Taxonomy and description
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:
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).
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.The types of VMS range from simple one or two line text message signs to fully variable signs that can include graphical displays. The sign designer needs to consider a number of factors, including sign-size, character height, legibility, contrast and viewing angle. Messages must be comprehensible to the vast majority of drivers. Use of VMS is increasing in response to more complex traffic management requirements and the need for more information to be provided to drivers.
VMS currently employ three general types of technology (IHT, 1997):
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.
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: