What Are They?

Variable message signs, also known as dynamic or changeable message signs, are used to give travelers vital information while they are en route, usually while they are traveling at highway speeds. The message must be brief and convey information which a driver can react to and put to use quickly. Messages can be generated from a pre-existing library or customized for the situation. The signs can be fixed or portable, with most fixed signs deployed at facilities such as bridges, causeways, tunnels or toll plazas. Wireless communications developed over the last 10 years have made it much easier to program and change messages remotely. New designs using modular message blocks and “rail-mounted” connections that don't need wiring for each installation can make portable signs more adaptable. See our Telecommunications Diagram of VMS for more information.

Variable message signs are often used to direct travelers to more detailed information on highway advisory radio broadcasts. They can guide drivers through busy interchanges where they face many choices, such as New York City's double-deck George Washington Bridge. The signs are also frequently deployed around long-term construction projects, where road conditions are continually changing. On smaller streets, they can be used in combination with portable speed detectors to warn drivers when they are going too fast.

Message signs are used to communicate different types of information:

• about recurring events, such as alternate routes around bottlenecks during rush hours;
• about non-recurring events, such as construction, lane closures or detours;
• about roadside facilities and attractions such as parking availability and recommended routes for sporting and entertainment events;
• weather and other natural events such as rockslides or floods;
• traffic management operations such as the activation of HOV or HOT lanes, ramp meters or the presence of tolls and;
• the speed of a vehicle.

Key Results

A 1999 evaluation of variable message signs on the freeway network around Amsterdam showed that they caused traffic to flow more smoothly and thus increased the efficiency of the network.

Fiber optics and light-emitting diode (LED) lights have dramatically improved the usability and effectiveness of variable message signs, as have advanced sensors and algorithms. For example, the signs on the George Washington Bridge became dramatically more believable and effective when they incorporated loop detector data into a program to calculate and tell drivers real-time travel time estimates. Instead of saying “BEST ROUTE UPPER” or “LOWER,” the signs presented messages such as, “UPPER APPROX 5 MIN”/ “LOWER APPROX 10 MIN.”

Recent developments in VMS technology include travel time prediction systems such as TIPS (Traffic Information & Prediction System). TIPS is a portable automated system for predicting and displaying real-time travel time for motorists in advance of and through work zones. TIPS uses microwave radar sensors to detect vehicles in freeway lanes, a micorcontroller to calculate traffic volume and occupancy for each lane, and radio signals to transmit this traffic information to a personal computer. The PC runs TIPS software, which uses an intelligent traffic algorithm and travel-time estimation model to calculate travel times; the travel time information is then sent via radio signals to a VMS. A typical message generated by TIPS might read "28MIN TO END OF WORKZONE." According to a TIPS evaluation (which included 660 returned surveys), 97% of motorists felt that a system that provides real-time travel information in advance of work zones is helpful.

How They Affect Driver Behavior

A study conducted on the 600-km freeway network around Paris showed that message signs could cause a significant number of drivers to change their routes. For example, the longer the queue length posted on the message sign, the greater the number of drivers who diverted. Credibility and clarity are very important if variable message signs are to have an effect on drivers' behavior. Less information or no information at all is better than incorrect information. Also, messages that tell drivers what action to take, “prescriptive” signs, are more likely to cause drivers to change their behavior than messages that simply describe the situation. Drivers respond strongly to the selection of words, their sequence and format and the location and spacing of the signs.

Benefits

In general, it is difficult to measure the benefits of variable message signs, especially ones used in non-recurring situations, because there is no control to compare the results to. Their impact is usually too small to show up on conventional measures of an entire system's efficiency. Also, they are often used in conjunction with other systems, so their contribution is difficult to single out.

Costs

The signs themselves can be costly, especially in permanent fixtures. To be visible under all conditions, the signs need to use both light-emitting and light-reflecting technologies. Light-emitting devices degrade in heat, and reflectors degrade from exposure to ultraviolet rays, so both types require maintenance.

Implementation Challenges

The placement and the design of the message are critical to its usefulness. If a sign is not located sufficiently upstream from a decision-point, drivers won't be able to take action in response to the message. Too far from the decision-point, and drivers won't be able to remember what they were told. Extremely subtle differences can affect how well a message works. Texas researchers found, for example, that “NEXT RAMP CLOSED AUG 25-26” was not as useful to drivers as “US 290 EXIT CLOSED MON-WED.”

Where are they implemented?

As with highway advisory radio, the frequent companion technology for variable message signs, variable signs are often used to tell drivers about conditions in closed or fixed systems such as turnpikes, tunnels, bridges, airports and causeways. They are also used to tell drivers about changing conditions in long-term construction sites which are visited by many drivers repeatedly.

Author: Phyllis Orrick