4.3 TMS-02 Variable Speed Limits

4.3.1 ITS service at a glance

4.3.2 ITS service profile

4.3.2.1 General ITS service description

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. 

4.3.2.2 What is the vision?

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.

4.3.2.3 What is the mission?

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

Harmonisation of traffic flow to improve safety

• Harmonization 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, limited visibility or slippery roads due to ice and snow, it is recommended to adjust the speed limits to the surface conditions and/or stopping sight distance.  

Environmental criteria 

• Speed control can be used to mitigate the negative environmental effects of traffic, like pollution or noise, which has been proved in evaluations. Reduction of the allowed speed and smoother traffic flow can help to achieve the directive on air quality in populated areas.   à 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, tunnels, 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 unexpected incidents. 

4.3.2.4 Distinctiveness from other ITS services

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: 

• Hard shoulder running, where VSL is an important ingredient (TMS-04).
• Dynamic lane management, where lane signals and VSL often share hardware (TMS-01).
• VSL may be combined with other variable road signs, like warning signs (for example fog, road works or queue, TMS-06) or prohibitory traffic signs (i.e. HGV overtaking ban TMS-05) with potential variable text signs.
• Ramp metering systems may co-operate with VSL (TMS-03).
• Speed Limit Information: Speed Limit Information services are implemented to keep the driver regularly informed of the regulatory speed limit. This can be achieved using roadside signing and in-vehicle systems which is described in TTIS-04 Speed Limit Information.

4.3.3 Harmonization Requirements and advice

4.3.3.1 ITS service definition

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.

4.3.3.2 Functional requirements and advice

4.3.3.2.1 Functional architecture

Figure 51 shows the typical functional architecture of a VSL system. Red arrows show possible interfaces to other services.

4.3.3.2.2 Functional requirements and advice

Functional requirements

• FR1: Functional decomposition and the provision of standardised interfaces should be carried out to ensure interoperability in cases where the service is carried out by more than one organisation (and is in any case recommended to be prepared for an easy functional decomposition, as could be the case in the future). Control and algorithms may be done through local (roadside) or central systems.
• FR2: Traffic Management Centre Operators should be trained in supervising the system, be able to control the system manually and override automatic operation. Exception: Local VSL systems sometimes can operate independently and need no supervision regarding current signposted speed limits.
• FR3: The central control system should have the ability to supervise and control each individual system. Exception: Local VSL systems sometimes can operate independently and need no supervision regarding current signposted speed limits.
• FR4: The signs must display the speed limit that the control system has requested, and functionality must be monitored continuously.
• FR5: To avoid unanticipated deceleration, gradual speed reductions should be shown as 20-40 km/h increments, dependent on operating environments, context, speed and road topology (note: 20 km/h reduction between two gantries is common practice on motorways).
• FR6: VSL systems should have a log that stores data about signposted speed limits, error messages, etc.
• FR7: Signs should report to the control system if message activation was successful or not and possible error messages.
• FR8: Automatic and semi-automatic systems should contain models and algorithms that calculate the speed limit and transmit it to the signs. These models and algorithms can be implemented in a central control system or at the roadside. 
• FR9: Collected data must be of sufficient quality to provide adequate input in the control system, in some cases this can necessitate the installation of additional roadway sensors. Exceptions: For systems using clock and/or calendar control, sensors are replaced by the system clock. For manually controlled systems at road works, sensors are usually replaced by a keypad (local control unit) or similar. 
  • Note: Systems may include both manual and automatic functions as well as several types of sensors. This requires well defined hierarchical rules and priorities.
• FR10: Detector data updating frequency should be adapted to the required response times. For instance, a normal updating frequency for traffic data is from real-time to one minute.
• FR11: The systems should have predefined handling of situations like power failure, disruptions in communications and other functional problems to avoid functional inconsistencies in the service. System parameters and error states should be disseminated in real-time to on-duty staff.
• FR12: If VSL systems interact with other services like hard shoulder running, dynamic lane management or HGV overtaking ban (or adjacent VSL systems), interfaces should be implemented either at roadsides or in central control systems. In practice, this can often be internal interfaces in the same system.

Functional advice:

• A general rule to achieve a good understanding and observance of VSL is that the speed limits have to be relevant for road users.  Not only to achieve the individual but especially the collective relevance. This sets the requirements for data collection and control principles. It is relatively easy for the driver to understand that the speed limit is reduced when there is congestion or bad weather, but it is more difficult to communicate speed reductions due to, for example, environmental reasons. Therefore, road authorities can consider including a warning sign with a pictogram or a text showing the reason for the displayed speed limit.
• Automatic control shall be used whenever possible. You may also consider the option of using a semi-automatic mode where you set the maximum speed limit manually but let the system vary the displayed speed limit up to this threshold. This can be especially useful for long road works.
• When a VSL system is used to decrease the risk of rear-end collisions due to congestion, detector spacing shall be adequate to the function. The distance between detector and gantry needs to be adjusted.
•  The control algorithms shall result in speed limits that are relevant to achieve the desired effects and responds quickly enough in critical situations
• The algorithms shall be constructed in an appropriate and stable way to avoid unnecessary switching of the speed limits. 
• The components in roadside equipment have to be of the correct environmental class to cope with the environmental conditions. For maintenance reasons, it is an advantage if the components of the systems are easy to reach and replace.
• When VSL is supplemented by speed enforcement, the monitoring technology used by private as well as public road operators, which generates evidence of speeding, has to meet the national legal requirements.

4.3.3.3 Interface requirements

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),

Interface requirements:

• IFR1: The Variable Speed Limit service must provide at interface 1 (see Figure 51) information on the current speed limit defined in DATEX II Recommended Service Profile including the mandatory data of following classes:
  • Common information
  • Location Referencing information
  • Road Traffic Data
  • Situation information
  • VMS, includes VMS panel information
• IFR2: If interface 2 is implemented the Variable Speed Limit Service must provide at interface 2 (see Figure 51) C-ITS coded information relevant to this ITS Core service.
• IFR3: When relevant, the Variable Speed Limit service should collect at interface 3 (see Figure 51) C-ITS coded information on Probe Vehicle Data (microscopic traffic situation) relevant to this ITS Core service.

4.3.3.4 Organisational Requirements

Organisational advice:

• The responsible organisation has to establish a good cooperation with the police and good communication with the public for the reasons and benefits of VSL. This is a key to success and encourages a positive attitude from drivers.
• VSL is mostly a concern of the Road operators (road authorities and motorway companies and its subsidiaries). However, these stakeholders can also be considered:
  • Municipalities and cities: At boundaries between state and municipal roads or when these systems influence traffic flow on municipal roads. Municipalities and cities may also implement VSL on their own networks.
  • Public transport authorities and operators: When these systems influence accessibility and schedules for public transport. It is also possible to consider priority for public transport in separate lanes in conjunction with VSL.
  • The Police: To enforce the speed limits for better compliance by road users. The police are generally an important partner when speed limits are enforced using automatic speed enforcement systems. Depending on national regulations, the police may also need to accept VSL projects formally. 
•  A cost-benefit analysis, as well as an analysis of the achieved effects in relation to the objectives, can be carried out when new VSL systems are deployed, unless similar projects already have been thoroughly evaluated.

4.3.3.5 Common Look & Feel requirements

• CL&FR1: Speed limits (maximum) should be displayed in one of the following ways:
  • Discontinuous signs (character matrix): White, off-white or yellow figures on a black background enclosed by a red ring. Discontinuous VMS can also be used without colour inversion if national regulations allow or require this..
  • Continuous signs (full matrix): Sign surface similar to fixed mandatory speed signs according to national regulations.

• CL&FR2: Advisory speed (recommended) signs should be displayed in one of the following ways:
  • Discontinuous signs (character matrix): White, off-white or yellow figures on a black background. The sign can have a white rectangular border, but no red or white ring. Discontinuous VMS can also be used without colour inversion if national regulations allow or require this.
  • Continuous signs (full matrix): Sign surface similar to fixed advisory speed signs according to national regulations

• CL&FR3: Compulsory minimum speed should be displayed as a continuous sign (full matrix): Sign surface similar to static minimum speed signs according to national regulations.
• CL&FR4: To avoid driver confusion about which speed limit is valid, static and variable speed limit signs should never be mixed along a particular roadway segment. Variable and static speed limit signs must not be located on the same cross-section.
• CL&FR5: It should be obvious to the drivers when a section with VSL ends and what the valid speed limit is after that. Normally this is done using fixed speed limit signs.
•  CL&FR6: For certain roadway segments speed limit signs may be active only when a reduced speed limit is set. In other cases, they may be blank. In some circumstances this makes it easier for the drivers to notice conditions that require a lower speed.
• CL&FR7: Signs should be located either above the carriageway or on the verge of the road. If signs are located on the verge, they should be signs on the near side of the road with possible supplementary signs to the offside. If there is more than one lane in the direction of travel, it is recommended to have signs on both sides.
• CL&FR8: If signs are mounted above the carriageway, each lane may have one speed limit sign located above (indicating speed limit of that lane only) or a single speed limit sign integrated in a larger VMS, applicable to all lanes.

• CL&FR9: Speed limits should be repeated at least after every entry slip road, and the distance should never exceed 10 km on long stretches or according to national guidelines and operating context.

Common Look & Feel advice:

• It is common that VSL is integrated in motorway control systems, where the variable speed limit signs are mounted above the carriageway. In these cases, VSL can be combined with, e.g. lane control and warning signs. The requirements and advice regarding VSL in this service description are still applicable but need to be combined with requirements and advice from other ITS Core services like TMS-01 (Dynamic Lane Management) and TMS-05 (Incident warning and Incident Management).
• Signs mounted above each lane can display different speeds for respective lanes, but only in limited cases and after proper testing and evaluation. In these cases, the greatest difference in speeds between adjacent lanes shall not exceed 20 km/h. This requires that the traffic management system algorithms perform separate calculations for each lane while respecting a maximum speed difference of 20 km/h between adjacent lanes throughout the length of the roadway segment.
• Side mounted VSL can remain in operation also when single lanes are closed. On the contrary, gantry mounted VSL over closed lanes can be switched off, unless they are used in a multilane control system where a red cross is displayed. 
• Yellow flashing lights can be added to improve driver awareness of the set speed limit . In Motorway Control Systems yellow flashing lights can be used to alert the driver that he/she enters a section with a lower speed limit.

For road and weather condition controlled VSL, many different types of sensors can be used to monitor weather in general and road surface locally.

• For weather controlled VSL, automatic control with supervision from TMC operators ought to be used as much as possible. Much effort needs to be put into design of the weather control model and its algorithms. Note: National legislations may prevent road operators from using automatic weather controlled VSL.

VSL on motorways

• For VSL on motorways, sensors are usually placed at each sign gantry. Inductive loops and microwave detectors are commonly used technologies. Also video, laser, light barriers and piezoelectric sensors are used.

• VSL on motorways are often automatic and based on sensor data. Manual override is used mainly in case of accidents or road works or, in wintertime, warnings concerning slippery and snowy roads. The algorithms should be designed both for increased throughput and safety (“queue warning function”).

Information provision standards

• IPS1: If a Variable Speed Limit service is implemented at interface 1 (see IFR1), it must be profiled based on CEN/EN 16157-3 using the DATEX II Recommended Service Profile for Speed Limits or any international machine-readable format fully compatible and interoperable with DATEX II.
• IPS2: If interface 2 is implemented, Variable Speed Limit information (see IFR2) must be profiled in an IVIM (Infrastructure to Vehicle Information Message) based on ISO 19321 using the C-ROADS C-ITS Message Profiles for the In-Vehicle Signage service, specifically the Traffic Sign Use Case.
• IPS3: When relevant, the Probe Vehicle Data (microscopic traffic situation) information (see IFR3) should be collected, which is profiled based on ETSI EN 302 637-2 using the CAR2CAR Communication Consortium Basic System Profile.

Variable message sign standards

• SS1: Discontinuous signs (i.e. LED) must follow the European standard EN 12966:2014+A1:2019 or their national counterparts. Continuous signs (retro-reflective, i.e. prism signs) must follow the European standard EN SS-EN 12899-1:2007 or their national counterparts where applicable.

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

• EN 12899-1:2007 Fixed, vertical road traffic signs – Fixed signs. This standard can be partly used for continuous (retro-reflective) variable signs, like prism signs.

4.3.3.7 Level of Service definition

4.3.3.7.1 Level of Service criteria

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

Coverage

• A – spot coverage: The VSL system covers only a short stretch of road where specific conditions prevail, for example hazardous intersection.
• B – Section coverage: The VSL system covers a longer stretch of road, but does not cover an entire corridor. A corridor in this case is the total road from one important point to another, e.g. between two cities. 
• C – Total corridor coverage: The VSL system covers an entire corridor as described above. 

Level of coordination

• A – No or limited coordination with other systems on a section. The VSL system does not operate in a coordinated way with other Traffic Management measures like hard shoulder running or lane signals.
• B – Coordination with other TM measures for section control: The VSL system is coordinated with other measures, often integrated in a Motorway Control System. 

Monitoring and control

• A – Manual monitoring: Traffic Management staff change the speed limit manually when there is a need. The operators can either discover the problem through a CCTV system or get information from partners, like the police and rescue services. 
• B – Clock and/or calendar control: The speed limit is set automatically bases on the time of day and/or year. This option is most suited for situations where you have daily or seasonal recurring problems.  
• C – Specific sensors monitoring: Automatic control with sensors which detect the situation that calls for a reduced speed limit. This is generally the best solution, since it is not dependent on manual supervision and the displayed speed limit is in most conditions seen as relevant by the drivers.

4.3.3.7.2 Level of Service Criteria related to Operating Environment

Level of service requirement:

• LoSR1: In the case that pre-deployment surveys / evaluations provide the necessary evidence to proceed with the deployment of the ITS-service “Variable Speed Limits”, the minimum and optimum LoS should respect the following Level of Service to Operating Environment mapping table.

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)