A variable- message sign, often abbreviated VMS, CMS, or DMS, and in the UK known as a matrix sign,
is an electronic traffic sign

Traffic sign

Traffic signs or road signs are signs erected at the side of roads to provide information to road users. With traffic volumes increasing over the last eight decades, many countries have adopted pictorial signs or otherwise simplified and standardized their signs to facilitate international travel...

 often used on road

Road

A road is a thoroughfare, route, or way on land between two places, which typically has been paved or otherwise improved to allow travel by some conveyance, including a horse, cart, or motor vehicle. Roads consist of one, or sometimes two, roadways each with one or more lanes and also any...

ways to give travelers information about special events. Such signs warn of traffic congestion

Traffic congestion

Traffic congestion is a condition on road networks that occurs as use increases, and is characterized by slower speeds, longer trip times, and increased vehicular queueing. The most common example is the physical use of roads by vehicles. When traffic demand is great enough that the interaction...

, accidents, incidents, roadwork zones

Construction

In the fields of architecture and civil engineering, construction is a process that consists of the building or assembling of infrastructure. Far from being a single activity, large scale construction is a feat of human multitasking...

, or speed limit

Speed limit

Road speed limits are used in most countries to regulate the speed of road vehicles. Speed limits may define maximum , minimum or no speed limit and are normally indicated using a traffic sign...

s on a specific highway segment. In urban area

Urban area

An urban area is characterized by higher population density and vast human features in comparison to areas surrounding it. Urban areas may be cities, towns or conurbations, but the term is not commonly extended to rural settlements such as villages and hamlets.Urban areas are created and further...

s, VMS are used within parking guidance and information

Parking guidance and information

Parking guidance and information systems, or car park guidance systems, present drivers with dynamic information on parking within controlled areas. The systems combine traffic monitoring, communication, processing and variable message sign technologies to provide the service...

 systems to guide drivers to available car parking spaces. They may also ask vehicles to take alternative routes, limit travel speed, warn of duration and location of the incidents or just inform of the traffic conditions.

A complete message on a panel generally includes a problem statement indicating incident, roadwork, stalled vehicle etc; a location statement indicating where the incident is located; an effect statement indicating lane closure, delay, etc and an action statement giving suggestion what to do traffic conditions ahead. These signs are also used for AMBER Alert

AMBER Alert

An AMBER Alert or a Child Abduction Emergency is a child abduction alert bulletin in several countries throughout the world, issued upon the suspected abduction of a child, since 1996...

 and Silver Alert

Silver Alert

A Silver Alert is a public notification system in the United States to broadcast information about missing persons - especially seniors with Alzheimer's Disease, dementia or other mental disabilities - in order to aid in their return....

 messages.

In some places, VMSes are set up with permanent, semi-static displays indicating predicted travel times to important traffic destinations such as major cities or interchanges along the route of a highway.

On the interchange of I-5 and SR 120

California State Route 120

State Route 120 , in northern California, runs between the Central Valley near Manteca, through Yosemite National Park, and ends at U.S. Route 6 in Mono County.-Route description:...

 in San Joaquin County, California, an automated visibility and speed warning system was installed in 1996 to warn traffic of reduced visibility due to fog (where Tule fog

Tule fog

Tule fog is a thick ground fog that settles in the San Joaquin Valley and Sacramento Valley areas of California's Great Central Valley. Tule fog forms during the late fall and winter after the first significant rainfall. The official time frame for tule fog to form is from November 1 to March 31...

 is a common problem in the winter), and of slow or stopped traffic.

VMSes were deployed at least as early as the 1960s. The current VMS systems are largely deployed on freeways or trunk highways.

Typical messages provide the following information:

• Crashes, including vehicle spin-out or rollover
• Stalls affecting normal flow in a lane or on shoulders
• Non-recurring congestion, often a residual effect of cleared crash
• Closures of an entire road
• Downstream exit ramp closures
• Debris on roadway
  Road debris
  Road debris, a form of road hazard, is debris on or off a road. Road debris includes substances, materials, and objects that are foreign to the normal roadway environment...
  
  
• Vehicle fires
• Short-term maintenance or construction lasting less than three days
• Pavement failure alerts
• AMBER Alert
  AMBER Alert
  An AMBER Alert or a Child Abduction Emergency is a child abduction alert bulletin in several countries throughout the world, issued upon the suspected abduction of a child, since 1996...
  
  s and weather warnings via the warning infrastructure of NOAA Weather Radio
  NOAA Weather Radio All Hazards
  NOAA Weather Radio All Hazards is a network of radio stations broadcasting continuous weather information directly from a nearby National Weather Service office. It is operated by the NWS, an agency of the National Oceanic and Atmospheric Administration within the United States Department of...
  
  's SAME
  Specific Area Message Encoding
  Specific Area Message Encoding or SAME is the protocol used to encode the Emergency Alert System and NOAA Weather Radio's SAME Public Warning System in the U.S...
  
   system
• Travel times
• Variable speed limit
  Speed limit
  Road speed limits are used in most countries to regulate the speed of road vehicles. Speed limits may define maximum , minimum or no speed limit and are normally indicated using a traffic sign...
  
  s

The information comes from a variety of traffic monitoring and surveillance systems. It is expected that by providing real-time information on special events on the oncoming road, VMS can improve vehicles' route selection, reduce travel time, mitigate the severity and duration of incidents and improve the performance of the transportation network.

Portable variable-message signs

Truck-mounted VMSes are sometimes dispatched by highway agencies such as Caltrans to warn traffic of incidents such as accidents in areas where permanent VMSes aren't available or near enough as a preventative measure for reducing secondary accidents. They are often deployed in pairs so that the second VMS truck can take over when the traffic queue overtakes the first truck, forcing the first truck to circle around and mobilize itself further upstream from the queue where it is effective. An optional third truck, the team leader, may be utilized for driving by and monitoring the incident itself, traffic patterns and delay times, in order to make strategic decisions for minimizing delays—analogous to spotter planes used in fighting forest fires.

Trailer-mounted variable-message signs are used to alter traffic patterns near work zones, and for traffic management for sporting events, natural disasters, and other temporary changes in normal traffic patterns. The messages displayed on the sign can be programmed locally on the unit's control panel, or units equipped with a cellular modem can be programmed remotely via computer or phone. Most manufacturers produce trailers which comply with the National Transportation Communications for Intelligent Transportation System Protocol

National Transportation Communications for Intelligent Transportation System Protocol

The National Transportation Communications for Intelligent Transportation System Protocol is a family of standards designed to achieve interoperability and interchangeability between computers and electronic traffic control equipment from different manufacturers.The protocol is the product of a...

 (NTCIP) which allows the portable trailer to be integrated with an intelligent transportation system

Intelligent transportation system

The term intelligent transportation systems refers to information and communication technology that improve transport outcomes such as transport safety, transport productivity, travel reliability, informed travel choices, social equity, environmental performance and network operation...

. Trailer-mounted VMS can be equipped with radar, cameras, and other sensing devices as part of a smart work zone

Smart work zone

A smart work zone is a road construction zone in which technology is employed to increase safety and provide information. Smart work zones often use radar guns or other sensors to detect the presence and speed of vehicles approaching a work zone, in order to display an appropriate message on one...

 deployment.

External links

• Changeable Message Sign Operation and Messaging Handbook. Operations Office of Travel Management, Federal Highway Administration
  Federal Highway Administration
  The Federal Highway Administration is a division of the United States Department of Transportation that specializes in highway transportation. The agency's major activities are grouped into two "programs," the Federal-aid Highway Program and the Federal Lands Highway Program...
  
  , August 2004. Retrieved 2009-12-07.