Knowledge Resources

An examination of databases and a sampling of locations reveal at least 12 metropolitan areas that are providing travel time messages on DMS. But at least 25 other metropolitan areas are gathering travel time data and have DMS deployed. A list of all of these metropolitan areas can be found at the end of this lesson. Travel time messages are not appropriate for every location, but they have proven successful in regions or corridors that experience periods of recurring congestion - congestion generally resulting from traffic demand exceeding available capacity and not caused by any specific event such as a traffic incident, road construction or a lane closure. The DMS can provide dynamic travel time information instead of providing generic messages such as "congestion ahead" or "stay alert."

While travel time messages may be overridden by traffic incident or road construction messages, they can provide valuable motorist information in conjunction with the event messages, as well as after the incident or construction has been cleared if there is residual congestion. Also, special events that typically generate traffic demand that exceeds capacity - fairs, concerts, sporting events - provide additional opportunities for providing travel time information to motorists.

The areas that have been providing travel time messages have found solid public support for the messages. Their experiences have provided a number of recommendations summarized here from the guidance report referenced below.

• Seek feedback from and educate the public before starting to post travel time messages. A campaign of public awareness is critical in order for the time messages to have an initial positive effect. New types of messages often cause motorists to slow down, so any efforts to reduce those "surprise" effects will help motorists more easily adapt to the new messages. Also engaging the public and the media in helping determine destinations and message forms will improve the quality of service and help achieve a positive response to the messages.
• Generate travel times automatically. Travel times should be generated automatically and not require a human operator to manually enter travel time data. All but one of the locations surveyed that provide travel time messages use automated processes to calculate the travel times. They use different technologies to measure the traffic flow, including loop detectors, video detection systems, automatic vehicle identification transponders and toll tags. The traffic data are processed to produce travel time over specified links between identified destinations. It is important to note that effective travel time messages do not require the data to be 100% accurate. Research has indicated that data with error rates of 20% produce useful traveler information. When presenting a range of travel times on DMS the acceptable error rate may be even higher.
• Construct travel time messages to benefit more than the local commuter where there is a mixture of types of travelers. Successful practices from the Atlanta area demonstrate that a relatively simple change to local information can benefit unfamiliar travelers as well. By including the distance to the destination in addition to the travel time, even those travelers unfamiliar with the area can determine the approximate level of congestion ahead.
• Maintain credibility of DMS messages by changing them in a timely manner and by validating the message displayed (1). Displaying messages that are inaccurate leads to motorist confusion and can adversely affect both traffic flow and the transportation agency’s credibility.

As stated in the January 19, 2001, Policy Memorandum, "Use of Changeable Message Sign (CMS)" (http://www.fhwa.dot.gov/legsregs/directives/policy/pame.htm) and reiterated by subsequent policy memorandums in 2002 ("AMBER Alert Use of Changeable Message Sign" - http://www.fhwa.dot.gov/legsregs/directives/policy/ambermemo.htm) and 2003 ("Use of Changeable Message Sign for Emergency Security Messages" - http://www.fhwa.dot.gov/legsregs/directives/policy/securmemo.htm), FHWA supports the use of dynamic message signs as traffic control devices to safely and efficiently manage traffic by informing motorists of roadway conditions and required actions to perform. It is FHWA policy that the appropriate use of DMS and other types of real-time displays should be limited to managing travel, controlling and diverting traffic, identifying current and anticipated roadway conditions, or regulating access to specific lanes or the entire roadway. But it is also important that these assets and investments be used more effectively to provide motorists with meaningful and useful information. Providing travel time information is an excellent method of notifying motorists about current conditions in a manner that can be easily interpreted and understood. Following these guidelines for providing travel times and ensuring the DMS messages are credible will increase the use of the information provided by Dynamic Message Signs as well as increasing customer satisfaction with this information.

Metropolitan Areas Providing Travel Time Messages on Dynamic Message Signs:

References:

(1) Johnson, Christine M., Policy Memorandum on the Use of Changeable Message Signs, January 19, 2001.
Document:http://www.fhwa.dot.gov/legsregs/directives/policy/pame.htm