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:

• automatic fault reporting of traffic detectors, controllers, communications links and signal lamps that mean that maintenance can be carried out promptly and with less signal down-time
• signal timing options that allow traffic engineers to develop queue management strategies to regulate traffic volumes through critical intersections and links – for example through traffic “gating” facilities that restrict numbers entering a sensitive areas
• signal timings that can be adapted in real-time to give active priority to public transport over other traffic
• signal plans and control strategies that are augmented by electronic VMS panels to warn of congestion and road closures or provide variable direction signs
• camera enforcement systems that deter red-light running at junctions with high accident rates
• improved detection methods that enable signal timings and control strategies to be adapted to meet the needs of vulnerable road users, such as cyclists and pedestrians
• air pollution monitoring and control strategies that relate to environmental targets

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.

Shared lanes

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 lane of a busy five-lane artery alternately for moving traffic, parking and delivery of materials during different times of the day.

Permitted use is shown by VMS at the start of the restricted lane and LED Changeable Message Signs (CMS) at intervals along the way to indicate when loading and unloading is active.

Contraflow Lanes

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).

Turn Restrictions

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 -hand side of the road this mean banning a turn at the intersection. When traffic drives on the , the restrictions apply to -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:

• on a major arterial road which is subject to traffic variations (peaks linked to seasonal or especially weekend travel)
• on a road temporarily used as an alternate route
• in an area where the traffic management strategy changes over time

The procedure consists of:

• analysing traffic flows to be addressed
• gathering specific traffic data (directional counts and turning movements, in particular)
• drawing up plans for required signals
• implementing them in the signal controller, which may require the addition of remote detection or control equipment

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:

• manually
• on a programmed schedule
• by an automated device that may detect local traffic conditions
• by a central system which takes into account not only the local conditions but also the traffic conditions on a zone or even on the whole network

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:

• monitoring and correctly maintaining the traffic monitoring and control equipment, especially traffic sensors
• periodically adjusting controls to traffic and/or to the traffic management policy
• occasionally reconciling purely local issues and those related to through traffic

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.

Railway and other Crossings

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:

• adjust the overall length of the traffic signal cycle times at each junction
• vary the amount of green time given to different signal phases at any one junction (the signal timing ‘splits’)
• alter the offset times between the green phase at successive junctions in order to minimise overall delays

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:

• extend the green time to allow the bus to pass if the signals are already at green, or
• change the signals away from the opposing traffic to favour the bus after the minimum green time has passed.

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.

Regional traffic control and management systems most commonly communicate with drivers via VMS. These usually comprise two or three rows of characters to form a message. Often, these are augmented with lane control signs and in some countries, with pictograms.

At the tactical level VMS provides the means of informing drivers of the need to be aware of approaching conditions. VMS also has a key role at the strategic level as part of a regional control tool. VMSs are limited to the display of short messages – and even with pictogram enhancement cannot convey the amount of information that is possible using radio, social media and in-vehicle driver information systems. 

Further Information

EasyWay ITS Deployment Guideline VMS-DG01 Principles of VMS Design available for download at: http://dg.easyway-its.eu/DGs2012

Ramp metering is a form of tactical management widely used in North America. It is used to a lesser extent in Europe and the rest of the world due to the practical problems of comparatively short motorway on-ramps with limited queuing capacity. Detection is needed on the freeway or motorway both upstream and downstream of the merge point. Merging traffic is held on the ramp to be released at a rate typically controlled by the volume of through traffic on the main carriageway.

Ramp meters reduce the likelihood of flow breakdown by preventing traffic levels on the main carriageway reaching unstable levels. Once flows become unstable and stop-start conditions set in, there is a significant loss of capacity and queues develop on the motorway due to the volume of traffic. The aim of ramp metering is to prevent or delay the onset of flow breakdown, to maximise throughput. One goal of metering is to encourage diversion of short-distance trips off the motorway. This is achieved by:

• regulating the flow of additional traffic onto the motorway that, if unchecked, would trigger flow breakdown and lead to critical shockwaves
• monitoring and managing the merging traffic to achieve an even distribution and avoid large platoons of vehicles entering the motorway at once, which increases the risk of flow breakdown

Ramp metering is implemented by installing traffic signals on the on-ramps to regulate the flow of traffic joining the motorway during peak or congested periods. The signals control the discharge of vehicles from the on-ramp, holding back the merging traffic and breaking up platoons of vehicles as required. It is important to have sufficient capacity for queuing vehicles so that the adjacent motorways and access roads are not disrupted by queuing traffic waiting to merge.

Timing of on-ramp traffic signals is generally dependent on the prevailing traffic conditions on both the main carriageway and the on-ramp. Access can be regulated in isolation (each ramp regulated independently) or centralised, with the flows admitted at consecutive ramps being computed by a comprehensive traffic management system. The system requires:

• control signals (which can be two-light or three-light) located towards the end of the access ramp before the main carriageway, activated during peak periods
• signs informing users of activation – or impending activation – of the ramp meter
• a traffic monitoring system (automated with several sensors) to determine current traffic flows on the main carriageway and traffic demand on the access ramp
• holding capacity on the access ramp to ensure that vehicle queues do not disrupt traffic on the surrounding arterial road network

In practice, ramp metering systems are located upstream of recurrent bottleneck congestion points – and have a safety role in addition to relieving main-line congestion. Ramp meters may be deployed individually or in combination as a dynamic system.

Most of the ramp metering systems that have been deployed are based on demand/capacity or occupancy rate algorithms and are not coordinated on an area-wide basis. More sophisticated systems account for conditions over a long section of the motorway, not just at the individual interchanges. One co-ordinated solution (ALINEA in France, for the Paris ring road and Île-de-France motorways) consists of imposing target downstream occupancy rates on the motorway for each of the local metering systems. The set of occupancy rates are optimised at the area-wide level. 

Ramp metering systems have proved to be a very effective way of maintaining good levels of service for traffic on the motorway, at the expense of those vehicles waiting to enter. This can be regarded as paying a small “time toll” in order to enjoy the benefits of a relatively free-flowing motorway. By spacing out the merging traffic, there is less queuing in the acceleration lane and smoother merges, which permits more stable flow and increases overall throughput. Motorway mainline speeds may be increased by as much as 50%. There is a disbenefit to traffic queuing on the ramp but the delay incurred at the signal is offset by the benefits to mainline traffic, both upstream and downstream where the merging traffic is included. More advanced ramp metering algorithms are even more effective.

Ramp metering is not deployed to directly deter drivers making short trips but can have the added benefit that it may discourage drivers who do make short trips – from using the motorway network when suitable alternatives exist.

Further Information

EasyWay ITS Deployment Guideline TMS-DG03 Ramp Metering Available for download at: http://dg.easyway-its.eu/DGs2012

The purpose here is to adapt the use of available running lanes to traffic circumstances. This most often involves handling recurring gridlock on a section of road linked to insufficient capacity of the section, or gridlock when one or more lanes are unavailable. This measure covers the following areas of use:

• sections with a restricted cross-section that is difficult to widen (overpasses, underpasses, tunnels, etc)
• convergent or divergent sections that are usually adequate but are poorly adapted to some traffic configurations
• high-risk areas (tunnels, sections subject to rock-slides or high winds);
• construction sites

Examples of specific uses include:

• reverse direction assignment of a median lane of a two-way road
• use of an emergency stopping strip (hard shoulder) as an additional lane in rush hours
• contra-assignment of a lane on a divided highway (lane used in rush hour or reserved for public transport)
• fast and intermediate lanes banned to heavy goods vehicles (HGV)
• reserved lanes for public transport, sometimes linked to “Park and Ride” services or Bus Rapid Transit
• High Occupancy Vehicle (HOV) lanes

The introduction of lane control requires resources such as automated controls to verify the consistency of instructions provided by various lane assignment signals, a modular barrier, or temporary marking (cones or beacons).

Regardless of the method used, the operation may be cumbersome and complex. All control systems must be maintained in fail-safe condition. For example:

• the various operating configurations must be compatible with existing mandatory signs and regulations – this often requires replacement of some static signs (especially instructions) with variable signs
• in a reversible lane on a two-way road or in a contraflow lane on a divided highway the risk of head-on collision between opposing flows leads to the installation of a vertical separation devices (beacons or other separators)
• in addition, automatic incident detection methods are very often needed in order to rapidly react to any abnormal situation
• automatic enforcement systems may also be introduced in order to reinforce efficiency and safety for lane control systems

Managed lanes (“HOT” lanes)

Managed lanes are either newly-built lanes or existing High-Occupancy Vehicle (HOV) lanes that are converted to High-Occupancy Toll (HOT) lanes. These operate as toll lanes using Electronic Toll Collection (ETC) or Open Road Tolling (ORT). The toll is applied to Single-Occupant Vehicles (SOVs) and in some cases low-occupant vehicles. Carpools (car-sharing by two or more occupants) generally can use the HOT lanes without charge to encourage greater use of HOVs. Single-occupancy vehicles are charged a variable toll that is dependent on the time of day, level of congestion on general-use lanes and the occupancy of the HOT lanes. Shifting demand from the general-use lanes to the HOT lanes makes better use of the available capacity in the HOT lanes while improving flow for all.

Work Zones

Narrow lanes, hard shoulder running and contra-flow systems often operate during construction, maintenance, widening and reconstruction of motorways. They have become commonplace on the motorway network and often create serious bottlenecks which require tactical measures to warn of delays ahead. Mobile generator-powered VMSs are often used to give warning of disruption and display average journey times. Speed controls, often with camera enforcement, are used to minimise accident risk and smooth flows.

Heavy Goods Vehicle (HGV) Overtaking ban

An HGV Overtaking ban is a means to channel trucks and HGVs onto a single lane (the slow lane). Implementation of a HGV overtaking ban is one of the measures allowing traffic managers and road operators to improve smooth running of traffic during peak periods. This traffic control measure can improve the co-existence of heavy goods vehicles, light vans and private cars on networks with high levels of traffic.

Its objectives are to:

• monitor and manage the HGV traffic flow on the highway network
• improve journey times for light vehicles
• improve safety by reducing vehicle queues caused by slow lorries overtaking
• ensure a better acceptance of heavy goods vehicles by the other road users.

Further Information

EasyWay ITS Deployment Guideline TMS-DG06 HGV Overtaking Ban available for download at: http://dg.easyway-its.eu/DGs2012

Dynamic lane management enables the flexible allocation of traffic lanes, which can be modified by means of Variable Message Signs, traffic guidance panels, permanent light signals, multiple-faced signs, LED road markers, and closing and directing installations. Fundamental applications of this service are: tidal flow systems, lane allocation at intersections, lane allocation at tunnels.

Dynamic reversal of lane direction can be made with systems using gantries but should always be subject to the operator’s validation. The implementation of the tidal flow remotely by the operator, can be eased by the use of video cameras (close the lane, wait until no car remains in it, then open it in the opposite direction).

Reversible lanes

Having reversible, tidal flow or contraflow lanes on a motorway – or having an entirely separate, reversible roadway – permits otherwise unused capacity in the off-peak direction to be used in the peak direction of flow. ITS can facilitate its operation through use of VMS, lane control signals, remote-controlled gates, CCTV and sensors. Reversible roadways in a number of US cities and in Barcelona in Spain have proved very effective. A motorway “tidal flow” system leading in to Birmingham in the UK has been in operation for over 30 years.

Movable median barriers

To add capacity during peak periods, moveable barriers can be deployed. Functionally, this is similar to contraflow, but instead of shifting traffic to the other side of a median, the median itself is moved. This effectively adds a lane to the peak direction. Lane control signals and Portable Dynamic Message Signs (PDMSs) can help with this technique, although the added lanes are usually separated by normal pavement lane lines and the movable barrier.

Further Information

EasyWay ITS Deployment Guideline TMS-DG01 Dynamic Lane Management available for download at: http://dg.easyway-its.eu/DGs2012

Hard-shoulder running enables temporary use of a motorway hard shoulder with the aim to increase road capacity when necessary. The name comes from motorway shoulder lanes used in emergencies - which originally had a relatively weak pavement but are upgraded and “hardened” to take running traffic. The goal of hard-shoulder running is to increase traffic flow to minimise or prevent heavy congestion and reduce the probability of congestion-related incidents.

Hard-shoulder running is similar to the creation of an extra lane, but have specific safety issues when vehicles stop when a breakdown occurs. It adds capacity whilst safety is maintained by ITS devices such as CCTV with image processing to detect a stationary vehicle, lane control signals and VMS. Safe refuge areas are normally provided for vehicles needing to stop when the hard shoulder is open to traffic.

Hard Shoulder Running can be at fixed times, or triggered automatically by traffic demand, or initiated by manual request from the control room operator. The measure can be applied to bottlenecks, locations with poor safety records (black spots) and recurring lack of capacity during peak periods. Some authorities allow buses and, in some cases, general (mixed) traffic to use the shoulder lane during peak periods.

The illustrations below shows the hard-running shoulder on I-66 in Fairfax County, Virginia, USA – and a close-up of the sign and lane control signal.

Hard-Running Shoulder on I-66 USA

Hard-Running Shoulder on I-66

Further Information

EasyWay ITS Deployment Guideline TMS-DG04 Hard Shoulder Running available for download at: http://dg.easyway-its.eu/DGs2012

Although different from other highway control methods, the control of commercial vehicles, especially freight vehicles, can be an important part of traffic control. A freight control system will typically use GPS and mobile phones to manage the exact location of a vehicle and its freight at any given time. By extending this system, it can help to control the traffic by routeing the freight to a less congested route.

Driving difficulties for trucks and HGVs during snow for example, may lead to traffic congestion on the whole network. A possible solution may be to organise HGVs in convoys. This type of action requires:

• preparation work to select the appropriate safe parking zones for trucks and HGVs
• close co-operation with traffic police to stop HGVs
• organising the convoys and escorting them
• a good communications plan to provide driver information

Further Information

EasyWay ITS Deployment Guideline TMS-DG06 HGV Overtaking Ban available for download at: http://dg.easyway-its.eu/DGs2012

The common objective of speed controls is to encourage drivers to travel at a safe speed or to improve traffic flow. They are a means to help drivers to travel at an appropriate, consistent speed taking account of the prevailing traffic or weather conditions. Persuading drivers to adopt more realistic speeds can have a calming effect and reduce erratic lane changing. The smoother traffic flow permits more throughput. In some cases these systems are also used to mitigate environmental effects, such as pollution or noise.

The measures are usually applied to sections of motorway that experience recurring congestion with gridlock and stop-start traffic conditions, including sudden stops that can be a source of accidents. It works:

• in heavy but smooth traffic flows, by imposing or recommending a driving speed equal to the average speed of traffic
• during periods of heavy congestion, by imposing progressively slower speeds to gradually slow vehicles on the approach to a traffic jam area

Apart from handling recurring periods of congestion, speed regulation is a tool that can be used in all traffic conditions to gradually slow vehicles approaching a traffic incident or accident area.

Speed control systems are more common in Europe than in the USA or Japan. The major benefits relate to traffic smoothing, with improved throughput and a reduced rate of accidents. Displayed speeds are generally mandatory, rather than advisory - and enforced by speed cameras. They are aimed at reducing the range of individual speeds in non-congested situations and protecting the end of queues when congestion appears. In some systems the variable speed limit display is coupled with an automated enforcement system involving video cameras recording licence plate numbers, which issues citations to motorists who exceed the speed limit by a predetermined threshold. (See Enforcement)

The major benefits of speed control are improved safety and better journey times. Smoother traffic flow yields a slight increase in capacity, and a reduction in the number of accidents, especially rear-end accidents. Capacity effects are small and unable to solve bottleneck congestion – but in sections where demand approaches capacity, speed control is likely to delay the onset of stop-start conditions. In some cases, depending on highway conditions and capacity limitations, traffic flow breakdown may be prevented. These benefits are obtained by reducing the speed differential between vehicles and engaging driver attention.

This measure requires:

• consistent and sufficiently dense traffic monitoring and data gathering (flow rates, vehicle speeds and lane occupancy rates)
• a high-performance algorithm to detect the onset of unstable traffic flows
• variable message signs at intervals allowing a continuous visibility of the signs for drivers
• consistent information initiatives (local information campaign to explain the operation, periodic reminders of instructions)

Traffic monitoring is indispensable: the operation can be fully automated but an operator must be able to recover control at any time if an unexpected event occurs – such as an accident or sudden deterioration in weather conditions. This measure is particularly suited to urban and suburban motorways, where the high proportion of routine drivers facilitates user compliance. The short length of controlled roadway (typically about 10 kilometres) also promotes adherence to speed limits.

Further Information

EasyWay ITS Deployment Guideline TIS-DG04 Speed Limit Information available for download at: http://dg.easyway-its.eu/DGs2012

US Federal Highways Administration Engineering Speed Limits available for download at: http://safety.fhwa.dot.gov/speedmgt/eng_spd_lmts/

Speed limits that are legally enforceable may require the use of special VMS that closely resembles the official speed-limit sign mounted on overhead gantries. They can be supported by speed enforcement systems that use camera images to identify speeding vehicles and drivers. For some legal jurisdictions the approved VMS may need to be authorised in legislation in order to be enforceable.

Speed control is often associated with lane control because both measures generally use the same display equipment (gantries and signs). It can also be used for incident management or traffic control through work zones.

A similar effect may be achieved through variable speed advisory signs, which do not have to be legislatively enabled or enforced, although such signs may have lower levels of compliance. Motorists may claim confusion over the mixture of regulatory (fixed) and advisory (VMS) speeds.

The success of VSL requires that drivers understand, and comply with, the reasons behind changing the speed limit and the associated benefits. In most cases, the displayed speed limit should match the conditions that drivers encounter. There will be some cases when circumstances call for a reduced speed limit for which the reason is not obvious – for example environmental reasons, or problems downstream such as incidents or work zones. A study in the UK showed that when a reason is displayed on VMS alongside the speed restriction, there was a 20% increase in driver compliance with the restriction.

Further Information

EasyWay ITS Deployment Guideline TMS-DG02 Variable Speed Limits available for download at: http://dg.easyway-its.eu/DGs2012

US Federal Highways Administration Variable Speed Limits available for download at: http://safety.fhwa.dot.gov/speedmgt/vslimits/

Controlled Motorways involve the application of several Automated Traffic Management techniques in combination – such as mandatory Variable Speed Limits (VSL), measures to reduce the frequency of lane switching through messages such as “Stay in lane”, speed controls and other traffic calming measures. In some systems (for example, in UK on the motorway around London and Birmingham), the variable speed limit display is coupled with an automated enforcement system (involving video cameras recording licence plate numbers), which issues citations to motorists exceeding the speed limit by a predetermined threshold.

Further Information

Dowling R.G and Elias A. Active Traffic Management for Arterials - A Synthesis of Highway Practice NCHRP Synthesis 447 Transportation Research Board , Washington D.C. USA, 2013 download at http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_syn_447.pdf

Federal Highways Administration Active Traffic Management available for download at: http://ops.fhwa.dot.gov/atdm/approaches/atm.htm

EasyWay ITS Deployment Guideline TMS-DG04 Hard Shoulder Running available for download at: http://dg.easyway-its.eu/DGs2012

UK Government gateway to information on Managed Motorways / Smart Motorways available for download at: https://www.gov.uk/government/collections/smart-motorways

One of the most frequent uses of automatic enforcement is the enforcement of speed limits using speed cameras. This requires a specialised speed camera that will measure the speed of each vehicle. In many cases, the system simply displays the speed to bring it to the driver’s attention, which is often enough to encourage them to reduce speed. In other situations, the system will take a picture of the offending vehicle for follow-up action. This would usually include capturing the number-plate of the car – but in some cases, it is necessary to include the picture of the driver, since in many countries, the vehicle owner may not be liable for actions by someone else using the vehicle.

The speed enforcement system can be permanent or a mobile unit. The information acquired by it will be used by the enforcement agency to apply penalties or (if necessary) prosecute the offender. Point-to-point journey times can be monitored by installing two cameras that are linked. Average speed monitoring is now commonplace, especially to regulate traffic speeds over extended sections, such as work zones. (See Speed Management)

Red light evasion ("red light running") at traffic signals can lead to serious accidents and it is extremely important that the traffic lights are observed. If non-compliance is high police presence or automated camera enforcement are two options. Automatic systems use a speed sensor (inductive loop) embedded in the road and a camera installed under the traffic signals. The red light and the vehicle sensor together will activate the camera and a picture taken of the offending vehicle. More advanced systems use the camera both to detect violations and to photograph the offence.

Where a railway (railroad) crosses a road or highway at a level crossing (“at-grade" crossing) there is always a potential safety issue. The level of protection varies considerably between different countries and regions. Where they are protected by gates, flashing lights or barriers – there is often in addition enforcement systems aimed at improving safety. For example, rather like red light traffic signal evasion, if a road vehicle enters a rail intersection during a warning period, it will be photographed.

ITS technologies are being explored to develop systems which can detect potential road/rail vehicle conflicts and help prevent collisions. For example, an Australian team is trialling an in-vehicle collision alert system which uses GPS and Dedicated Short Range Communications (DSRC) to warn vehicles if a train is approaching a crossing.

Heavy vehicles (trucks in particular) are often driven continuously by a single driver, accidents often result from fatigue. In many countries, there are regulations that specify the maximum amount of continuous driving time that is permitted. In order to enforce this, a black-box or electronic tachograph that detects the axle movement and possibly the location of the vehicle along time is often used. (See Driver Safety)

Since the road can only take a certain amount of weight per axle, it is important to ensure that all vehicles are not overloaded and are within legal limits. Otherwise, there will be significant deterioration of the road structure. Weigh-in-Motion (WiM) systems use a sensor on the road to measure each vehicle’s axle weight. When an overloaded vehicle passes, it will be notified to the enforcement agency which will take appropriate measures. (See Weight Screening)

Since most of violations may be contested in court, the systems must be adapted to meet the legal requirements of each country. In some cases, only the vehicle number-plate (licence plate) is necessary to catch the offending vehicle, while in others, the actual driver must be identified. For these systems to be effective and have the desired deterrent effect, they need to collect information that can stand up in court. Equipment must be type approved to certify that it operates to the required standard of accuracy and reliability. Type approval is important to avoid a legal challenge based on inadequate equipment and procedures. (See Equipment Certification)

Privacy Issues

Privacy is another important issue. It is extremely important to keep records confidential. In the case of identifying and notifying offenders, it may be necessary to capture photographs of offenders, which may lead to privacy concerns. (See  Privacy)