1. Case Study: Human Factor for Road Systems “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 1.
Table 1. Three-step method for calculating base reading time.

2. 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 maneuver
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 2 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 2. Maneuver distances required for preparatory lane changes.
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:

3. 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 legibility
distance is calculated using the formula below:
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.).
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 3 contains daytime legibility
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.
Table 3. Daytime legibility distances of five symbol types by age group.

4. 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 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 bellow.