Variable Message Signs (VMS) are capable of displaying pre-defined or freely programmable messages which can be changed remotely with individual pixel control. They consist of:
A Dynamic Message Sign (DMS) is any sign or graphics board that can change the message (text or pictogram) conveyed to the viewer. It may be either a Variable Message Sign (VMS) or a Changeable Message Sign (CMS) where:
See Roadside DMS
Roadside and overhead VMS are a basic traffic management tool for road owners and authorities – to provide information and advice to drivers and riders independent of any in-vehicle systems. In some cases VMS signs can replace fixed roadside signs to inform drivers of speed limits – if necessary with the flexibility of changing mandatory speed limits in response to traffic or road conditions where the legislation allows.
Research evidence shows that road user acceptance and compliance is substantially increased when the reason for a new, lower than normal, speed limit is displayed on the VMS in addition to the prevailing speed limit.
VMS can be used to convey maximum advisory (non-mandatory) speeds — to advise drivers and riders to slow down in fog, high winds and icy conditions or on the approach to an incident or slow-moving traffic ahead.
Evaluation of an icy road surface warning system installed on a mountain road in California indicated that, when the warnings of icy conditions were activated, mean speeds were more than 5 mph (8 km/h) lower than when no warnings were shown (Veneziano and Ye, 2011). In specific situations where ice was present but not likely to be expected by drivers (such as cold, clear and not-dry conditions), the reduction in speed was somewhat less — under 3 mph to around 3 mph (4.8 km/h). The speed data was collected at the VMS sign – rather than in the subsequent curve where the driver response may have been greater.
A fog-warning system on the A16 in the Netherlands had two levels of lower speed limit available, depending on the level of visibility. It was found that when the system was activated, average speeds decreased by 9-10 km/h. When the very low visibility speed limit was displayed, average speeds were higher than appropriate for the road conditions indicating that the advisory speed limit of 60 km/h was not low enough to influence driver behaviour sufficiently.
Dynamic management of speed limits in response to traffic flows has been shown to reduce speed variability and smooth flows, reduce numbers of accidents and increase overall throughput. It also has environmental benefits through the reduction of emissions. It is mainly applied on motorways. When used in construction zones or for temporary narrow lanes, the safety impacts are considerable, particularly as regards the protection of roadside workers. Dynamic speed limits are often combined with camera-based speed enforcement. (See Work Zones)
Roadside VMS is commonly used, particularly on urban roads, to inform drivers that they are speeding in the hope of encouraging them to slow down. These VMS are linked to speed detectors. When triggered by a speeding vehicle they display the speed limit or a message such as “Slow down”. The devices can also be mounted on a trailer for portability – for use, for example, in work zones. Portable Dynamic Message Signs (PDMS) have been shown to have an effect on the proportion of drivers speeding and on average traffic speeds. Some of that impact may be a novelty effect — since few evaluations have looked at the persistence of changes.
“Smart” or “Managed” motorways are becoming more common as a means of managing high traffic flows on motorways. (See for example http://www.highways.gov.uk/our-road-network/managing-our-roads/improving-our-network/smart-motorways/).
Typically, as traffic builds up towards the critical 2,000 vehicle per lane per hour, speed limits are reduced. The aim is to reduce speed variance and shock waves caused by traffic slowing down and speeding up - which can lead to incidents and accidents. The overall objective is to maintain traffic throughput - but the schemes have also brought about reductions in accidents and secondary incidents. Speed limits are enforced with automatic camera-based technology (time over distance cameras in the more recent schemes). Another common feature is the use of the hard shoulder for traffic at times of high congestion (“hard shoulder running”) with appropriate VMS signing. (See Case Study: Active Traffic Management Pilot M42 (UK))
National and regional standards are likely to apply to the design and installation of VMS. There are also applicable international standards for regulatory signs. Sixty three countries across the world have ratified the Vienna Convention on Road Signs and Signals. This states that road signs conveyed on VMS should generally conform to the text and symbols used on standard road signs. The Convention allows variations in colour in the interests of legibility. For example, it permits the use of dark text on a light background (the norm for speed signs) to be replaced by light text on dark background in the case of VMS - although the border must remain red.