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:
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:
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.