ITS-Service Strategy

General Service Description

Variable speed limit (VSL) system use Variable Message Signs (VMS) to display speed limits (maximum, minimum or advisory) for road users on specific road section. Speed limits adapted to the particular weather, road and/or traffic conditions. Background system use gathered information and make decisions according the pre-defined rules and control signs on the road using telecommunication connections.

On motorways, VSL is mostly used to harmonise traffic flow and thus increase throughput and safety. Environmental reasons could also be a main goal. Weather controlled VSL has the aim to help drivers travelling at a safe speed according to the prevailing weather and road surface conditions. Environmentally controlled VSL systems operate in a similar way as weather controlled systems, but with different detectors and control models. VSL can also be used at intersections to improve safety when conflicting traffic occur. The speed reduction has a safety effect in itself, but drivers are also alerted by the system, and are therefore more observant when driving through the intersection.

The basis for most variable speed limit systems is the detection of the current traffic conditions as well as the weather and road conditions through suitable sensors. A distinction has to be made between the detection systems that are embedded in the road surface and as the so-called “contactless” systems (i.e. microwave). The collected data are normally combined and analysed in the local control stations or the sub-centres.

Some advanced motorway control systems make predictions of the future traffic flow. Many systems also have prioritisation as well as horizontal and vertical comparison of display sites taking place to avoid conflicting, illegal or traffic-hazardous display combinations. Illegal display combinations can in addition be locked-up in the signs on site. This enables a reasonable and continuous display consecution for the road user.

In some cases, VSL is supported by Speed Enforcement (SE), which mostly uses cameras to identify speeding vehicles and/or drivers.

Speed enforcement (SE) covers violations of speed limits either on a spot or over a defined section of the road, also called section control. Depending on the strategy, mobile and/or stationary speed enforcement is used.

VSL solutions have been implemented and tested in more or less all European countries. The implementations vary from small tests to broader large scale implementations. The purpose of VSL use is different from case to case. The general overall purpose is for safety reasons, to decrease speed and accidents, and to harmonize the traffic flow for increased throughput. VSL is mainly used on motorways but also on other roads like trunk roads. The systems are mainly automatic but supervised by a Traffic Control Centre (TCC) or Traffic Management Centre (TMC).

What is the Vision?

The main purpose of VSL is to help drivers to travel at an appropriate speed considering the prevailing traffic or weather conditions. Sensitive road segments, like tunnels, are often subject to VSL deployment for safety reasons. VSL can also be used to mitigate negative effects for society in general, like pollution or noise and to increase throughput.

What is the Mission?

Regulating the speed limits so that the objectives of the specific deployments are met.

• Harmonisation of traffic flow
  • armonization algorithms determine the optimal speed depending on the traffic volume on the main carriageway to avoid flow breakdowns and to increase throughput.
• Speed control dependent on rain, slippery roads or visibility
  • In the case of heavy rain, slippery roads due to ice or snow or limited visibility it is recommended to adjust the speed limits to the surface conditions and/or stopping sight distance.
• Environmental criteria
  • Speed control could be used to mitigate the negative environmental effects of traffic, like pollution or noise, which has been proved in evaluations (see examples in annex).  Reduction of the allowed speed and smoother traffic flow can help to achieve the directive on air quality in populated areas.
  • User acceptance is a key factor to achieve the desired results. If the drivers understand the reasons for the displayed speed limits and experience them as relevant, they are more motivated to observe them.
• Other applications

VSL can also be used for example at bus stops, bridges subject to strong winds, intersections and in order to help protecting vulnerable road users. The service can also increase traffic safety by alerting and slowing down traffic approaching road works and incidents.

Distinctiveness from other ITS-services

In practice, VSL is often an integrated part in a larger traffic management system, especially on motorways. These systems are often referred to as ‘Motorway Control Systems’, MCS.

Relevant complementary information, which is not the content of this Deployment Guideline and will be covered by other chapters are:

• Hard shoulder running, where VSL is an important ingredient (TMS-DG04).
• Dynamic lane management, where lane signals and VSL often share hardware (TMS-DG01).
• VSL may be combined with other variable road signs, like warning signs (for example fog, road works or queue) or prohibitory signs (i.e. HGV overtaking ban TMS-DG06).
• Ramp metering systems may co-operate with VSL (TMS-DG03).
• Speed limit information (TIS-DG04): Speed limit information services are implemented to ensure that the driver is always informed of the regulatory speed limit. This can be achieved using roadside signing and road markings, and in-vehicle systems which is described in TIS-DG04 Speed Limit Information.

VSL may be complemented by Speed Enforcement (SE). The effects of both systems may be increased by combining them. You may enforce the normal (not reduced) speeds, decreased speeds or all signposted speeds. It is recommended to enforce at least the reduced speeds in order to achieve the desired effects. When combining VSL and SE it is especially important to display relevant speed limits and communicate the reasons for reduced speeds in order to maintain the confidence of the public. It is also paramount that the VSL and SE systems co-operate in a reliable way, so that the enforced speed limits correspond to the speed limits displayed by the VSL systems.

[HKPK1]Which exampels in Annex?

Contribution to overarching European ITS objectives

Service radar

The common main objective of VSL is both to support drivers in travelling at a safe speed and to improve traffic fluency. In some cases, speed limits are also used to mitigate environmental effects, such as pollution or noise.

In most cases, the displayed speed limit should correspond to the conditions the drivers encounter, and therefore will be experienced as relevant and thus the acceptance is increased. This will result in better safety, better mobility, smoother traffic, increased comfort and reduced impact on the environment. However, there are cases when circumstances call for a reduced speed limit where the reason is not obvious to the drivers, i.e. environmental reasons, problems downstream like incidents or work zones.

VSL evaluation objectives, methodologies and methods of data collection differ from country to country. The figure below, Figure 1, provides a quantification of “Variable Speed Limits” services added value regarding the three main overarching European ITS objectives which are: safety, efficiency and environment. The applied scales for the service radars are based on expert view and not on specific scientific analysis.

Safety

The deployment of speed control offers the opportunity to optimize traffic safety depending on capacity and weather conditions. Traffic-related and/or weather-related speed control reduces the risk of congestion and accidents. The impact analysis of existing facilities confirms the positive effect on traffic safety.

VSL on motorways are expected to reduce accidents in the interval 15-40% or even up to about 60% during periods with heavy traffic loads (Bundesministerium für Verkehr, Bau- und Wohnungswesen, Heft 787, 2000), dependent on factors like traffic flow, congestion duration and severity, and speed limit without active VSL.

VSL at intersections have very different effects due to the situation, traffic flows, etc., but if locations are chosen wisely, a decrease in personal injuries and fatalities in the range of 15-40% can be expected (Results from Swedish trials with VSL, Swedish Road Administration, 2008).

VSL used in foggy conditions is proved to reduce accidents by up to 80% when used in combination with other Traffic Management measures like Dynamic Route Information Panels (Bundesministerium für Verkehr, Bau- und Wohnungswesen, Heft 787, 2000).

Environmental impact

VSL systems on motorways positively affect the flow of traffic and reduce traffic-related congestion and accidents (and the consequence of further congestion development). Improving the free flow of traffic reduces noise and pollutant emissions (emissions reduced by between 2 % and 8 % depending on the individual pollutant considered with VSL on the M25 in UK). VSL can also be used for environmental purposes, with a reduced speed limit to mitigate noise and emissions when there is no congestion.

VSL at intersections have negligible effects on the environment.

Network efficiency

Demand-oriented speed control improves the flow of traffic in the complete network area concerned. The duration of congestion, and thus the loss in operational costs and time costs, is considerably reduced as the existing section capacity is optimally used. For motorways, traffic flow and throughput can be increased by up to about 15%. Another effect is a more even flow, which has a positive effect on both traffic safety and throughput. For example, on the French A7 motorway, individual speed variability has decreased from 7 km/h to 2 km/h with VSL.

[HKPK1]Who modifies the service radar and does it need modification???