Critical Length of Grade

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Critical Length of Grade

• Additional Material Provided for Your Benefit
• Fall 2008

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Critical Length of Grade

• Maximum grade in itself is not a complete design
  control
• Critical length is used to indicate the maximum
  length of a designated upgrade on which a loaded
  truck can operate without an unreasonable
  reduction in speed
• Should consider using less steep grades or added
  lanes if the critical length is too long

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Critical Length of Grade

• The design values are determined by the following
  factors
• Size and power of vehicle use weight/power ratio
  equal to 200lb/hp
• Speed at entrance to the critical length should
  use the average running speeds of trucks (need to
  adjust for potential upgrades and downgrades)
• Minimum speed on the grade below in which
  interfere to following vehicles is considered
  unreasonable this depends on whether or not the
  facility is a two-lane or multilane highway the
  time period other vehicles are unable to pass
  (note gt minimum 25 mph)

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Crash Involvement Rate
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Critical Length of Grade

• A common basis for determining the critical
  length of a curve is based on the reduction in
  speed of trucks below the average running speed
• In the past, 15 mph below the average would be
  acceptable. Now, should use 10 mph as the a rule
  of thumb (see previous graph)
• Exhibit 3-59 can be used to estimate the critical
  length of curve

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Critical Length of Grade
3-59
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Critical Length of Grade
Speed 60 mph, 4, reduction of 10 mph
1,200 ft
3-59
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Critical Length of Grade
Example What is the critical length of a 5
upgrade approached by a 2 upgrade with a length
equal to 1,650 ft?
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Critical Length of Grade
6 mph
1,650 ft
3-59
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Critical Length of Grade
325 ft
4 mph 10 mph 6 mph
3-59
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Critical Length of Grade
Recreational Vehicle
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Critical Length of Grade

• Other design elements
• Must design for trucks that increase speed if the
  entrance is downgrade (10 to 15 mph)
• When design controls are exceeded, need to
  consider adding an additional lane (next overhead)