Over the years, a wide variety of traffic management systems have been developed for urban traffic control. Some of the more common methods are shown in the display box below. Among them, computerised traffic signal control, also known as Urban Traffic Control (UTC), has become the norm for large towns and cities. In dense urban networks there are clear benefits from using computers to harmonise traffic control to balance demands and flow. Other methods involve the planned management of roadspace through lane assignment, parking controls, turning bans, one-way street systems and tidal flow schemes. The needs of pedestrians, cyclists, the elderly require special attention. (See Safety of Vulnerable Road Users)
The design and management of urban road networks is a vast subject. ITS-related measures have an important part to play.
Computerised UTC systems allow signal plan changing in response to varying traffic conditions. Dynamic control systems have brought substantial benefits, mainly through improvements in mean speeds and reductions in travel time in the range 10% to 20%. They include SCOOT (UK), SCATS (Australia), MOTION (Germany), PRODYN (France), UTOPIA (Italy), and STREAM (Japan). All too often these benefits are quickly eroded by traffic growth with the result that the public may be unaware of the scale of benefit until there is a serious system failure resulting in widespread congestion. (See Urban Traffic Control)
Urban Traffic Control systems have been getting “smarter” by adopting ITS in various ways – for example traffic delay and congestion monitoring, Automatic Incident Detection (AID), knowledge-based control systems and dynamic origin-destination estimation. Improved traffic detection and network monitoring with CCTV, supported in some cases by real-time monitoring of link and network journey times using vehicle tracking and reports from probe vehicles (floating cars). (See Probe Vehicle Monitoring)
A number of other advanced features are also available:
Traffic control strategies are no longer solely about maximising vehicle throughput. They can be designed to achieve deliberate traffic restraint – for example through very high levels of bus priority at the expense of other traffic or by introducing queue management policies and deliberate area access control. These developments, give traffic engineers and network controllers the means to implement a very adaptable form of urban traffic management – that respond to transport policies and management priorities and their acceptability to the public and local politicians.
This involves the use of a lane for different functions at different times of the day- for moving traffic or loading, unloading and parking. For example, Barcelona uses the left lane of a busy five-lane artery alternately for moving traffic, parking and delivery of materials during different times of the day.
Contraflow lanes on urban streets are generally restricted to bicycles, buses and taxis. On high speed arterial roads a contraflow lane may be reserved only for buses or Bus Rapid Transit (BRT).
This strategy applies to traffic that is turning across the opposing traffic stream at a signalised intersection. In the case of countries that drive on the right-hand side of the road this mean banning a left turn at the intersection. When traffic drives on the left, the restrictions apply to right-turning traffic. The measure involves taking turning-traffic through the intersection and diverting it in a loop so it can approach the intersection from a different direction. In this way cross-movement is eliminated. Turn restrictions are sometimes supported with VMS if they apply only at certain times of the day.
The restriction of specific movements may bring a shift in traffic patterns and changes to individual movements that are a source annoyance to local residents or businesses. While inconvenient for some – because U-turns, rerouting or other actions are needed – the overall operation of the intersection is greatly enhanced. The inconvenience may be lessened by properly selecting places for U-turns along the corridor or by using a “ground loop” or “jug handle” loop - although this requires turning vehicles to transit the junction twice.
Traffic signal optimisation has been applied to urban networks for many years. The practice of coordinating traffic signals at successive intersections to accommodate the progression of vehicle platoons in a “green wave” is a highly cost-effective method of increasing traffic throughput – reducing delay, stops and fuel consumption. But many traffic control systems run sub-optimally since in most cities signal timings need to be regularly checked to ensure that specific junction timings are optimised to accommodate changing demand patterns.
The purpose of the measure is to adapt the operation of traffic signals to match traffic flows or to impose a specific regulating policy such as bus priority. This may be used at an intersection located:
The procedure consists of:
The use of microscopic simulation models allows quick testing for various strategies at the same time as visualisation of their impact – while verifying quantitatively that various criteria are satisfied. These may include total vehicle time spent on the network or delay time for different categories of users.
Signal plans can be activated in various ways:
Traffic signal control in urban areas should take into consideration the needs of all road users, including pedestrians, two-wheeled vehicles and public transport. This requires:
Adaptive Traffic Signal Control can eliminate the retiming issue and can often achieve greater efficiency, since the systems adjust the traffic signal phase timings cycle by cycle in near real-time – at least for some of the day. Traditional control strategies are still needed for times when the adaptive system fails or becomes untenable. Fixed plans can be useful for traffic gyratories where unexpected demand patterns can cause the gyratory to “lock” in a way that requires a “clearance plan” to return the junction to free flow.
Often when trains, swing-bridges or other modes have priority at traffic signals and block traffic, signal timings at nearby intersections continue as if the blockage had not occurred. Signal timings should be adjusted to accommodate re-routing and VMS be deployed to warn drivers of the delays. Adaptive Traffic Signal Control strategies may be able to react to this situation automatically.
Adaptive Traffic Signal Control is concerned with computerised systems that adapt themselves to actual traffic measurements and situations. They may do so either through the on-line choice of predetermined control plans or through on-line calculation of tailor-made control plans in real-time. Combinations of the two are possible. Adaptive systems measure current traffic conditions and dynamically adjust how much time is allocated to the different traffic streams according to the measured traffic volumes and queue lengths. They:
In saturated conditions, shorter cycle lengths will generally achieve more traffic throughput, because the signals are processing queues at the maximum service rate (called the saturation flow rate) for all movements.
Since adaptive control systems are to some extent “self-optimising,” there is less need to coordinate signal timings in the traditional sense. This is because the signal control algorithms adapt automatically to changes in demand (for example traffic diverted from a motorway incident), eliminating the need for explicit proactive adjustments in the signal timing plans. Developments in recent years allow the use of “gating” strategies that emphasise queue management by controlling the total volume of traffic entering a heavily congested area. These strategies depend on expert systems and dynamic modelling.
As the traffic flow regime changes, the operational objectives and priorities may change as well. For example, as traffic demand increases the level of service drops until saturation flows are reached and congestion sets in. During this transition, the control objective may change from free flow, to maximising vehicular throughput and finally to queue management.
For urban arterial networks, “gating” strategies can be applied to store queues of vehicles where they do the least damage. These strategies provide a way for determining where queuing traffic should be held to cause least disruption, so that traffic flows freely within the gated area. It is also necessary to minimise the impact that queuing traffic may have through “blocking back” which affects upstream intersections.
More sophisticated urban traffic control systems will take account of traffic parameters such as queue lengths at junctions, point-to-point travel times and traffic delays. These can be measured using ITS-based techniques such as automatic video queue length measurements, automatic video license plate readings and probe vehicle data.
It is important to understand that even fully adaptive systems are generally not completely autonomous. Competent control room staff with experience of traffic management are still needed to deal with complex situations whenever they arise. Sometimes better progression can be achieved using traditional offline optimisation routines than the automated systems offer. Furthermore, no traffic signal system is able to deal successfully with oversaturated traffic conditions.
As traffic control efficiency reaches its practical limit, further benefits in urban travel management will come mostly from the development of demand management policies inducing modal and time shifts (such as controlled access to urban areas, teleworking and car sharing). (See Demand Management)
Modern UTC systems generally include priority management of public transport and emergency vehicles – and increasingly, traveller information systems such as park-and-ride directions, parking occupancy, arrival time of the next bus. (See Information Dissemination)
This concept of traffic signal pre-emption uses sensors and/or transponders to detect public transport vehicles (trams or buses) approaching an intersection and special control software to either:
This is also a demand management strategy – to encourage the use of public transport – and has been shown not to interfere with other traffic unreasonably, so improving overall throughput. (See Transport Demand Management)
The centre lanes on urban arterial roads are sometimes used for moving traffic in one direction in the morning and the opposite direction in the afternoon. For example Barcelona in Spain, uses reversible lanes, referred to as “tidal flow,” on three of seven lanes of a major artery – which are controlled by VMSs on overhead gantries. Birmingham (UK) has a similar arrangement on the Aston Expressway. In other cities the centre lanes have been reserved for express public transport during peak periods. Cross-traffic turns are usually prohibited when this use is active but permitted off-peak – again controlled by overhead lane control signals.
These systems are put into operation on a daily basis during peak periods where there is recurrent congestion with available capacity in the opposite direction (one lane minimum) . Sometimes this involves the use of special equipment to move the central safety barrier. Many tidal flow schemes simply use signing systems such as variable lane assignment signs placed on gantries. For practical reasons, the lane direction changing is generally made on a fixed-time basis (each day at predetermined hours), although several cities in the United States have implemented a concept that allows more dynamic reversal of lane directions.