903.12 Changeable Message Signs (MUTCD Chapter 2L)

903.12.1 Description of Changeable Message Signs (MUTCD Section 2L.01)

Support. A changeable message sign (CMS) is a traffic control device that is capable of displaying one or more alternative messages. Some CMS have a blank mode when no message is displayed, while others display multiple messages with only one of the messages displayed at a time (such as OPEN/CLOSED signs at weigh stations).

Dynamic message signs (DMS) are a type of CMS which are stationary traffic control devices capable of displaying one or more alternative messages that provide travelers with real-time, traffic-related variable messages. DMS are used to warn, regulate, route and manage traffic.

The provisions in EPG 903.12 apply to both permanent and portable changeable message signs with electronic displays or the electronic display portion of an otherwise conventional static sign. Additional provisions that only apply to portable changeable message signs (PCMS) can be found in EPG 616.12.5. Additional MoDOT provisions that apply to permanently-located DMS can be found in EPG 910.3.

The CMS is a traffic control device at all times regardless of the type of message being displayed. Accordingly, the limitations on design, format, and manner of display of a message conveyed on a conventional sign apply to CMS regardless of the type of message being displayed at any given time. Some of the general provisions regarding traffic control devices are reiterated in EPG 903.12. However, EPG 903.12 is not an independent or stand-alone reference for CMS. Users of CMS are expected to consult the other articles in the EPG for criteria on how to develop effective messages that comply with the EPG and that meet the expectancy and limitations of the road user. In this regard, the engineering processes applied to decisions about whether to use a particular sign, for example, are no different for the decisions about the type and content of the message under consideration for display on a CMS. The other limited-use messages allowed on CMS as provided for in EPG 903.12 likewise fall under the same MUTCD provisions as the primary-use traffic operation regulatory, warning, and guidance messages except as stated otherwise in EPG 903.12.

CMS messaging can be subject to habituation, a phenomenon by which repeated exposure to a stimulus results in diminished response. CMS habituation can occur through repeated exposure to messages, especially those messages that might not be perceived as having relevance to the road user, resulting in diminished responsiveness of the road user to that message. Because messages can be changed or extinguished, the effectiveness of CMS is tied more to the messages displayed thereon, the frequency of displayed messages, and the relevance to the road user, rather than to the installation of the signs themselves.

Guidance. Changeable message signs should be used judiciously to avoid habituation and preserve their effectiveness during the display of real-time messages about traffic conditions or traffic advisories.

Standard. No items other than inventory or maintenance-related information (see EPG 903.1.4) shall be displayed on the front or back of a CMS or portable CMS. Names or logos of the manufacturer, brand, or model shall not be displayed on a CMS or portable CMS, either in the message display itself or on the exterior housing.

Guidance. Blank-out signs that display only single-phase, predetermined electronic-display legends that are limited by their composition and arrangement of pixels or other illuminated forms in a fixed arrangement (such as a blank-out sign indicating a part-time turn prohibition, a blank-out or changeable lane-use sign, or a changeable OPEN/CLOSED sign for a weigh station) should comply with the provisions of the applicable Article for the specific type of sign, provided that the letter forms, symbols, and other legend elements are duplicates of the conventional messages as detailed in the “Standard Highway Signs” publication (see EPG 911 (MUTCD Section 1A.05)). Because such a sign is effectively an illuminated version of a conventional sign, the size of its legend elements, the overall size of the sign, and the placement of the sign should comply with the applicable provisions for the conventional version of the sign.

903.12.2 Applications of Changeable Message Signs (MUTCD Section 2L.02)

Standard. CMS shall display only traffic operational, regulatory, warning, and guidance information except as otherwise provided in EPG 903.12 and EPG 910.3. Advertising or other messages not related to traffic control shall not be displayed on a CMS or on its supports or other equipment.

Option. CMS may display traffic safety campaign messages (see EPG 903.12.7), transportation-related messages, emergency homeland security messages, and America’s Missing: Broadcast Emergency Response (AMBER) and Blue Alert messages, all as provided for in EPG 903.12 and 910.3.

Transportation-related messages for the purpose of improving traffic conditions, such as those providing information on alternative means of transportation, electronic toll collection, or carpooling may be displayed to remind or inform drivers of relevant options or opportunities for transportation.

Support. Messages regarding broader transportation items not related to improving traffic conditions, such as reminders of driver’s license or vehicle registration renewal, vehicle recall information, and vehicle maintenance, do not meet the purpose of a transportation-related message.

Examples of transportation-related messages include “STADIUM EVENT SUNDAY, DELAYS NOON TO 4 PM” and “OZONE ALERT—USE TRANSIT.”

Guidance. A CMS should not be used to display a transportation-related message if doing so could adversely affect respect for the sign. “CONGESTION AHEAD” or other overly simplistic or vague messages should not be displayed alone. These messages should be supplemented with a message on the location or distance to the congestion or incident, delay and travel time, alternative route, or other similar messages.

CMS should not be used in place of conventional signs for conditions that do not change, except for blank-out type signs used to display regulatory, warning, and guidance information that routinely reoccurs, but only on a part-time basis. Similarly, when only certain elements of a message on a non-changeable sign are subject to change, only those elements of the sign should be in an electronic display.

Support. The purpose of CMS is to provide real-time traffic regulatory, warning, or guidance messages as follows:

A. Incident management and route diversion;

B. Warning of adverse roadway travel conditions due to weather;

C. Special event applications associated with traffic control or conditions;

D. Lane, ramp, and roadway control;

E. Priced or other types of managed lanes;

F. Travel times;

G. Warning situations;

H. Traffic regulations;

I. Speed control or warning;

J. Variable destination guidance;

K. Supporting temporary traffic control; or

L. Active Traffic Management.

CMS provide significant flexibility and capability in communicating many types of real-time traffic control messages to road users. While their intended purpose is the display of traffic regulatory, warning, or guidance information, other limited uses are also allowed under certain conditions, as provided in EPG 903.12. Their integrity as an official traffic control device rests significantly on their judicious use and proper messaging format and content, regardless of the message type being displayed.

Standard. MoDOT shall issue and maintain a policy regarding the use and display of all types of messages to be used on their CMS. The policy shall define the types of messages that will be allowed, the priority of messages, the proper syntax of messages, the timing of messages, and other important messaging elements to ensure messages displayed meet the basic principles that govern the design and use of traffic control devices in general (see EPG 911 (MUTCD Section 1D.01)) and traffic signs in particular as provided for in the EPG.

Guidance. When CMS are used at multiple locations to address a specific situation, the message displays should be consistent along the roadway corridor and adjacent corridors, which might necessitate coordination among different operating agencies.

AMBER and Blue Alerts (see the first option paragraph of this Article), when displayed, should not preempt messages related to traffic or travel conditions. AMBER and Blue Alert messages should be kept as brief as possible and, when possible, direct road users to another source, such as broadcast or highway advisory radio, for detailed information about the alert.

Support. EPG 910.3.2.15 contains procedures for Child Abduction, Blue Alert, and Ozon Alert messages.

Standard. Types of “alert” messages other than AMBER and Blue alerts that are unrelated to traffic or travel conditions shall not be displayed on CMS.

The format of CMS displays shall not be of a type that could be considered similar to advertising or promotional displays.

Support. In times of a declared state of emergency, it might be appropriate to display messages related to evacuation, homeland security, or emergency information. Traffic patterns, movement, or other situations might be atypical due to the emergency, necessitating unique messaging not specifically related to traffic conditions.

Standard. Homeland security and emergency messages shall only be displayed in declared states of emergency when there is an imminent threat to the general population. Generic security or personal safety messages shall not be displayed when there is no context of a declared state of emergency or known imminent national security threat. Homeland security and emergency messages shall not be promotional or advisory in nature, including the message design, layout, or manner of display.

Guidance. Homeland Security and emergency messages should undergo significant levels of scrutiny prior to being approved for broadcast to ensure accuracy and consistency with emergency conditions. These messages should be designed to convey a clear and simple meaning in a similar format to traffic control messages.

Support. EPG 903.2.20 contains information regarding the design of CMS that are used to display variable speed limits that change based on ambient or operational conditions on the variable Speed Limit (R2-1) sign.

EPG 903.3.13 contains information regarding the design of CMS that are used to display the speed at which approaching vehicles are traveling on the Vehicle Speed Feedback (W13-20 and W13-20aP) sign and plaque.

EPG 912 (MUTCD Section 5B.01) contains provisions for LEDs used in electronic-display signs to accommodate driving automation systems.

903.12.3 Legibility and Visibility of Changeable Message Signs (MUTCD Section 2L.03)

Support. The maximum distance at which a driver can first correctly identify letters and words on a sign is called the legibility distance of the sign. Legibility distance is affected by the characteristics of the sign design and the visual capabilities of drivers. Visual capabilities, and thus legibility distances, vary among drivers.

For the more common types of CMS, the longest measured legibility distances on sunny days occur during mid-day when the sun is overhead. Legibility distances are much shorter when the sun is behind the sign face, when the sun is on the horizon and shining on the sign face, or at night.

Visibility is the characteristic that enables a CMS to be seen. Visibility is associated with the point where the CMS is first detected, whereas legibility is the point where the message on the CMS can be read. Environmental conditions such as rain, fog, and snow impact the visibility of CMS and can reduce the available legibility distances. During these conditions, there might not be enough viewing time for drivers to read the message.

Guidance. CMS used on roadways with speed limits of 55 mph or higher should be visible from ½ mile under both day and night conditions. The message should be designed to be legible from a minimum distance of 600 feet for nighttime conditions and 800 feet for normal daylight conditions. When environmental conditions that reduce visibility and legibility are present, or when the legibility distances stated in the previous sentences in this paragraph cannot be practically achieved, messages composed of fewer units of information should be used and consideration should be given to limiting the message to a single phase (see EPG 903.12.5 for information regarding the lengths of messages displayed on CMS).

The electronic display of standardized regulatory and warning signs used individually or as part of the legend for a larger sign should meet the size and legend requirements for those specific signs in EPG 903.2 and 903.3.

903.12.4 Design Characteristics of Messages (MUTCD Section 2L.04)

Standard. Except as provided in the following paragraph, messages shall not include animation, flashing, dissolving, exploding, scrolling, or other dynamic display elements.

When a portable CMS is used as an arrow board that uses a flashing or sequential display for a lane or shoulder closure, the display and operation shall be considered that of an arrow board and shall comply with the provisions of EPG 616.12.5 and 616.12.6.

Guidance. In developing messages for display on CMS, the provisions of EPG 911 (MUTCD Section 1D.01) should be consulted for the principles of an effective traffic control device.

Standard. All message displays on CMS, whether for traffic operational, regulatory, warning, or guidance information, or for the other allowable message types as defined in EPG 903.12, shall follow the same design and display principles found in the EPG used for other traffic control signs, except as provided elsewhere in EPG 903.12.

Guidance. Except in the case of a limited-legend CMS (such as a blank-out or a part-time regulatory sign display) that is used in place of a conventional regulatory sign or an activated blank-out warning sign that supplements a conventional warning sign at a separate location, the signs should be used as a supplement to and not as a substitute for conventional signs and markings unless otherwise provided for in the EPG.

Support. When CMS are overused for messages not directly associated with real-time driving conditions, road users might pay less attention to the sign, thereby limiting their effectiveness as traffic control devices.

Guidance. Warning Beacons (see EPG 902.18.3) should not be installed on CMS, rather CMS should be used predominately to display messages that are critical to real-time travel conditions. CMS word messages should be limited to no more than three lines, with no more than 20 characters per line.

The spacing between characters in a word should be between 25 and 40 percent of the letter height. The spacing between words in a message should be between 75 and 100 percent of the letter height. Spacing between the message lines should be between 50 and 75 percent of the letter height. Table 903.12.4 contains information for spacing between characters, words, and lines of text.

Table 903.12.4 Spacing between Message Characters, Words, and Lines of Text

Height of Letters Used on CMS	Spacing between Characters in Words	Horizontal Spacing between Words	Vertical Spacing between Lines of Text
12	3 - 5	9 - 12	6 - 9
18	4 ½ - 7	13 ½ - 18	9 - 13 ½
Note: All units are in inches

Except as otherwise provided in the EPG, word messages on CMS should be composed of all upper-case letters. The minimum letter height should be 18 inches for CMS on roadways with speed limits of 45 mph or higher. The minimum letter height should be 12 inches for CMS on roadways with speed limits of less than 45 mph. When a message is composed of two phases and higher informational load (see EPG 903.12.5), the letter height should be 18 inches, regardless of the speed limit, to optimize legibility distance and available viewing time.

Option. CMS used to replicate a conventional sign may use the character size of the conventional sign being replicated.

Support. Using letter heights of more than 18 inches will not result in proportional increases in legibility distance.

Guidance. The width-to-height ratio of the sign characters should be between 0.7 and 1.0. The stroke width-to-height ratio should be 0.2.

Support. The width-to-height ratio is commonly accomplished using a minimum font matrix density of five pixels wide by seven pixels high.

Standard. CMS shall automatically adjust their brightness under varying light conditions to maintain legibility.

Guidance. The luminance design of a CMS should meet industry criteria for daytime and nighttime conditions. Luminance contrast design should be between 8 and 12 for all conditions.

Support. CMS maintenance and replacement practices might need to account for the reduction of LED luminance and luminance contrast that occurs naturally over time and might substantially impact legibility.

Guidance. Contrast orientation of CMS should always be positive, that is, with luminous characters on a dark or less-luminous background.

Support. Legibility distances for negative-contrast CMS are likely to be at least 25 percent shorter than those of positive-contrast messages. In addition, the increased light emitted by negative-contrast CMS has not been shown to improve detection distances and might visually overwhelm the darker characters of the sign legend.

Standard. The colors used for the legends and backgrounds on CMS shall be as provided in Table 903.1.6.

Guidance. When displaying color messages use of full background color should be avoided in nighttime conditions. Full background color at night can cause a washout effect of the text or other image being displayed.

Standard. Except as provided for in the following paragraph, if a black background is used, the color used for the legend on a CMS shall match the background color that would be used on a standard sign for that type of legend as specified in Table 903.1.6.

Option. CMS that use only yellow or amber LEDs may display a yellow or amber legend that does not match the background color used on a standard sign for that type of legend as specified in Table 903.1.6.

Standard. If a green background is used for a guide message on a CMS or if a blue background is used for a motorist services message on a CMS, the background color shall be provided by green or blue lighted pixels such that the entire CMS would be lighted, not just the white legend.

Support. Some CMS that employ newer technologies have the capability to display a near duplicate of a standard sign or other sign legend using standard symbols, the Standard Alphabets and letter forms, route shields, and other typical sign legend elements with no apparent loss of resolution or recognition to the road user when compared with a conventional version of the same sign legend. Such signs are of the full-matrix type and can typically display full-color legends. Figure 903.12.4 shows comparative examples of the effects of varying pixel densities on legend form.

Guidance. If used, the CMS described in the previous paragraph should not display symbols or route shields unless they can do so in the appropriate legend and background color combinations. Where an LED matrix is used to form the changeable legend, signs with pixel spacing greater than 20 millimeters should display only word legends and no symbols or route shields.

For a single-phase message where the Standard Alphabets and other legend elements of standard designs are used, the lettering style, size, and line spacing should comply with the applicable provisions for the type of message displayed as provided elsewhere in the EPG. For two-phase messages, larger legend heights should be used as described previously in this Article because of the need for such messages to be legible at a greater distance. Regardless of the number of phases, the CMS should comply with the legibility and visibility provisions of EPG 903.12.3.

Support. EPG 910.3.2.8.1 includes additional information regarding Design Characteristics of Messages for DMS.

Figure 903.12.4 Example of Capability to display Sign Legends Based on Pixel Pitch

Notes:

1. Pixel pitch is the distance from the center of a pixel to the center of an adjacent pixel.
2. The pixel pitch is described in Metric units because sign manufacturers only use Metric units.

903.12.5 Message Length and Units of Information (MUTCD Section 2L.05)

Guidance. The maximum length of a message should be dictated by the number of units of information contained in the message, in addition to the size of the CMS. A unit of information, which is a single answer to a single question that a driver can use to make a decision, should not be more than four words.

Table 903.12.5 Example of Units of Information

Question	Answer	Number of Information Units
What happened?	MAJOR CRASH	1
Where?	AT EXIT 12	1
Who is the advisory for?	Drivers heading TO NEW YORK	1
What is Advised?	USE ROUTE 46	1
Note: the following is an example of a two-phase message that could be developed from the four information units shown in this table:

Support. In order to illustrate the concept of units of information, Table 903.12.5 shows an example message that is comprised of four units of information.

The maximum allowable number of units of information in a CMS message is based on the principles described in this Article, the current highway operating speed, the legibility characteristics of the CMS, and the lighting conditions.

Standard. Each message shall consist of no more than two phases. A phase shall consist of no more than three lines of text. Each phase shall be understood by itself, and the meaning of the entire message shall be the same, regardless of the sequence in which the phases are read. Each line of legend shall be centered on the sign. If more than one CMS is visible to road users, then only one sign shall display a sequential message at any given time.

Option. A legend on a CMS that replicates a legend on a conventional sign that would not normally be center justified may be left justified or right justified as appropriate, such as a travel time display.

Standard. Abbreviations displayed on CMS shall comply with the provisions of EPG 911 (MUTCD Section 1D.08).

Guidance. When designing and displaying messages on CMS, the following principles should be used:

A. The minimum time that an individual phase is displayed should be based on 1 second per word or 2 seconds per unit of information, whichever produces a lesser value. The display time for a phase should never be less than 2 seconds.

B. The maximum cycle time of a two-phase message should be 8 seconds.

C. The duration between the display of two phases should not exceed 0.3 seconds.

D. No more than three units of information should be displayed in a message phase.

E. No more than four units of information should be in a message when the traffic operating speeds are 35 mph or more.

F. No more than five units of information should be in a message when the traffic operating speeds are less than 35 mph.

G. Only one unit of information should appear on each line of the CMS.

Support. Table 903.12.5 provides an example of the number of units of information in a message.

Option. A unit of information consisting of more than one word may be displayed on more than one line. An additional CMS at a downstream location may be used for the purpose of allowing the entire message to be read twice.

If more than two phases would be needed to display the necessary information, additional CMS may be used to display this information as a series of two distinct, independent messages with a maximum of two phases at each location, in accordance with the provisions of the first Standard paragraph of EPG 903.12.5.

Support. Figures 913.12.5.1 and 903.12.5.2 provide examples of message construction for CMS. Each example shows the message content, layout, and phasing for a potential message and an improved message. The improved message for each example has been optimized for recognition, comprehension, and effectiveness.

See EPG 910.3.2.8.1 for additional information regarding Message Length and Units of Information for DMS.

Figure 903.12.5.1 Examples of Message Construction for CMS

*Examples shown are for single-color CMS with pixel spacing greater than 20 mm and use all upper-case lettering. Multi-color, fullmatrix CMS with pixel spacing of 20 mm or less should use upper- and lower-case lettering where appropriate and proper legend and background colors.

Figure 903.12.5.2 Examples of Message Construction for CMS

*Examples shown are for a portable CMS where the display width is generally limited to 8 characters per line of legend.

903.12.6 Travel Time Messages (MUTCD Section 2L.06)

Support. Travel times provide road users useful information about the level of congestion on segments of highways where motorists experience frequent incidents that slow traffic. Travel times are only helpful to the road user if they have a general understanding of the length of the road segment the travel time is related to so that they can compare that to the time it takes them to travel a similar distance on a highway without congestion. However, travel time messages require road users to read and process a significant amount of information and careful consideration is needed to ensure the overall message is not overloading the motorist.

Guidance. Travel times should be tied to the distance to a particular destination or junction so that road users can estimate the level of congestion based on the time to travel that distance. When travel times are displayed on CMS, such as during peak traffic conditions, the message should comply with the provisions of Sections EPG 903.5.47 and 903.5.48. If both a travel time and a distance are displayed, the sign should display only one destination. A distance displayed as part of a travel time message should be rounded to the nearest whole mile.

Option. When comparative travel time displays are used providing travel times on different routes to one destination, distances to that destination may be eliminated.

A reference-location-based exit number (see EPG 903.5.22) may be displayed in lieu of a destination name or junction thereby providing the necessary distance information to the road user. If reference-location-based exit numbers are displayed, then up to two travel times may be displayed provided that the distance to the exit is not also displayed.

Support. EPG 910.3.2.12 includes information regarding Congestion Messages for DMS.

903.12.7 Traffic Safety Campaign Messages (MUTCD Section 2L.07)

Support. An allowable ancillary use of CMS is the display of traffic safety messages in conjunction with a traffic safety campaign that includes other forms of media as the primary communication and education mechanism.

Standard. Traffic control messages shall have priority over traffic safety campaign messages.

Guidance. When a CMS is used to display a traffic safety campaign, the message should be simple, direct, brief, legible, and clear (see EPG 911 (MUTCD Section 1D.01)). Traffic safety campaign messages should be relevant to the road user on the roadway on which the message is displayed. For example, messages regarding school bus stop safety should not be displayed on freeways where school bus stops are not found.

A CMS should not be used to display a traffic safety campaign message if doing so could adversely affect respect for the sign. Messages with obscure or secondary meanings should not be used as they might be misunderstood or understood only by a limited segment of road users and require greater time to process and understand. Similarly, slogan-type messages and the display of statistical information should not be used.

The broad traffic safety campaign marketing message should be appropriately shortened or otherwise modified to comply with the provisions of EPG 903.12.5 when a traffic safety campaign message is displayed on a CMS.

Traffic safety campaign messages should emphasize the applicable regulation or warning and should reference any penalties associated with violations of the regulation. Traffic safety campaigns using CMS should include coordinated enforcement efforts where penalties or enforcement type warnings are part of the message displayed on the CMS.

Traffic safety campaign messages should not be displayed on CMS unless they are part of an active, coordinated safety campaign that uses other media forms as the primary means of outreach. For consistency on a national level, traffic safety campaigns should be coordinated with those on the National Highway Transportation Safety Administration’s annual communications calendar or similarly planned sate campaigns.

Support. Examples of traffic safety campaign messages include “UNBUCKLED SEAT BELTS FINE + POINTS” and “IMPAIRED DRIVERS LOSE LICENSE + JAIL.”

EPG 910.3.2.16 includes additional information related to Driver Safety Campaigns.

903.12.8 Permanently-Located Changeable Message Signs (MUTCD Section 2L.08)

Support. See EPG 910.3.2.6 for information regarding Permanently-Located Changeable Message Signs and Dynamic Message Signs (DMS).