A number of different control methods are applied on regional networks of motorways, freeways, expressways and other arterial roads. Their aim is to enhance or increase capacity and/or stabilise the traffic flow and prevent the onset of stop/start conditions. Flow stability also has a beneficial effect on traffic throughput. The methods include:
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.
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:
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:
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.
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:
Examples of specific uses include:
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:
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.
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.
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:
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).
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.
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.
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.
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:
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:
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:
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.
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.
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.
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
UK Government gateway to information on Managed Motorways / Smart Motorways available for download at: https://www.gov.uk/government/collections/smart-motorways