Road Network Operations
& Intelligent Transport Systems
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Roadside Dynamic Message Signs

Some definitions

A Dynamic Message Sign (DMS) – is any sign or graphics board that can change the message to the viewer (text or pictogram). It may be a Variable Message Sign (VMS) or a Changeable Message Sign (CMS) where:

  • VMS - is a sign capable of displaying pre-defined or freely-programmable messages that can be changed remotely
  • CMS - is a sign capable of displaying pre-defined fixed messages, the content of which cannot be changed remotely
  • PDMS – is a portable DMS that may be relocated, that is usually vehicle- or trailer-mounted


Dynamic message signs – and more specifically changeable message signs – have been around for many years. Initially these often were conventional road signs with rotating prism elements within, where 3 options of message could be chosen. These allowed simple diversion route strategies to be implemented or warnings to be issued at known problem locations, however this type of sign is limited in what can be displayed and also needs careful maintenance to ensure rotating elements do not seize up.

Developments in technology then led to the available of magnetic flip disk (or flip dot) signs to be developed where a sign is made up of an array of disks with one black face and one yellow retro reflective face and passing a current through relevant disks can switch the face of the disk displayed. This type of sign uses very low power consumption, but can suffer from disks occasionally sticking and may not be as clearly visible as LED of Fibre Optic Signs depending on the lighting conditions. Fibre optic signs have a light source and fibre optic strands which illuminate pixels or a simple image.

The majority of roadside DMS are now LED as these are generally reliable and flexible in operation.

Changeable Message Signs are used when either a warning is to be given for a specific hazard or behaviour or when there are only a limited number of options available. A good example might be a speed activated warning sign associated with a bend in the road. The sign is illuminated if a vehicle approaches at high speed. These type of signs can be used to support road safety campaigns and can be very effective in reducing incidents are specific accident prone locations.

Variable Message Signs are used for a wider range of purposes including:

  • Hazard/Queue warning (there is a potential hazard)
  • Incident warning (an incident has occurred)
  • Speed limit reduction/Harmonisation (advisory or mandatory)
  • Lane Control (including Hard Shoulder Running)
  • Tactical incident management
  • Strategic incident management and Diversion
  • Campaign Messages
  • Major Event messages
  • Provision of current level of service information including journey time information
  • Toll levels
  • Junction Control
  • Pre-event warning messages


VMS are often used to control and shape the flow of traffic within a given area, be it a region or a single corridor of road. One such purpose is that of controlling traffic flow in tunnels, or busy stretched of motorway, where capacity issues can be a serious cause for concern. Often these VMS systems, as well as allowing for manual operation from a control room, can be triggered by readings on automatic sensors up the road. Such systems include variable speed limit systems on motorways, which help to keep traffic flowing when there is congestion. Traffic loops built into the road surface detect slow-moving or stationary traffic and, through a computer programme, the system works out the most appropriate speed limit further back on the network in order to let traffic clear or to assist in allowing traffic to flow smoothly. This can often be implemented lane by lane, allowing the opening of hard shoulders or closing lanes as required, much as outlined for tunnels.

A useful synopsis of Active Traffic Management experiences in the USA and Europe can be found on the US DoT Federal Highway Administration website


As noted above VMS are used to disseminate traffic situation and incident information in for several purposes. It is important to review the use cases and propose equipment that can be used to deliver the expected information messages. This in practice, means that if a sign is to be used for diverting traffic in needs to be placed in a location where the driver can see and act on the information to be provided. It also means that the sign has to have the ability to present the information types. Thus if pictograms are required the sign must be capable to presenting these and if words are required the sign must be able to display sufficient words for the driver to comprehend. Considerable research has been undertaken on the amount of text that can be assimilated by the driver. 

The size of the sign required is, in part dependent on the speed of traffic on the link in question. The faster the road speed the larger the sign needs to be to display the same amount of information. In addition, on high speed roads, the amount of information that can be assimilated in the available time is small. In the UK approximately seven words can be displayed on the largest VMS signs on motorways which are 3 line 18 character VMS.

Portable Dynamic Message Signs are particularly useful at locations where a permanent dynamic message sign is not justified. This may be during medium term roadworks, for a specific event, or perhaps on a motorway with significant seasonal traffic demand. Pre-planning of the location for such signs is vital in order to ensure they can be safely deployed and retrieved. Increasingly such signs are solar or wind powered and in locations where such signs are periodically deployed, provision of hardstanding for these signs should be considered.

In Europe there are technical standards for VMS design that must be followed. In addition there are guidelines for the Principles of VMS message design that provide more detail on how and when it is appropriate to use VMS.


Reference sources

TRL (2002) Safety and Effectiveness of the Wider use of VMS. TRL report 526 ISBN 1-84608-525-X