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OCR for page 41
HFG CHANGEABLE MESSAGE SIGNS Version 1.0
DETERMINING APPROPRIATE MESSAGE LENGTH
Introduction
Determining the appropriate message length for a CMS refers to choosing a message length that drivers have the time to
comprehend as they pass the sign. Controlling message length is extremely important because there is a limited amount of time
to present information to drivers. Message length is described not only by the absolute length in the number of words, but also
by the number of information units included in these words. Information units are a measure of the message load, or total amount
of information in the message. If there are too many words or information units in a message, it may need to be split into two
phases. Dudek (1) provides additional guidance for reducing message length and splitting long messages.
Design Guidelines
Message
Guidelines Example (from Dudek (1 ))
Property
Information Use no more than:
Units: 2 information units per line
A measure of the Answer
amount of 3 information units per phase Question
(see below) (1 information unit)
information
presented in terms 4 information units per What is the problem? MAJOR ACCIDENT
of facts used to message read at speeds of
Where is the problem? AT US-23
make a decision; a 35 mi/h or more
single information 5 information units per Who is the message for? NEW YORK
unit consists of message read at speeds less What should they do? USE I-280 EAST
1 to 4 words than 35 mi/h
Length: Use no more than:
Number of words Eight words per message for
or characters in a drivers at high speeds
message, (based on the required reading
excluding time of 1 s per four- to eight-
prepositions character word, excluding
prepositions, or 2 s per Acceptable message length because the preposition "to" does
information unit, whichever is not count.
longest)
Phases: Two phases maxi mum per Poorly designed message:
Similar to a page message
of a book, a phase Each phase must be able to be
is the text that is understood alone
displayed at a
single point in time One line should not contain
parts of 2 information units
but may contain 2 whole Improved message:
information units
When dividing messages
between two phases,
compatible information units
should be kept on the same
phase
Based Primarily on Based Equally on Expert Judgment Based Primarily on
Expert Judgment and Empirical Data Empirical Data
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OCR for page 42
HFG CHANGEABLE MESSAGE SIGNS Version 1.0
Discussion
Information units: The recommendations for the number of information units that are appropriate for display are based on
research and operational experience (1). Dudek (2) summarizes that 1 s is needed per four- to eight-character word excluding
prepositions or 2 s per information unit, whichever is longer. Using this assumption, the required viewing distance for different
numbers of information units, for drivers traveling at different speeds, are included below.
REQUIRED VIEWING DISTANCE PER NUMBER OF INFORMATION UNITS AT VARYING SPEEDS (FROM DUDEK (1 ))
However, the MUTCD (3) states that the minimum phase display time should be based upon 1 s per word or 2 s per information
unit, whichever is shorter. This direct contradiction of Dudek (2) causes practical consequences for drivers. If the longer time is
used (2) and the message includes many short words that do not all need to be remembered, the phase display time could be
unnecessarily inflated. If the shorter time is used (3) and the message includes information units composed of multiple important
words, drivers may not have time to read all of them. If longer messages need to be provided, they should be shown in
conjunction with other information media (See When to Use Changeable Message Signs, 19-2).
Length: Dudek (1) states that the appropriate absolute message length is affected by (1) the amount of time that the driver is in
the legibility zone of the CMS, considering travelling speed and environmental conditions; (2) the driver workload including all
driver activities such as reading signs, lane positioning, etc.; and (3) message familiarity because drivers take more time to read
unfamiliar content or unusual messages
The eight-word maximum for high speeds is based on the legibility distance, or the distance at which the words on the sign
become legible, as well as the speed that the driver is travelling. This recommendation assumes drivers are traveling at 55 mi/h
and the legibility distance of the sign is 650 ft (which is the approximate legibility distance for a lamp matrix sign with 18-in.
character heights) (2). If the message is too long for drivers to read at normal speeds, it is likely that some drivers will slow
down to be able to read the message, affecting the traffic flow (1). In general, the message length should be reduced as much as
possible without losing the message intent (1). The message length can be reduced by the use of alternative phrases or
appropriate abbreviations and the removal of redundant and unimportant information.
Phases: Dudek (1) reports that research has shown drivers have difficulty reading messages that are on more than two phases.
Because either the first phase or the second phase may be read first by a passing driver, each phase should make sense by itself.
This is accomplished by keeping compatible information units in the same phase. In addition, portions of two different
information units should not be displayed on a single line because it is confusing to drivers and increases reading time (1).
Design Issues
The legibility distance for a CMS is affected by a number of factors. If the sign is placed off to the side of the roadway rather
than directly over the travel lanes, additional sight distance is required (1) because a driver's field of view is assumed to be
between 10° right and left of head-on. Proffitt and Wade (4) support rotating the CMS 5° to 10° toward the roadway to increase
the amount of time that roadside signs are at an optimal reading angle. However, conflicting ideas exist regarding the assumed
angular range of the legibility distance. This distance is also affected by lighting conditions, sun position, vertical curvature,
horizontal curvature, spot obstructions, rain, fog, and trucks in the traffic stream (1). If the legibility distance of the sign is
reduced, then the time that the driver has to read the sign is reduced, necessitating a reduction in the number of information units
contained on the sign.
Cross References
Key Components of Sight Distance, 5-2 Presentation to Maximize Visibility and Legibility, 19-4
Changeable Message Signs, 13-6 Composing a Message to Maximize Comprehension, 19-8
When to Use Changeable Message Signs, 19-2 Displaying Messages with Dynamic Characteristics, 19-10
Key References
1. Dudek, C.L. (2004). Changeable Message Sign Operation and Messaging Handbook. (FHWA-OP-03-070). College Station: Texas Transportation Institute.
2. Dudek, C.L. (1992). Guidelines on the Use and Operation of Changeable Message Signs . (FHWA-TX-92-1232-9). College Station: Texas Transportation
Institute.
3. FHWA (2009). Manual on Uniform Traffic Control Devices for Streets and Highways . Washington, DC.
4. Proffitt, D.R., and Wade, M.M. (1998). Creating Effective Variable Message Signs: Human Factors Issues. (VTRC 98-CR31). Charlottesville: Virginia
Transportation Research Council.
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