Limit CMS message length to allow for adequate reading time at high speeds.
Best practices for use of CMS shared from several states.
Made Public Date


United States

Amber, Emergency, and Travel Time Messaging Guidance for Transportation Agencies


In May 2004, FHWA published guidance on the use of changeable message signs (CMS) which included: recommendations on the display of AMBER alerts, emergency messages, and information on travel times. The purpose of this study was to provide operational guidance to transportation agencies as to the use of changeable message signs to display travel time information, emergency or security alerts and child abduction alerts. Information was gathered through a series of interviews conducted with representatives from State Departments of Transportation (DOTs) and Federal Highway Administration Division Offices.

One issue that was addressed in the document was the use of AMBER alerts and how the majority of states were broadcasting these alerts using CMS. The purpose of the guidance was to provide best practice information to transportation agencies to improve the use and consistency of CMS messaging in the United States.

Lessons Learned

Changeable Message Signs (CMS) are clearly an important device in aiding in the safe and efficient movement of people and goods through the transportation network. CMS is an outstanding example of ITS using computing and communications technologies to support traffic management and provide travel information directly to the audience that needs it most. While CMS have been in use for years, improving technology and a changing climate has necessitated, or provided the opportunity for, greater and more diverse uses of CMS. However, there is a balance to be struck between the variety of new uses possible for CMS with practices that are best suited to the use of these devices. CMS for the use of travel times, homeland security and AMBER Alerts are still, to varying extents, new applications for these devices. One overall theme in the document was to recognize the requirements of the motorists and respect them. Others can benefit from the experience of those that have deployed CMS by limiting the length of CMS messages to allow adequate reading time at high speeds.

Based on the experiences of several states, a few lessons have emerged related to providing an adequate and reliable message length that can serve as a basis for guidelines on CMS operations:

  • Recognize the expectations of travelers when receiving information via CMS. We live in an information world and are constantly bombarded with information delivered through many different media. Consumers have come to recognize bogus information and quickly label the source of the bogus information as unreliable. To work in this environment successfully, agencies are urged to maintain CMS to keep them in top condition (no burnt out sections on CMS) so that they function properly.
  • Provide adequate information to ensure proper driver action. When providing information that you want the driver to respond to, don’t just tell them what to do, but why the action is needed. An example: "right lane closed" versus "right lane closed- accident ahead" provides the driver with a sense of urgency and full understanding of the required actions.
  • Limit messages to account for restricted driver reading times. The majority of CMS are located on high speed facilities, as a result agencies need to limit the use of paging (requiring drivers to read more than one screen of information) and recognize the limitations of drivers to read and react to long messages when traveling at high speeds. Recommendations have been made that drivers require 1 second per word (not including prepositions) to read a CMS message. An Arizona State University study found that fiber optic CMS provide average legibility at 835 feet. That number is reduced to 685 feet when taking into account the loss of visibility due to the roof of the car, leaving about 6-7 seconds of reading time when traveling between 65 and 75 mph, suggesting a maximum message length of 6-7 words (not including prepositions) to provide adequate reading time to drivers.
  • Always work to build credible and useful information. The value of CMS and the messages they display significantly influences their credibility.

This lesson suggests that agencies should take these guidelines into consideration when designing, deploying and operating CMS to achieve optimal benefits. Optimal benefits will help achieve ITS goals that include: improved safety, mobility and customer satisfaction by providing adequate, concise messages that can be read at high speeds allowing the traveling public to make informed decisions based on reliable information.

Amber, Emergency, and Travel Time Messaging Guidance for Transportation Agencies

Amber, Emergency, and Travel Time Messaging Guidance for Transportation Agencies
Publication Sort Date
US Department of Transportation

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