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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 22-39
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HFG TUTORIALS Version 1.0 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) 22-40
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HFG TUTORIALS Version 1.0 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 22-41
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HFG TUTORIALS Version 1.0 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. 22-42