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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
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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
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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
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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
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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
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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