The sight of electronic signs has been commonplace on all UK motorways since their introduction in 1969. Their installation was designed to help motorists by providing advance warning of impending problems on the motorway ahead, therefore minimising disruption to traffic and improving safety.
The signs themselves are referred to as "variable message signs" - or VMS - as the messages displayed can be altered depending on the nature of the event.
A limited range of messages can be displayed on the VMS signs in order to provide a prompt and efficient communication to drivers using that particular road. However, by using different types of messaging systems, a greater range of messages can be conveyed to drivers.
Types of VMS Signals
Mark 1 Message Sign
These signs were initially introduced on the M4 in 1969, positioned on the approaches to the original Severn Bridge (now part of the M48). They proved to be both successful and highly useful, and so were soon rolled out across the UK motorway network. They are also to be found on the UK's most newest stretch of motorway, the M6 Toll, where they are usually sited on the longer stretches between junctions.
Slip Road Post Mounted Signal
Gantry Mounted Signal
They have an advantage over the matrix sign as they can control more than three lanes of traffic, as well as traffic leaving the motorway at junctions. Each lane has its own sign, so different messages can be displayed that are relevant to each lane. These signs are usually spaced 1/2 mile apart, shortened to 2/3 mile if junctions are closely spaced.
Types of VMS Signs
Compared with the signals shown above, the VMS Signs can display more detailed information, conveying a variety of important messages to motorists. These signs have been introduced in the past few years, generally appearing on gantry signs and on tall cantilever poles at the side of the road. The signs can be manually controlled or activated automatically by the Motorway Incident Detection Automated Signalling (MIDAS) system. Again, a variety of signs can be found, as listed here.
|Enhanced Message Sign
These signs are commonly found on alongside gantry signs, and are usually used to provide motorists with additional information on top of the gantry sign displays, thus giving drivers more knowledge of what to expect ahead.
They generally feature two lines of text each displaying up to 12 characters, although some more recent examples feature a third line or have longer lines (up to 18 letters). Although most of the time they advise drivers of why the signals have been switched on, they can also give motorists advance warning of delays on other routes.
|Gantry Message Sign
Somewhat quite elderly now, these are possibly the oldest VMS signs on the motorway network. They are common on the motorways around Glasgow, especially the M8 and M74.
They are limited to a single line of text, but can hold up to 22 characters owing to the sign spanning all three lanes of traffic. However, they can be switched to operate in a similar manner to gantry mounted signals.
(photo courtesy of Thomas Nugent)
Mark 2 Message Sign
Found on some stretches of motorways and major trunk routes, these signs are generally found on rural motorways where gantry signs are not found. The signs are located at the side of the road on a tall cantilever post. They are able to display two lines of text each displaying up to 12 characters, and have an additional Enhanced Matrix Indicator (slightly larger than the matrix sign). Their roll-out was stopped with the release of the Mark 3 sign.
Mark 3 Message Sign
This sign is more common than its predecessor, the Mark 2, as they were released about a year or so after the roll-out of VMS signs was introduced. In fact, these signs are often found alongside the Enhanced Message Signs in urban areas. They can display the same information as a Mark 2 sign, but can provide more detail as they feature three lines holding up to 18 characters of text. They don't have a separate matrix indicator, but on some versions, matrix displays can be still be set in using the right-hand portion of the sign.
Mark 4 Message Sign
The newest of the lot, this sign is currently found on the M3, M4 and M42. It can be cantilever or gantry mounted, and can display text in upper case and lower case, as well as being able to show diagrams of road signs (such as speed limits or warning signs). They are also available in a multi-coloured form, especially useful when displaying signs.
Mobile Variable Message Sign
These trailer-mounted signs appear in areas where the fixed VMS system has not been installed, particularly on trunk routes. They are used in the same manner as fixed VMS signs, but are generally manually operated from the unit itself.
Approved VMS Displays
The messages that are displayed on the VMS Signs (including mobile signs) are known as "legends". These have to be pre-approved before they can be used, and are categorised into one of four groups, dependent on the message.
Matrix and gantry signs can be set to display the following messages:
(images for illustration only - not all VMS signal types can display all messages)
advisory speed limit
(can be varied)
(can be reversed)
|leave motorway at next exit||risk of fog ahead||risk of ice ahead||
lane closed ahead
(can be varied)
lane closed ahead
all lanes closed
(can be varied)
all lanes closed
lane closed - do not
proceed in this lane
|end of restriction||
end of restriction
(no longer used -
replaced by "End")
The red cross message is always accompanied by red flashing lights. This message is a mandatory instruction - passing the sign can result in a £60 fine and three penalty points.
- Tactical legends advise drivers of a hazard ahead, in order to advise them to slow down.
- Tactical diversion legends advise drivers of an upcoming diversion, due to a closure or restriction.
- Site specific legends are authorised to support a local or area-specific contingency plan, for example the closure of an airport.
- Link legends advise that the road ahead is closed, but don't give directions or instructions.
- Network legends advise drivers of a diversion elsewhere on the motorway / trunk network.
This type of legend id used to inform drivers of delays on a particular road, and suggest alternative routes to take. They generally use a series of strategically place Mark 3 sign (shown above), and are used to distribute traffic across the network to avoid adding more congestion to where the problem is and use roads that are free-flowing.
A specific layout is used, for example:
|Line 1 - where the problem is (road, junction number) and what is causing it|
|Line 2 - intended destination (main town or city the road is heading towards)|
|Line 3 - suggested alternative route|
These signs are set by the National Traffic Control Centre in Birmingham, and can be used to display other incident management or driver incident displays when not in use.
This type of legend is used prior to road works, in order to protect workers when setting out or removing cones, barriers and signs. They can also be used to advise motorists of mobile lane closures in the road ahead. The message displayed is usually "workforce in road - slow".
Travel and Delay Times
A new feature that has recently been trialled in the Midlands and South West, the Highways Agency has introduced a system whereby drivers can be advised of roughly how long it will take to reach a certain point, or how long you will be sat in a queue for. By doing this, drivers know if they are likely to encounter any significant hold-ups and, if necessary, plan ahead to take an alternative route.
They are given a low priority, so if incident warnings and information needs to be displayed, the signs will be changed accordingly.
How the system
The signs work by using Automatic Number Plate Recognition cameras sited next to the motorway, which collects data to build profiles of how long a vehicle has taken to travel between two points. The data collected is averaged out to give an approximate travelling time between those two points. The signs refresh every five minutes in the event of traffic flows slowing or speeding up, thus keeping the information current.
The Highways Agency don't have access to the recognition data as the cameras are connected to an off-site system that only stores the data for profile building. Once that has been completed, the data is overwritten by the next set of profiles.
What is displayed
In normal circumstances, the message displayed advises drivers of how long it should take to reach a certain point. How much information is displayed depends on the size of the VMS sign, as the smaller signs will not be able to provide as much detail.
On the larger three line MS2/MS3 signs, the destination, distance and time can be shown, whereas on the two line gantry displays, the distance is removed. Research carried out by the Highways Agency found drivers preferred to see distances as this helped them to understand the journey or delay times.
|Line 1 - reference point, usually junction number followed by the destination or road number|
|Line 2 - distance (omitted from two-line displays)|
|Line 3 - time taken to reach the reference point|
If something causes a delay that passes above a pre-determined time, the signs automatically change from displaying journey time to delay time. An example of this is:
Only junction numbers are used, not town or city names. This is because the Highways Agency only collates data for the roads they run, motorways and trunk routes, and as drivers may take different routes into a town centre, or go to different parts of the town, accurate data cannot be collated. In order to help drivers who may not know where the junction shown is, the larger VMS signs usually display the main primary destination or motorway served by that junction.
Accuracy and Calculations
The times displayed are constantly monitored by the National Traffic Control Centre, taking into consideration any delays caused by road works or accidents. This allows times to be kept as accurate as possible. In the event of a major event, such as sports match and concerts, the Highways Agency liaise with the organisers to ensure they are aware of what may cause additional traffic to use the roads. This may mean that traffic times may not be accurate due to fluctuations in traffic volumes.
The Highways Agency does not use speed and distance to work out the journey times, but uses the information collected by the Automatic Number Plate Recognition system. This allows an accurate reading to be given (updated every five minutes), reflecting circumstances that may affect traffic flow, such as peak times. However, travel times are capped so that they reflect the maximum legal speed limits that drivers should be adhering to.
Alongside displaying the above types of information, VMS Signs are also used for government national road campaigns. They can be displayed whenever and wherever, and can be set to run for a short period or for a number of days. They are given a low priority, so if incident warnings and information needs to be displayed, the signs will be changed accordingly.
Each sign has to be pre-approved, and is displayed in accordance with what
campaign is in progress. Examples include:
- "THINK! DON'T DRINK AND DRIVE"
- "THINK! DON'T DRIVE TIRED"
- "THINK! DON'T PHONE WHILE DRIVING"
- "TAKE A BREAK"
- "WATCH YOUR SPEED"
- "KEEP YOUR DISTANCE"
Automatic VMS Systems
The VMS system has been developed so that signs can be switched on or amended automatically, without the input of control room staff. This system, known as Motorway Incident Detection and Automated Signalling (MIDAS) uses a series of induction loops spaced 500m apart, which detects the speed of vehicles and the formation of queues. This system allows VMS signs to warn drivers of pending conditions on the road ahead, together with gantry signs displaying the advised maximum speed. The system turns the signs off shortly after the traffic flow has returned to normal.
Control room staff have the ability to over-ride the MIDAS system in the event of a severe malfunction, thus possibly causing a safety hazard, or in the event of road works taking place (so the system can be switched off whilst the works are being carried out). At the moment, the MIDAS system is only in place on the busier parts of the motorway network.
The MIDAS system itself, alongside the control room support staff, control the VMS network through the Highways Agency's Control Office Base System (COBS). The system prioritises what is displayed on each VMS sign, so in the event of an operator of MIDAS requesting incident information be shown on a sign displaying a campaign, the data will automatically be changed. This system also controls the emergency telephones.
There are two other automatic systems used also. Firstly is Controlled Motorways, currently used on the M25 between Junctions 10 and 16. This system uses MIDAS and COBS to monitor traffic flow by raising or reducing the speed limit accordingly, thus preventing stop-start traffic conditions. Speed limits on this section are mandatory, with speeds displayed on the gantries in red circles and cameras enforcing them. The second system is Active Traffic Management, which uses the Controlled Motorway system, but adds in a few extra details. A full in-depth feature on this scheme can be found at CBRD.
Some photos by Highways Agency and is used under an Open Government license.