Transportation
Passing Sight Distance
AASHTO four-component passing sight distance calculator for two-lane highways, computing d₁ through d₄ with speed-interpolated parameters.
Passing Sight Distance
Passing sight distance (PSD) is the minimum sight distance on a two-lane, two-way highway that allows a driver to safely pass a slower vehicle. The passing driver must accelerate to passing speed, complete the maneuver in the opposing lane, and return to the right lane before meeting an oncoming vehicle.
AASHTO defines PSD using four distance components: d₁ — initial maneuver (perception-reaction + acceleration); d₂ — time in opposing lane at passing speed; d₃ — clearance to oncoming vehicle at end of maneuver; d₄ — distance covered by the opposing vehicle during the maneuver (taken as ⅔ d₂). Parameters t₁, t₂, and acceleration are speed-dependent per AASHTO Table 3-6.
PSD = d₁ + d₂ + d₃ + d₄
d₁ = 1.47·t₁·(V − m + a·t₁/2) d₂ = 1.47·V·t₂
d₄ = ⅔·d₂ V in mph, t in s, a in mph/s
Passing Sight Distance Calculator
Imperial units — mph, feet · two-lane highways only