Notes on Roadway Characteristics and Sight Distance

Roadway Characteristics

• Sight Distance: The length of roadway a driver can see ahead at any time.
  • Critical for making evasive maneuvers after observing an object in the path.

Types of Sight Distance

• Passing Sight Distance (PSD): Minimum sight distance needed for safe passing maneuvers.
• Stopping Sight Distance (SSD): Minimum sight distance required to stop a vehicle safely after seeing an object in its path.
• Decision Sight Distance (DSD): Needed when the driver must react to unexpected stimuli.

Passing Sight Distance

• Also referred to as overtaking sight distance.
• Components of PSD:
  • d₁: Distance traversed during perception-reaction time and initial acceleration.
  • d₂: Distance travelled while occupying the left lane during passing.
  • d₃: Distance from the end of the maneuver to the opposing vehicle.
  • d₄: Distance traversed by the opposing vehicle during the passing maneuver.
• Calculation:
  $PSD = d₁ + d₂ + d₃ + d₄$

Factors Impacting Passing Sight Distance

• Speed and position of the passing vehicle.
• Speed and position of the vehicle being passed.
• Speed and position of opposing vehicles.

Road Design Implications

• Safe stopping distances must be adhered to all points along the road alignment.
• No passing zones should be established when adequate passing distances cannot be maintained.
• Avoid frequent changes in speed limits to reduce sight distance requirements.

AASHTO Recommendations for Passing Sight Distance

Stopping Sight Distance

• Required to stop after perception of an object in the path.
• Longer than merely braking distance due to the perception-reaction time included.
• Components of SSD:
  • Reaction Distance: Distance travelled during perception-reaction time.
  • Braking Distance: Distance travelled while braking until the vehicle stops.

Table of Stopping Sight Distances on Level Roads

Example

• A motorist traveling at 88 km/h sees an obstacle and stops 9 m from it. The initial distance when he observed the crash can be calculated using the stopping distance equation and affects on perception-reaction time.

Decision Sight Distance

• Defined by AASHTO for situations where unexpected stimuli are present.
• Longer than SSD due to the need for longer perception-reaction times in complex environments.
• The sight distance is affected by the type of maneuver required and the road's location (urban vs. rural).

Table of Decision Sight Distances for Different Maneuvers

Key Considerations for Drivers and Vehicles

• Discuss characteristics related to driver vision, including static and dynamic characteristics of vehicles for highway design.
• Differentiate between passing, stopping, and decision sight distances to understand their unique roles in roadway design and safety.