AASHTO 01

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AASHTO 01

• CHAPTER III
• Instructor Dr. Nedal Ratrout

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Elements of Design

• Sight Distance
• Four aspects of sight distance are discussed
  below
• The sight distance needed for stopping.
• The sight distance needed for the passing of
  overtaken vehicles.
• The sight distances needed for decisions at
  complex locations
• The criteria for measuring these sight distance
  for use in design.

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Stopping Sight Distance

• Sight distance is the length of the roadway ahead
  that is visible to the driver. The available
  sight distance on a roadway should be
  sufficiently long to enable a vehicle traveling
  at or near the design speed to stop before
  reaching a stationary object in the path.
• Stopping sight distance is the sum of two
  distances
• The distance traversed by the vehicle form the
  instant the driver sights an object necessitating
  a stop to the instant the brake are applied
• The distance needed to stop the vehicle form the
  instant brakes application begins. These are
  referred to as brake reaction distance and
  braking distance, respectively.

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Brake Reaction Time

• The study of reaction time by Johansson and Rumar
  
• Referred to the median reaction-time value for
  these drivers was 0.66 s. with 10 percent using
  1-5 s or longer.
• Another study, found 0.64 s as the average
  reaction time, while 5 percent of the drivers
  needed over 1 s.
• In a third study, the values of brake reaction
  time ranged form 0.4 to 1.7 s
• The recommended design criterion of 2.5 s for
  brake reaction time exceeds the 90th percentile
  of reaction time for all drives and has been used
  in the development of Exhibit 3-1

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Braking Distance
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Design Values

• The sum of the distance traversed during the
  brake reaction time and the distance to brake the
  vehicle to a stop is the stopping sight distance.
  
• In computing and measuring stopping sight
  distances, the height of the drivers is 600
  mm2.0 ft, equivalent to the taillight height of
  a passenger car.

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Effect of Grade on Stopping

• The stopping distances needed on upgrade are
  shorter than on level roadway those on
  downgrades are longer. The stopping sight
  distances for various grades are shown in Exhibit
  3-2.

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Variation for Trucks

• Trucks as a whole, especially the larger and
  heavier units, need longer stopping distances
  form a given speed than passenger vehicles.
  However, the truck driver is able to see
  substantially farther beyond vertical sight
  obstructions.
• Separate stopping sight distances for Turks and
  passenger cars, therefore, are not generally used
  in highway design.

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Decision Sight Distance

• Decisions sight distance is the distance needed
  for a driver to detect an unexpected or otherwise
  difficult-to-perceive information source or
  condition in a roadway environment that may be
  visually cluttered.
• Because decision sight distance offers drivers
  additional margin for error and affords them
  sufficient length to maneuver their vehicles at
  the same or reduced speed, rather than to just
  stop, its values are substantially greater than
  stopping sight distance.
• Drivers need decision sight distances whenever
  there is a likelihood for error in either
  information reception, decision-making, or
  control actions.

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Passing Sight Distance for Two-Lane Highways

• Passing sight distance for use in design should
  be determined on the basis of the length needed
  to be complete normal.
• Sight distance should be determined for a single
  vehicle passing a single vehicle.

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Passing Sight Distance for two-Lane Highways

• The minimum passing sight distance for two-lane
  highways is determined as the sum of the
  following four distances ( shown in Exhibit 3-4)
• Distance traversed during perception and reaction
  time and during the initial acceleration to the
  point of encroachment on the left lane.
• Distance traveled while the passing vehicle
  occupies the left lane.
• Distance between the passing vehicle at the end
  of its maneuver and the opposing vehicle.
• Distance traversed by an opposing vehicle for
  two-thirds of the time the passing vehicle
  occupies the left lane, or 2/3 of d2 above.
• In Exhibit 3-5. Time and distance values were
  determined in relation to the average speed of
  the passing vehicle.

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• THANK YOU