ITS service definition
Variable speed limit (VSL) system use Variable Message Signs (VMS) to display speed limits (maximum), advisory speed (recommended) or compulsory minimum speed, to guide drivers to travel at a speed suitable to the prevailing traffic, road or weather conditions.
ITS service objective
The common main objective of VSL is to support drivers travelling at a safe speed or to improve traffic fluency. In some cases, these systems are also used to mitigate environmental effects, such as pollution or noise.
In most cases, the displayed speed limit should correspond to the traffic, road and weather conditions the drivers encounter, and therefore will be experienced as relevant. The drivers are then more likely to adhere to the speed limits. This will result in better safety, better mobility, smoother traffic, increased comfort and a reduced impact on the environment. However, there are cases when circumstances call for a reduced speed limit for which the reason is not obvious to the drivers, i.e. environmental reasons or problems downstream like incidents or work zones.
ITS service radar
ITS service key words
Variable speed limit (VSL) system use Variable Message Signs (VMS) to display speed limits (maximum), advisory speed (recommended) or compulsory minimum speed for road users on specific road sections.
Speed limits are adapted to the particular weather, road and/or traffic conditions. Background system use gathered information and take 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 capacity and traffic safety. 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. Data from sensors are collected by local control systems and analysed in a control algorithm to make decisions according the predefined rules. The local control systems are mainly automatic but supervised by a Traffic Control Centre (TCC) or Traffic Management Centre (TMC).
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 using VSL is different from case to case. The general overall purpose is for safety reasons, to decrease speed and accidents, to harmonize the traffic flow for increased throughput and to adapt to the weather and road conditions. VSL is mainly used on motorways but also on other roads like trunk roads.
In some cases, VSL is supported by Speed Enforcement (SE), which mostly uses cameras to identify speeding vehicles and/or drivers. 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. 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.
The main purpose of VSL is to help drivers to travel at an appropriate speed considering the prevailing traffic, road 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 traffic throughput.
Regulating the speed limits so that the objectives of the specific deployments are met.
Harmonisation of traffic flow to improve safety
Speed control dependent on rain, slippery roads or visibility
In practice, VSL is often an integrated part of 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 the VSL service and will be covered by other chapters are:
VSL system consists of variable speed limit signs, maximum, minimum or advisory, to guide drivers to travel at a speed suitable to the prevailing traffic or weather conditions, in some cases supported by Speed Enforcement, which mostly uses cameras to identify speeding vehicles and/or drivers.
Figure 51 shows the typical functional architecture of a VSL system. Red arrows show possible interfaces to other services.
Note: Interfaces are identified in general architecture (see Figure 51). DATEX II Recommended
Service Profile is valid only for variable speed limit signs (maximum speed limit),
Common Look & Feel advice:
For road and weather condition controlled VSL, many different types of sensors can be used to monitor weather in general and road surface locally.
VSL on motorways
Information provision standards
Variable message sign standards
Note: The standards allow several levels of performance to be selected due to i.e. the environment where the signs are used.
Standards for Fixed traffic signs
Table 28 gives the Level of Service recommendations for a Variable Speed Limits service. The background of this concept is descripted in chapter 2.6.
Table 28: Level of Service recommendations for Variable Speed Limits
Level of coordination
Monitoring and control
Level of service requirement:
Note: The Level of Service to Operating Environment mapping table is not an outcome of a specific scientific analysis but an expert view output.
Table 29: Level of Service to Operating Environment mapping table (see also chapter 2.5.3 and ANNEX C)