siemens Automotive (1995) SCOOT in Toronto. Traffic technology international, Spring'95, 8-30.

Venglar, S. and Urbanik II, T. (1995) Evolving to real-time adaptive traffic signal control. Proceedings of the Second World Congress Intelligent Transport Systems, Yokohama.

Wood, K. (1993) Urban traffic control, systems review. PR41. Crowthorne. TRL.

Links

SCOOT (http://www.scoot-utc.com/)

UTMS (http://www.utms.or.jp/)

UTOPIA (http://www.utopia-net.it/en/)