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HFG TUTORIALS Version 1.0
Tutorial 5: Determining Appropriate Sign Placement
and Letter Height Requirements
When determining the appropriate sign placement, it is important to consider a number of
driver-related factors. The Traffic Control Devices Handbook (Pline, 2001) describes a process that
utilizes these factors and is the basis for the steps described below. This method is mostly focused
on guide and informational sign applications.
Step 1. Calculate the Reading Distance
The reading distance is the portion of the travelling distance allotted for the driver to read the
message, based upon the time required to read it (reading time). The Traffic Control Devices
Handbook outlines two methods for calculating the reading time. The first method, used by the
Ontario Ministry of Transportation, is described in the following three steps:
1. Allocate 0.5 s per word or number and 1 s per symbol, with a 1-s minimum for the total
reading time. This time should only include critical words. Drivers do not need to read
every word of each destination listed on a sign to find the one they are looking for. For ex-
ample, assume they are reading a sign with two destinations: Mercer St. and Union St., each
with a direction arrow. Drivers only need to read the word Mercer to realize that is not the
street they are looking for and the word Union to know that is their destination. They then
only need to look at the arrow for Union St.
2. "If there are more than four words on a sign, a driver must glance at it more than once, and
look back to the road and at the sign again. For every additional four words and numbers,
or every two symbols, an additional 0.75 s should be added to the reading time." (Ontario
Ministry of Transportation Traffic Office, 2001)
3. If the maneuver does not begin before the driver reaches the sign, add 0.5 s to the reading
time. This extra time is to account for the extreme viewing angle immediately before the
driver passes the sign, which prohibits reading. If the maneuver has already begun, the
driver does not need to continue to read the sign, and thus does not need more time.
These three steps are summarized in Table 22-7.
Table 22-7. Three-step method for calculating base reading time.
Step 1 Step 2 Step 3
Does the maneuver initiate
Base Reading Time (BRT) Are there more than 4 words?
before passing the sign?
BRT (s) = 0.5x + 1y Yes: Add time based on the BRT Yes: Add 0 s
where: 2 < BRT 4 Add 0.75 s
x = the number of critical words/ 4 < BRT 6 Add 1.50 s
numbers in the message 6 < BRT 8 Add 2.25 s
y = the number of critical symbols ...etc
in the message
No: Add 0 s No: Add 0.5 s
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Another method for calculating reading time, cited in previous studies, applies to complex
signs in high-speed conditions. The formula provided is:
Reading Time (s) = 0.31 (Number of Familiar Words) + 1.94
After finding the reading time, convert it into a reading distance by multiplying by the travel
speed.
Step 2. Calculate the Decision Distance
The decision distance is the distance required to make a decision and initiate any maneuver,
if one is necessary. After reading the sign, the driver needs this time to decide his/her course of
action based upon the sign's message. Decision times range as follows:
· 1 s for simple maneuvers (e.g., stop, reduce speed, choose or reject a single destination from
a D1-1 sign)
· 2.5 s or more for complex maneuvers (e.g., two choice points at a complex intersection)
After finding the decision time, convert it into the decision distance by multiplying by the
travel speed.
Step 3. Calculate the Maneuver Distance
The maneuver distance is the distance required to complete the chosen maneuver. The maneu-
ver distance depends on the course of action decided upon by the driver and the travel speed.
The sign placement should consider all of the maneuvers that could be chosen based upon the
message.
An example of required maneuver distances is provided in Table 22-8 for lane changes in
preparation for a turn. These distances do not apply to situations in which drivers must stop. For
high-volume roadways, more time may be needed to find a gap, while for low-volume roadways,
some of the deceleration distance may overlap with the lane change distance.
Table 22-8. Maneuver distances required for preparatory lane changes.
Operating Speed (mi/h) Gap-Search Distance (ft) Lane Change Distance (ft) Deceleration Distance (ft)
Non-Freeway Maneuver Distance Requirements
25 66 139 77
35 92 195 154
45 119 251 257
55 145 306 385
Freeway Maneuver Distance Requirements
55 218 306 308
65 257 362 462
70 277 390 549
Source: Pline (2001)
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Step 4. Calculate the Information Presentation Distance
The information presentation distance is the total distance from the choice point (e.g., inter-
section) at which the driver needs information. This distance is calculated using the following
formula:
Information Presentation Distance = Reading Distance + Decision Distance + Maneuver Distance
Step 5. Calculate the Legibility Distance
The legibility distance is the distance at which the sign must be legible. This distance is based
upon the operating speed and the advance placement of the sign from the choice point. The leg-
ibility distance is calculated using the formula below:
Legibility Distance = Information Presentation Distance - Advance Placement
Step 6. Calculate the Minimum Letter Height
The minimum letter height is the height required for the letters on the sign based upon the
legibility distance calculated above. It is also based upon the legibility index provided in the
MUTCD (30 ft/in.).
Legibility Distance (ft)
Minimum Letter Height (in.) =
Legibility Index (ft/in.)
Another consideration is the minimum symbol size. The minimum symbol size is based upon
the legibility distance of the specific symbol that is being used. Table 22-9 contains daytime leg-
ibility distances for five types of symbols based upon research (Dewar et al., 1994).
From these legibility distances, we can obtain two general trends: (1) legibility distances vary
by sign type and (2) legibility distances are greatly reduced for older drivers. Legibility distances
for symbols are generally greater than for word messages.
Example Application
As an example, a driver approaches an intersection on a 35-mi/h (51 ft/s) roadway. The
driver needs to read a simple designation sign (D1-1) that contains one destination word and
Table 22-9. Daytime legibility distances of five symbol types by age group.
Daytime Legibility Distances (ft)
Symbol Type Number of Signs
Young Middle-Aged Old Mean
Warning 37 736.4 714.7 581.5 677.6
School 2 573.3 634.7 501.2 569.7
Guide 21 472.3 461.5 366.0 433.3
Regulatory 12 464.4 437.9 367.4 423.1
Recreational 13 321.1 292.6 228.9 280.8
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one symbolic arrow. The sign is placed 200 ft in advance of the intersection. The legibility
index is assumed to be 30 ft/in. (FHWA, 2009). See Figure 22-9.
1. Reading Distance (ft) = [(1 s/word)(1 word) + (0.5 s/symbol)(1 symbol)](51 ft/s) = 77 ft
2. Decision Distance (ft) = (1 s/simple decision)(1 simple decision)(51 ft/s) = 51 ft
3. Maneuver Distance (ft) = Gap Search + Lane Change + Deceleration = 92 ft + 195 ft +
154 ft = 441 ft
4. Information Presentation Distance (ft) = Reading Distance + Decision Distance + Maneuver
Distance = 569 ft
5. Legibility Distance = Information Presentation Distance Advance Placement = 569 ft
200 ft = 369 ft
6. Letter Height = (369 ft)/(30 ft/in.) = 12 in. (when rounded to the nearest inch)
Information Presentation Distance (569 ft)
Reading Decision Maneuver Distance (441 ft)
Distance Distance
Gap Search Lane Change Deceleration
(77 ft) (51 ft) (92 ft) (195 ft) (154 ft)
Legibility Distance (369 ft) Advance Placement (200 ft)
Figure 22-9. Graphic illustrating the example application of a driver approaching
an intersection.
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