Variable Speed Limit
From Roader's Digest: The SABRE Wiki
|A 50mph variable speed limit has been set on this MS4 on the M1 alongside lane diversion symbols to prepare for a stranded vehicle in Lane 1 ahead.|
|Pictures related to Variable Speed Limit|
View gallery (49)
A variable speed limit is a speed limit which is set dynamically, usually by digital variable message signs, so that the maximum permitted speed changes according to road conditions. Most variable limits can be found on motorways, but there are some that can be found on the all purpose road network-rarely, if at all, in England and on a few major A roads in Scotland and Wales.
In most cases, a system of variable speed limits is designed to address issues with congestion at certain times of day, but they are also often used for safety reasons where a risk exists only at certain times of day.
Variable Speed Limits in the UK
Motorways and Smart Motorways
Variable Speed Limits are now widely used on congested sections of the British motorway network to prevent stop-start congestion. In many places this is part of a wider Smart Motorway scheme, with complex technology that attempts to manage congestion in various ways, and where the Variable Speed Limit is just one measure in use. In other locations (such as the south-western section of the M25 and the M20 around Maidstone) the Variable Speed Limit technology has been installed on its own. Generally, standalone Variable Speed Limit installations pre-date the introduction of Active Traffic Management and Smart Motorways, and new standalone installations are unlikely.
In Scotland, similar technology is employed, with one of the high-profile installations being on the M90 north of the new Queensferry Crossing. The term Smart Motorway is unique to England, and in Scotland these schemes are known as Intelligent Transport System.
In a typical motorway installation, overhead gantries are placed at regular and frequent intervals over the carriageway, with an electronic matrix sign mounted above each lane that is capable of showing a two-colour speed limit roundel. In full Smart Motorway installations, there will also be a matrix sign over the hard shoulder, and each matrix will also be capable of use as a lane indicator. In some later Smart Motorway installations in rural areas, there are no overhead gantries, and instead the speed limit is indicated only on a single MS4 matrix sign mounted at the roadside. The matrix signs are capable of showing a speed limit sign with any multiple of 10 between 20 and 60, plus a National Speed Limit sign when the system is not being used.
When the signs show a speed limit with a red circular border, the limit is legally binding, in the same way as a fixed speed limit indicated on a normal traffic sign. The signs are also capable of showing a speed limit without the red border, in which case the limit is only advisory. Typically, some of the gantries - at regular intervals - will have safety cameras mounted above each lane, connected to the speed limit system and capable of having their trigger speed adjusted in line with the indicated limit.
On English motorways, the system is controlled by Highways England from a central control room. It is monitored by loops cut in the road to monitor traffic speed and flow, and by CCTV fitted on each gantry, and limits are ordinarily set by computer automation, though the system is continually monitored and an operator can override the system if necessary.
Non-motorway Variable Speed Limits
Away from the motorway network, Variable Speed Limits similar to those on motorways exist in several places, though each installation tends to be unique to its location.
One of the most motorway-like is on the A470 to the north of the M4 in the Welsh valleys. This is a Variable Speed Limit system modelled on the standalone installations used on English motorways, and indeed given that it has overhead matrix signals is actually more like other UK motorway Variable Speed Limit systems than the one installed nearby on the M4 at Newport.
In Greater London, several high capacity urban roads operated by Transport for London have Variable Speed Limit capability, though it is rarely deployed and tends to be used during planned maintenance tasks. These systems are not normally used to manage congestion. Broadly similar installations exist on the A102 Blackwall Tunnel, the A406 Lea Valley Viaduct and on the A12 between Hackney Wick and Redbridge. In each case rotating prism variable message signs are used. On the A12 installation, the road has overhead and central reservation-mounted MS1 signals capable of indicating advisory limits which are occasionally used to manage congestion; the Variable Speed Limit constitutes a second traffic control system that does not use the electronic matrix panels. This is likely to be because all these schemes predate the installation of the first modern Variable Speed Limit and therefore do not follow the conventions of more modern installations.
A further non-motorway use of Variable Speed Limits is currently specific to Scotland.
School 20mph limits
In Scotland, a form of Variable Speed Limit is now widely used on main roads outside schools. It imposes a legally enforceable 20mph limit during the times that pupils are to be expected to be making their way to and from the school. At other times the road's ordinary speed limit applies; typically this is 30mph but these limits are used in some places on roads with higher limits.
The Scottish Government has granted Roads Authorities a blanket authorization to use a system of low cost electronic flashing light signs in order to set up the Variable Speed Limits. The systems consist of a sign with flashing amber lights combined with a school sign, a 20mph speed limit sign and a plate reading "when lights flash". The authorisation is contained in the document ETLLD Circular No. 1 /2004.
On roads with a normal limit that is higher than 30mph, advance warning signs are required. The advance signs are LED matrix-based, and when activated show a representation of the triangular school warning sign with the text "20mph limit ahead".
Perhaps the earliest consideration of an official Variable Speed Limit was in early readings of the Road Traffic Act 1934, which introduced the 30mph urban speed limit to British roads and which creates the definition that still exists today between "restricted" and "derestricted" roads. It was originally intended that the 30mph limit was to protect pedestrians and other vulnerable road users, and was not because drivers were thought to be behaving unacceptably on the roads, so the original proposal was for it to only apply during the day. Between midnight at 5am, urban roads were to have no speed limit. This measure was deleted, and the 30mph limit operated 24 hours a day when the new law was passed, but it would have effectively been a very early Variable Speed Limit.
The earliest attempt to regulate traffic speed by use of signals may have been the introduction, in 1965, of the "Motorwarn" signals (a pair of flashing amber beacons running off a battery, with signals placed at intervals of one mile on motorways). The emergency legislation introduced by transport minister Tom Fraser created the blanket 70mph speed limit where there had been none before, and created an advisory 30mph limit when the Motorwarn signals were activated. This limit was only advisory, and so was not a Variable Speed Limit in the modern sense, but it appears to have been the earliest use of technology comparable to systems in use today. It was followed several years later by the introduction of permanent motorway signals which were also capable of showing advisory limits.
The first true motorway Variable Speed Limits were installed by the Highways Agency on the south-western quadrant of the M25 in the early 1990s and were an experimental system to alleviate stop-start traffic conditions caused by heavy traffic flows. The intention was that, by reducing vehicle speeds, headways between vehicles could be reduced and the throughput of the road would be greater, and by enforcing a lower maximum speed there was less incentive for vehicles to speed up and then brake as gaps in traffic opened and closed.
The Highways Agency issued a leaflet explaining the scheme to motorists:
"Reducing speeds when traffic is very heavy should actually improve the flow of traffic. This may help you reach your destination sooner than under the stop-start conditions currently experienced on this section of motorway at busy times. It should also make driving safer."
Initially the Variable Speed Limit system was subject to a 12-month trial, but evidently this was considered successful as it was quickly rolled out to a wider area.
A further Variable Speed Limit scheme was installed on the M20 Maidstone Bypass shortly before the Highways Agency began experimenting with Active Traffic Management around 2003. Variable Speed Limits were a part of that trial, and the subsequent technology scheme known as Smart Motorways include Variable Speed Limits as one of the core features.
Roads subject to Variable Speed Limits
|-||No||Smart Motorway (Under construction)|
|-||No||Smart Motorway (Expected completion June 2017)|
|-||Yes||Will eventually start from J24|
|-||No||Smart Motorway (Under construction, opening late 2016)|
|-||Yes||Section through Heathrow was added in 1995 as the first variable speed limit in the country.|
|-||Yes||Activated in 2009 (Westbound only)|
|-||Yes||Section between J3A and 7 operates as part of the Smart Motorway system and was activated in 2005. The other sections were activated in 2009.|
|-||No||Smart Motorway (Under construction)|
|-||No||Smart Motorway (Under construction)|
|Southbound approach to the M4||Yes|
|J3-Broadway||Yes||Continues as A12 Westlink|
|Broadway-Divis Street||Yes||Continues on M1|