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HFG SIGNING Version 1.0
COMPLEXITY OF SIGN INFORMATION
Introduction
The complexity of sign information refers to the number of information units presented as part of a roadway sign
message. In this context, an information unit can describe geography (e.g., city), type of roadway (e.g., highway),
event causes (e.g., stalled vehicle), event consequences (e.g., traffic jam), time and distances, and proposed actions.
Therefore, information units can be described as the relevant words in a message. Much of the guideline
information presented below has been adapted from Campbell, Carney, and Kantowitz (1).
Design Guidelines
Messages that require an urgent action should be a single word or a short sentence with the fewest number of
syllables possible. Drivers should be able to understand the message immediately.
Messages that are not urgent or for which a response may be delayed can be a maximum of 7 units of
information in the fewest number of words possible. If the information cannot be presented in a short
sentence, the most important information should be presented at the beginning and/or the end of the message.
Navigation instructions should be limited to 3 or 4 information units.
Based Primarily on Based Equally on Expert Judgment Based Primarily on
Expert Judgment and Empirical Data Empirical Data
DETERMINING THE NUMBER OF INFORMATION UNITS
4 units Road Construction Ahead
at Jaspertown
8 units Road Construction on Interstate 5
for next 10 miles
Take Highway 99
11 units Interstate 80 closed for construction
between Iowa City and Cedar Rapids
Exit at West Liberty
and drive north on Highway 16
16 units Accident Ahead
Exit 215 closed to Dover
Traffic detoured to Exit 216
Follow Highway 46 to Chester
and turn east onto Inglenook Road
EFFECTS OF INFORMATION COMPLEXITY
Length of Message
3-4 units 6-8 units 10-12 units 14-18 units
Duration of Each Glance 1.08 s 1.18 s 1.20 s 1.35 s
Number of Glances 3.8 6.9 9.6 15.5
Memory Recall 100% 97.5% 75.4% 52.4%
Source: Labiale (2)
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HFG SIGNING Version 1.0
Discussion
The longer the message, the more processing time the driver requires. Therefore, messages that require the driver to
make an immediate response should be as short as possible. One-word messages informing the driver of the
appropriate action to take might work best in these situations. As the response required by the driver becomes less
and less urgent, the messages can become more detailed; however, an effort should still be made to make the
messages as concise as possible.
Zwahlen, Adams, and DeBald (3) analyzed the number of lane deviations that occurred while drivers were operating
a CRT touch screen. The results suggest that the number of glances away from the roadway should be limited to
three and that glance durations that exceeded 2 s in duration are unacceptable. Zwahlen et al. (3) examined the
amount/complexity of information necessary for evoking these unsafe glance frequencies and durations. The results
of this on-road study suggest that although the duration of glances does not increase dramatically as the number of
information units increase, the number of glances does. Therefore, the shortest information message (3 to 4 units)
would be the most appropriate for keeping drivers' attention on the forward roadway. The driver's ability to recall
information was also examined in Labiale (2): only 75% of a 10- to 12-unit message could be recalled, in
comparison to 100% of a 3- to 4-unit message and 98% of a 6- to 8-unit message. This finding is consistent with
Miller (4), which proposed that the maximum capacity of working memory is "seven, plus or minus two" chunks of
information. Again, this finding suggests that keeping the message short, 3 to 8 information units, would increase
the likelihood that it will be recalled by the driver.
Design Issues
Complexity is a function of how much information is being provided and how difficult it is to process. The phrase
"information units" is used to describe the amount of information presented, in terms of key nouns and adjectives
contained within a message.
High-Complexity Examples Low-Complexity Examples
> 9 information units 3-5 information units
Processing time > 5 s Processing time < 5 s
Examples: Topographical representations of a route, or Examples: Directions of turns, or estimates of travel
full route maps, or schedules for alternative modes of costs.
transportation.
Cross References
Driver Comprehension of Signs, 18-8
Key References
1. Campbell, J.L., Carney, C., and Kantowitz, B.H. (1998). Human Factors Design Guidelines for Advanced Traveler Information Systems
(ATIS) and Commercial Vehicle Operations (CVO) (FHWA-RD-98-057). Washington, DC: FHWA.
2. Labiale, G. (1996). Complexity of in-car visual messages and driver's performance. In A.G. Gale et al. (Eds.). Vision in Vehicles, 5
(pp. 187-194). Bron Cedex, France: INRETS.
3. Zwhalen, H.T., Adams, C.C., Jr., and DeBald, D.P. (1988). Safety aspects of CRT touch panel controls in automobiles. In A.G. Gale et al.
(Eds.). Vision in Vehicles, 2 (pp. 335-344). Amsterdam: Elsevier Science.
4. Miller, G.A. (1956). The magical number seven plus or minus two: Some limits on our capacity for processing information. Psychological
Review, 63, 81-97.
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