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Human Factors Guidelines for Road Systems - Collection C: Chapters 16, 17, 18, 19, 20, 22 (Tutorials 4, 5, 6), 23 (Updated), 24, 25, 26 (Updated) (2010) / Chapter Skim
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Tutorial 6: Calculating Appropriate CMS Message Length under Varying Conditions
Pages 70-75

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From page 70...
... , 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.
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From page 71...
... 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.
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From page 72...
... 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 rainfall. Therefore, signs utilizing other technologies should use fewer information units in heavy rainfall.
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From page 73...
... · 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.
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From page 74...
... · 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.
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From page 75...
... 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.
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Key Terms

  • lanes closed
  • led cmss
  • blanking time
  • legibility distance
  • information unit

  • follow detour
  • portable led
  • legibility distances
  • permanent cmss
  • cms message

  • design speed
  • speed reduction
  • permanent led
  • road closed
  • reduction action

  • fewer information
  • maximum information
  • combine information
  • portable message
  • travel lanes

  • blank screen
  • improves comprehension
  • base legibility
  • vertical curves
  • horizontal offset

  • roadside obstructions
  • reduced using
  • distances found
  • lighting conditions
  • closed due


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