Taxonomy and description

Terminology

Variable message signs (VMS) are an integral part of Intelligent Transportation Systems . VMS can be used where greater flexibility is required than can be offered by fixed direction or advisory signs. VMS are used to deliver on road information to drivers in real-time, in order to optimise traffic safety and efficiency.

In it simplest form a Variable Message Sign could be a manually operated device saying ‘full’ or ‘empty’ on a board, as seen on manned car parks. Normally VMS is a term employed to electronically signs, controlled via a computer. Whatever the sophistication of the technology, the message displayed on the VMS needs to be understood by all, so the quality of the message is most important.

VMS is also known by various other names including:

• DMS: Dynamic Message Signs;
• CMS: Changeable Message Sign;
• EMS: Electronic Message Sign;
• VAS: Vehicle Activated Sign.

A VAS is for example a sign which detects and warns speeding vehicles on the approach to bends or speed limits or high vehicles on approaches to low bridges (see DfT 2003 for more information).

Technology

VMS are generally linked to a manned control centre. The controller transmits information to be displayed in a coded way through one-to-one communication links, a local network or via radio communication.

VMS currently employ three general types of technology (IHT, 1997):

• Electro-Mechanical signs involve rotating planks with two faces or prisms with three faces which are usually used to give versatility to a standard fixed-face traffic sign.

• Reflective flip-disk signs are made up of a matrix of disks, one side black, the other fluorescent. The momentary application of an electrical current will magnetically ‘flip’ a disk between the ‘on’ and ‘off’ states. These signs are well suited to showing combinations of letters or symbols as a message.

• Light emitting signs normally use fibre-optic, magnetic flip disc or light-emitting diode (LED) technologies. The major advantage of these signs is that a greater range of message can be displayed than for reflective technology signs. LEDs, being solid-state devices, can also produce very good reliability with minimal maintenance.

Further developments may lead to applications based on Liquid Crystal Display (LCD), matrix band, liquid dot matrix and micro shutter technologies (ERTICO, 1998). It is feasible to combine technologies within the same sign. When used as warning signs, it is usual for them to be fitted with amber-flashing lanterns.

VMS is often used with other technologies for example a variable message display could show a different price for different times of the day or if connected to a camera monitoring device, a different price depending on the level of congestion on the road.
The two main restriction on the effectiveness of VMS is the limitation of the panel to display data (number and size of characters and pictograms) and the temporal correctness of the information provided. Only limited information can be presented to the driver. Another problem is that the messages displayed may not be completely understood by all drivers, and so pictograms are preferred to text messages (ERTICO, 1998).

Operation of VMS

The Highways Agency opened an English National Traffic Control Centre in 2003, which will deliver information through Variable Message Signs and in-car systems. The Scottish National Driver Information and Control System (NADICS) covers an area from Inverness to Gretna and Edinburgh to Glasgow in Scotland. A number of local councils have installed VMS as part of accident reduction strategies. Variable Message Signs can be operated by various organisations including:

• trunk road maintenance operators;
• local roads authorities;
• the police;
• and other interested parties