5.2 Key Components of Sight Distance

  • Formally, sight distance is the length of road a vehicle travels before the driver completes a maneuver in response to an element, hazard, or condition that requires a change in the vehicles speed and/or path.
  • Sight distance consists of two components — perception-reaction time (PRT) and the amount of time needed to safely complete a maneuver.
  • PRT is the amount of time a driver needs to detect a road element, decide how to respond, and initiate a maneuver (e.g., braking, steering). Table 5.1 explains factors that influence PRT.
Table 5.1 Effects of PRT Components on Sight Distance
PRT Component Factor Implications for Sight Distance
Seeing/Perceiving an Object Low Contrast Low-contrast objects take longer to perceive.
Visual glare Glare increases the amount of time motorists take to perceive objects.
Older age Older drivers are:
  • Less sensitive to visual contrast
  • Less able to deal with glare
  • More likely process information slowly when confronted with visual clutter
Object size/height
  • Drivers need to be closer to small objects to see them
  • Compared to smaller objects, larger objects do not appear to change in size or move as fast.
Driver expectations Drivers need more time to perceive unexpected objects.
Visual complexity When objects are embedded in visually cluttered background, PRT is longer.
Visual attention
  • Drivers have a 10-degree cone of vision to attend to main traffic
  • Shifting focus to a new position requires 0.15 – 0.33 seconds.
Driver experience/ familiarity Drivers react more quickly to objects and situations when they have greater experience and/or familiarity with the setting
Driver eye height PRT and/or decision times are longer when a driver’s eyes are positioned at a lower height.
Cognitive Elements Older drivers Decision making is slower for older drivers.
Complexity Drivers need more time to process complex information and to initiate more complex of calibrated maneuvers.
Driver State Fatigued drivers have longer PRTs than alert drivers.
Traffic Volume/road type Drivers need more time to process critical information when traffic volumes are high and/or vehicle speeds are high.
Initiating Actions Older age Older drivers need more time to execute vehicle control movements (e.g., braking, steering, accelerating). These drivers can have a limited range of motion.

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Chris VanDyke

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chrisvandyke@uky.edu