Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 70
HFG TUTORIALS Version 1.0
Tutorial 6: Calculating Appropriate CMS Message
Length under Varying Conditions
The amount of information that can be displayed on a CMS is limited by the amount of time
that the driver has to read the message. This amount of time in turn is determined by the legi-
bility distance of the sign and the traveling speed of the passing vehicle. The legibility distance is
the maximum distance at which a driver can first read a CMS message. According to Dudek
(2004), this distance depends upon a number of factors including:
· Lighting conditions
· Sun position
· Vertical curvature of the roadway
· Horizontal curvature of the roadway
· Spot obstructions
· Rain or fog
· Trucks in the traffic stream
These obstructions and visibility limitations reduce the amount of time that the sign is within
view or legible, ultimately requiring a reduction in the amount of information that is displayed
on the CMS. The information that can be displayed is measured in information units. An infor-
mation unit is a measure of the amount of information presented in terms of facts used to make
a decision. For example, the location of the problem, the audience that is affected by the problem,
and the recommended action to take are each 1 information unit. To determine the appropriate
number of information units for display on a CMS, the following steps should be considered.
Step 1. Determine the Legibility Distance for the CMS
The maximum legibility distance for a CMS depends on the design characteristics of the sign
(Dudek, 2004). These characteristics include the display type, character height, character width,
character stroke width, and the font displayed. The base legibility distances found in Table 22-10
are presented in Dudek (2004) and are based on the results of several studies. The distances are
based on all uppercase letters, 18 in. character heights, approximately 13 in. character widths,
and approximately 2.5 in. stroke widths. Note that all of the information for light-emitting diode
signs provided in this tutorial applies only to the newer aluminum indium gallium phosphide
(or equivalent) LEDs.
Table 22-10. CMS legibility distances for varying lighting conditions.
Legibility Distances (ft)
Lighting
Light-Emitting Diode Fiberoptic Incandescent Bulb Reflective Disk
Mid-Day 800 800 700 600
Washout 800 800 700 400
Backlight 600 500 400 250
Nighttime 600 600 600 250
Source: Dudek (2004)
22-43
OCR for page 71
HFG TUTORIALS Version 1.0
Step 2. Use the Driver Speed to Find the Base Maximum Number of
Information Units Allowed in a Message
The maximum number of information units is derived from the legibility distance of the CMS
(which depends on the technology used) and the speed of the passing vehicles. The faster that
the passing drivers are going, the less time they have to read the CMS message. Also, because the
legibility distance of the sign depends upon the technology used, the number of information
units also varies with the technology that is used. Finally, the diverse technologies perform differ-
ently under changing conditions. Table 22-11 presents the base maximum number of informa-
tion units that can be presented for assorted CMS technologies, under several ambient lighting
conditions.
Step 3. Adjust for Adverse Roadway and Environmental Conditions
There are many roadway and environmental conditions that reduce the visibility of CMSs and
thus require a reduction in information units. Dudek (2004) provides further guidance on the
exact number of information units that should be used under different conditions. The follow-
ing sections describe how various conditions and factors lead to trade-offs in the number of
information units that may be displayed.
Vertical Curves
The reduction in information units required for vertical curves depends on the design speed
of the curve as well as the CMS offset from the road and mounting height. The following general
relationships apply to CMSs on vertical curves:
· As the design speed of the curve decreases, the number of information units that may be
used decreases.
· As the horizontal offset from the road increases, the number of information units that may
be used decreases.
· As the mounting height of the CMS decreases, the number of information units that may
be used decreases.
Table 22-11. Maximum number of information units per message for various technologies at
different speeds.
Maximum Information Units per Message
Light-Emitting Diode Fiberoptic Incandescent Bulb Reflective Disk
Lighting
0-35 36-55 56-70 0-35 36-55 56-70 0-35 36-55 56-70 0-35 36-55 56-70
mi/h mi/h mi/h mi/h mi/h mi/h mi/h mi/h mi/h mi/h mi/h mi/h
Mid-Day 5 4 4 5 4 4 5 4 3 5 4 3
Washout 5 4 4 5 4 4 5 4 3 4 3 2
Backlight 4 4 3 4 3 2 4 3 2 2 1 1
Nighttime 4 4 3 4 4 3 4 3 3 3 2 1
Source: Dudek (2004)
22-44
OCR for page 72
HFG TUTORIALS Version 1.0
In general, permanent CMSs that are mounted over the roadway are not affected by crest ver-
tical curves (Dudek, 2004).
Horizontal Curves
The main concern with CMSs located on horizontal curves is the obstruction of the sign by
roadside objects. Permanent CMSs that are mounted above or adjacent to the travel lanes will
likely be high enough to be seen over any roadside obstructions. However, portable CMSs are
usually closer to the ground and more likely to be obscured by obstructions. In general, the num-
ber of information units that may be used decreases when:
· The obstruction gets closer to the roadway
· The curve radius decreases (i.e., for tighter curves)
Rain
Rain does not generally affect CMSs (Dudek, 2004). However, when the intensity of the
rainfall increases to 2 in./h or more, the visibility of the sign can be impacted. When the
operating speed of the roadway is over 55 mi/h, Dudek (2004) recommends that the number
of information units displayed on portable LED CMSs should be reduced by 1 information
unit. Portable LED CMSs often use fewer pixels per character, and thus have lower luminance
levels per character than permanent CMSs, which are relatively unaffected even in heavy rain-
fall. Therefore, signs utilizing other technologies should use fewer information units in heavy
rainfall.
Fog
Fog can affect visibility even more than heavy rain. Generally, Dudek (2004) does not recom-
mend a reduction in information units for permanent LED CMSs because of fog. A reduction is
not necessary because of the high character luminance and contrast of permanent LED CMSs.
However, portable LED CMSs require a reduction. The number of information units that may
be used decreases when:
· The visibility range decreases
· The offset from the road increases
Trucks on the Roadway
Large trucks pose sight obstructions for other vehicles on the roadway. When a driver's view
of a CMS is obscured by a truck, the driver has the option to change his/her traveling speed or
position to see around the truck. However, as the number of trucks on the roadway increases,
the amount of space that is available for drivers to do this repositioning decreases. Thus, the more
trucks that are on the roadway, the more likely they are to impair the view of a CMS for other
drivers.
Step 4. Adjust for Blanking Time
Greenhouse (2007) found that inserting a 300-ms blank screen between phase 1 and phase 2 of
a portable message sign improves comprehension. The study is further discussed in the guideline
22-45
OCR for page 73
HFG TUTORIALS Version 1.0
for Displaying Messages with Dynamic Characteristics. Although the blanking time was only tested
between phases 1 and 2 (not between 2 and 1), it is reasonably conceivable that drivers who see
a blank between phases 1 and 2, but not between phases 2 and 1, would reverse the order of the
phases and possibly have trouble understanding the message. Dudek (1992) recommends that
blank time and/or asterisks should be displayed between cycles of a message that contains three or
more phases (on one-word or one-line signs). Because one-word and one-line signs are more lim-
ited in the amount of information that they can display at one time, the phases may not make sense
independently and drivers who read later phases before phase 1 may not understand the message.
Thus, giving an indication of where the message is in the cycle gives drivers an idea of their
location in the cycle.
Overall, drivers may use the blanking time to determine where they are in the message cycle,
even before the message is legible to them. There are additional benefits in terms of message com-
prehension as shown by Greenhouse (2007). However, the insertion of blanking time reduces
the total available time for the driver to read the message, potentially requiring a reduction in
information units. Thus, there is a trade-off between the benefits of providing blanking time and
the number of information units that may be contained in the message.
Step 5. Display the Resulting Number of Information Units
After the calculations and adjustments from Steps 1 through 4 are performed, the result will
be the number of information units that may be displayed in the message. If there are still more
information units in the message than should be displayed, they should be reduced using the fol-
lowing steps, until the appropriate number of information units is reached (steps and examples
adapted from Dudek (2004)).
Step 5A: Omit and Combine Information Units
First, attempt to reduce the number of information units without losing content by following
the steps below.
· Omit unimportant words and phrases
Example:
Original Message: Shortened Message:
ROAD CLOSED AHEAD ROAD CLOSED
DUE TO CONSTRUCTION 1 MILE
FOLLOW DETOUR ROUTE FOLLOW DETOUR
The word "Ahead" is unnecessary as drivers will assume the closure is ahead. The reason is
less important than the location of the closure.
22-46
OCR for page 74
HFG TUTORIALS Version 1.0
· Omit redundant information
Example:
Original Message: Shortened Message:
MAJOR ACCIDENT MAJOR ACCIDENT
ON I-276 NORTH PAST I-80
PAST I-80 2 LEFT LANES CLOSED
2 LEFT LANES CLOSED
KEEP RIGHT
If the CMS is on I-276, the same freeway as the accident, the information is evident to the
drivers and may be omitted. The information units "2 Left Lanes Closed" and "Keep Right"
are redundant because drivers can assume that if the two left lanes are closed, they will need
to move to the right.
· Combine base CMS elements
Example:
Original Message: Shortened Message:
TRUCK ACCIDENT FREEWAY CLOSED
PAST I-80 EXIT AT I-80
ALL LANES CLOSED FOLLOW DETOUR
AT I-80
I-287 NORTH TRAFFIC
EXIT AT I-80
FOLLOW DETOUR
In the example above, the incident descriptor, incident location, and lanes closed mes-
sage elements are combined into the information unit "Freeway Closed". The location
of the closure can be eliminated because the action element "Exit at I-80" describes the
location.
Step 5B. Reduce the Number of Audiences in the Message
Example:
Original Message: Shortened Message:
I-76 CLOSED I-76 CLOSED
BEST ROUTE TO BEST ROUTE TO
PHILADELPHIA/I-95 PHILADELPHIA
USE RTE-73 NORTH USE RTE-73 NORTH
When using this reduction technique, message designers must use their judgment to decide
which audience is more important to address in the message. In the previous example, the
audience "Philadelphia/I-95" was reduced from 2 information units to 1 information unit,
"Philadelphia".
22-47
OCR for page 75
HFG TUTORIALS Version 1.0
Step 5C. Use Priority Reduction Principles
If the message still contains more information units than should be displayed, the information
units should be reduced in order of priority. The priority order is derived from the information
drivers need the most in order to make driving decisions. In Table 22-12, the information units
are listed in priority order, with number 1 being the highest priority information.
If the closure is due to roadwork, the effect on travel and good reason for following the
action should be eliminated. Even though the incident/roadwork descriptor is useful to driv-
ers, it may be replaced with the lanes closed element if necessary. When choosing information
units to eliminate, the designer should start deleting units from the bottom of these priority lists
first (i.e., element numbers 8 or 6). More examples of the application of these steps can be found
in Dudek (2004).
Table 22-12. Order of priority for information units.
Lane Closures Freeway/Expressway Closures
1. Incident Descriptor 1. Closure Descriptor
2. Incident Location 2. Location of Closure
3. Lanes Closed 3. Speed Reduction Action (if
4. Speed Reduction Action (if needed)
needed) 4. Diversion Action
5. Diversion Action (if needed) 5. Audience for Action (if needed)
6. Audience for Action (if needed) 6. Effect on Travel (if needed)
7. Effect on Travel (if needed)
8. Good Reason for Following
Action (if needed)
22-48
