As countries experience greater economic and commercial activity, with growth in vehicle ownership and rising expectations of personal mobility, a consequence is that – over time – parts of the road network become congested with traffic. Often the congestion is predictable and happens on a regular basis.
One of the principal tasks of Road Network Operations is to address these congestion situations and provide measures that will mitigate its worst effects. (See Operational Activities)
The operational methods that support congestion management include the vital importance of effective inter-agency working and the role of Traffic Control Centres. (See Traffic Control Centres)
Recurring congestion is defined as congestion that occurs on a frequent and regular basis due to traffic demand exceeding roadway capacity. A simple law of (traffic) physics is that when traffic demand approaches the roadway capacity, the quality of service rapidly diminishes, to the point where demand exceeds capacity and traffic flow breaks down completely. The array of processes, tools and practices used to mitigate the resulting congestion is often referred to as congestion management; namely managing the situation in a manner as to avoid or minimise the negative impacts of congestion.
A simple law of (traffic) physics is that when traffic demand approaches the roadway capacity, the quality of service rapidly diminishes, and then when demand exceeds capacity, traffic flow breaks down completely. The array of processes, tools and practices used to mitigate the resulting congestion is often referred to as congestion management – namely managing the situation in a manner as to avoid or minimise the negative impacts of congestion.
A number of commonly used measures are described below. They fall into three categories:
The purpose of this 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:
Signal plans can be activated in various ways:
Traffic signal control should also take into consideration all types of users and in particular pedestrians, two-wheeled vehicles and public transport.
This action requires:
(See Urban traffic Control)
Most large cities in the world are now equipped with traffic signal control systems. ITS is concerned with systems which adapt themselves to actual traffic measurements and situations, either through on-line choice of predetermined control plans or through on-line calculation of tailor-made control plans (and combinations in-between these two).
(See Urban Traffic Management)
The purpose here is to adapt the use of available lanes to 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 therefore covers the following areas of use:
Some examples of specific uses include:
The introduction of such a measure 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. Any control systems must be maintained in fail-safe condition. For example:
Dynamic lane management (DLM) enables the allocation of lanes to be modified on a temporary basis by means of traffic guidance panels, permanent light signals, multiple-faced signs (DMS), LED road markers, and overhead installation of VMS for signalising lane closures and lane directions.
Fundamental applications of this service are: for tidal flow systems, lane allocation at intersections, lane allocation in road tunnels, hard shoulder running. (See Highway Traffic Management)
These systems are put into operation on a daily basis during peak periods where there is recurrent congested with available capacity in the opposite direction (1 lane minimum) .This sometimes means in practice the use of special equipment to move the central concrete safety barrier, although some 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 known as “managed lanes”, that allows more dynamic reversal of lane directions.
As far as ITS is concerned, dynamic reversal of lane direction can be made with systems using gantries. Lane reversal should always be subject to operator’s validation. The implementation of the tidal flow remotely by the operator, can be eased by the use of video cameras (first, close the lane; wait until no car remains in it; then open it in the opposite direction).
(See Highway Traffic)
The purpose of this measure is to increase the capacity of an arterial and improve safety. It is usually applied to recurring gridlock on an expressway or motorway that experiences stop-start traffic flow and sudden stops that cause accidents. The measure works by imposing or recommending a driving speed:
Speed control systems have been more popular in Europe than in the US or Japan and the major benefit relates to traffic smoothing, with improved throughput and a reduced rate of accidents. Displayed speeds (generally mandatory) are aimed at reducing the range of individual speeds in non-congested situations and protecting the end of queues when congestion appears. The benefits of speed control systems include: smoother flowing of traffic, yielding slightly increased capacity, thus resulting in a postponed disruption time, and reduced number of accidents, especially rear-end accidents. These benefits are obtained through an effective reduction of observed speeds but also through an increase in driver attention.
This measure requires:
Traffic monitoring is indispensable: the operation is fully automated but an operator must be able to recover control at any time if an unexpected event occurs on the network (such as an accident or sudden deterioration in weather conditions). This measure essentially applies to urban ring roads, where the high proportion of routine drivers facilitates user compliance with operations and the short length of controlled roadway (typically about 10 kilometres) promotes adherence to speed limits.
Apart from handling recurring periods of gridlock, speed regulation is a tool that can be used to gradually slow vehicles entering an incident or accident area.
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 posted speed limit by a predetermined threshold.
(See Speed management)
Although different from the previous 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 a mobile phone system 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. (See Freight & Delivery Operations)
Driving difficulties for trucks and HGVs during snow for example, may lead to traffic congestion on the whole network. A solution to this situation may be to organise HGVs in convoys. This type of action requires:
Incident management is defined as the implementation of a systematic, planned and coordinated set of responsive actions and resources to prevent accidents in potentially dangerous situations and to handle incidents safely and quickly. It proceeds through a cycle of several phases: from incident detection to restoration of normal traffic conditions, including the use of immediate and advance notice of possible dangers or problems, such as warnings, in order to prevent accidents. Incident warning and management have two main goals:
(See Traffic Incidents)
This measure consists of spreading out traffic flows over time and space during heavy traffic periods such as long weekends or special national events, causing heavy traffic on a specific arterial or in a particular area. The principle is based on dissemination of information:
These operations require:
The documents distributed must always be up to date and occasionally must be in several languages. It is difficult to assess:
(See Planned Events)
Three different approaches are used with this measure, which makes use of Variable Message Signs:
Most systems are manually operated with the operator being aided by computer outputs such as travel times on the competing routes, or simply by other data from the field (such as video images), or by pre-determined strategies and traffic management plans designed off-line for a given number of situations. These systems can not generally handle complex situations such as incidents in a grid motorway network, multiple incidents situations, etc and are often unable to give an updated strategy, once the first one is implemented. They also generally do not include forecasts of the demand and of the impact of the current strategy, so they are far from producing an “optimal” strategy, and they can even produce situations worse than a “do-nothing” strategy (due to risk of creating over-diversion).
Some more advanced systems, such as MOLA (UK) or VISUM-online (Germany), are able to simulate the impact of a set of possible strategies designed off-line in order to help the operator to choose the best one.
One system, OPERA (France), is able to automatically generate guidance strategies based on forecasts and on a real-time expert system, thus adjusting itself to current traffic patterns and their forecasts.
Another “collective route guidance” system, in the sense that it induces changes in the users’ route choice is worth quoting: the adjustment of toll rates on competing inter-urban motorways in France during rush hour periods in order to achieve a better balance of traffic.
With this approach route guidance is provided to the individual user at each choice point by means of Satellite navigation which is adapted by an in-vehicle equipment receiving dynamic traffic data. The optimal route can either be calculated on-board on the basis of received data (generally link travel times) using a one-way area broadcast link or calculated centrally and downloaded to the equipment using a two-way short range link. (See In-vehicle Systems and In-vehicle Systems )
The purpose of this approach is to remove all traffic (or a single category of vehicles) from a road temporarily, in one direction or both. The measure is used following a current, planned or anticipated event that makes it impossible or especially hazardous to travel over the section of road in question. Access to the closed section is prevented via a physical barrier and in most cases, traffic is re-routed. Depending on the circumstances, the closure and detour may be:
In the case of a prepared closure, implementation follows several stages:
The closure of a road is a major operation that mobilises many resources: police forces, temporary signing, communications, control centre or at least a management structure. This measure imposes serious restrictions on the travelling public and services and may entail difficulties for some vehicle categories based on the limitations of the alternate route. The following should be noted:
The diversionary routes must be included in the global strategic management of the network using similar roadways or motorways for long distance traffic or local roads for short distance.
A clear distinction must be made between closure of access and regulated access:
Closing access consists of closing one or more access points to a divided highway that is subject to gridlock or closure following an accident for example, while ensuring that alternative routes allow users to complete their travel. The objective is to keep traffic on the roadway below a set threshold to maintain a smooth flow.
This measure can prove effective during peak weekend traffic or heavy travel on connecting highways or when random events reduce the capacity of a roadway to a level below demand. Implementation involves the following key points:
For planned and predictable operations an advance information campaign on likely closures and proper signing on-site can strongly influence user acceptance of the operation and thus attainment of the objective. This measure can entail some drawbacks:
For reasons of credibility and to avoid shifting traffic from one access point to the next, several consecutive access points may have to be closed simultaneously.
The purpose of this approach is to maintain a smooth flow on a through section or convergence point of a major arterial that is subject to recurring gridlock, by regulating traffic inflow from an access ramp. The objective is to optimise the flow on the major arterial. This type of operation is essentially used on urban highways, but the principle can also be applied in adapted form to intercity highways during periods of heavy traffic.
An information campaign and very accurate technical monitoring are indispensable to user acceptance and credibility of devices. This measure poses certain constraints:
Regulated access is extremely common in the United States and is growing in Europe. There are also a few types of static regulation; on the ring road around Paris, for example, the width of some entrance lanes has been narrowed by small beacons.
Where traffic problems arise on one of two normally concurrent routes (same origin, same destination, similar travel times), it may be effective at some point to direct all traffic to the route with better traffic conditions at that specific time. The principle consists of directing traffic for the destination to the route with better traffic conditions.
Traffic conditions must be monitored on both routes (traffic, construction and local events) and established guidelines must be followed.
Implementation is based on:
The potential for using this measure is very limited since truly concurrent routes are very uncommon. The impact is limited to through traffic, since local users rarely refer to directional signs; the effectiveness of this measure therefore depends on the traffic mix (local/through).
The purpose of this approach is to avoid gridlock of a section or area that is difficult to manage or poses a hazard if a traffic queue develops. Traffic entering the problem section is limited. This measure is generally used in the following cases:
The operation is usually carried out under a traffic management plan (co-ordinated among partners) and in all cases under the operational control of a co-ordinating structure. Traffic is screened at a suitable location (usually a toll booth or intersection, but occasionally in the middle of a roadway equipped with flow control signals to handle the conditions) and adjusted based on traffic conditions in the area to be protected. (See Traffic Management Plans)
Special efforts are made to inform users at the site and extensively upstream. The vehicle holding area must provide adequate capacity, safety and comfort. The tail of the traffic congestion should be marked. Provision must always be made for emergency vehicles.
In some cases, this type of measure may only apply to selected vehicle categories. When faced with major problems, operators may organise several levels of control, implemented in a cascade as a holding area approaches full capacity. Some drawbacks or constraints may appear:
This procedure removes HGVs from a problem area (snow-covered section of road with steep grades or high wind areas, for example) by temporarily holding them in specified areas provided for this purpose. This measure can:
The measure is implemented in the following stages:
This operation is often associated with measures to assemble trucks into convoys. Some problems can arise and include:
Advance, repeated explanatory media messages to associations representing professional drivers are important (annually, at the start of winter, for example).
The purpose of this approach is to promote anticipation of changes in direction and avoid hesitancy, excessively slow speeds and emergency manoeuvres that cause braking and insecurity, by making signs visible, legible, continuous, consistent and understandable by all. Recurring braking and traffic congestion at an intersection or diverging or merging roads cannot always be attributed to inadequate capacity, but may result from inadequate route guidance. Clarification of signs is implemented in several stages:
While the system operator is in fact qualified to verify compliance of signs with rules governing use and installation, the same is not true for understanding the causes of driver behaviour; it is important that this part of the analysis rely on people with no knowledge of the local area or traffic-related occupations.
The purpose of this approach is to improve the organisation of traffic by permanently or temporarily changing and enforcing traffic rules. This measure handles braking or traffic jams attributable to poor organisation of traffic, such as intrusive parking, disruptive movements, inconsistent traffic mix, weekly events. This measure is implemented through the following actions:
In some cases, compliance with (and effectiveness of) regulatory measures can only be achieved if accompanied by a minimum level of infrastructure changes (such as provision of parking, physical closures, new access routes or left-turn lanes).
Many traffic jams and slow moving traffic are linked to a temporary deficit in capacity (intersection gridlocked with returning traffic at the end of a weekend, for example). The purpose of this measure is to restore capacity to the same level as on the rest of the route by making temporary, low-cost improvements to geometry to cope with peak traffic flows or solve chronic dysfunctions. The sites affected are usually intersections and corrective measures include:
This operation includes the following steps:
Traffic flow must be monitored over the full length of a route (one bottleneck may conceal another) and safety must be maintained (avoid promoting higher speeds).
Beyond the limited improvements noted here, more extensive changes to the geometry of some critical intersections can sometimes be considered (layout of roundabouts, signal controlled junctions, overpass installation); these extend beyond the actual scope of operations and the discussion here.