AASHTO Passing Sight Distance Components
Passing sight distance (PSD) governs where passing zones may be marked on a two-lane, two-way highway — it is the minimum length of roadway a driver must be able to see ahead in order to safely initiate and complete a passing maneuver before meeting oncoming traffic.
The Four Distance Components
AASHTO's model sums four sequential distances. d₁ is the initial maneuver distance — the distance covered while the passing driver perceives the opportunity, reacts, and accelerates into the opposing lane. d₂ is the distance traveled while occupying the opposing lane at passing speed. d₃ is the clearance distance between the passing vehicle and an oncoming vehicle at the moment the maneuver completes — a safety margin, not a travel distance. d₄ is the distance the oncoming vehicle itself covers during the maneuver, taken as two-thirds of d₂ under AASHTO's assumption that the opposing vehicle is also traveling at the design speed.
Speed-Dependent Parameters
The maneuver time (t₁), time in the opposing lane (t₂), and acceleration rate (a) that feed into d₁ and d₂ are not fixed — AASHTO Table 3-6 tabulates each by design speed, and this calculator linearly interpolates between tabulated speeds. As design speed increases, both maneuver times lengthen and required clearance distance grows, so PSD grows faster than stopping sight distance does over the same speed range — which is why passing zones require dramatically more sight distance than a driver needs simply to stop.
Where PSD Applies (and Where It Doesn't)
This model applies exclusively to undivided two-lane, two-way rural highways where passing occurs in the opposing traffic lane. It has no application to divided highways, one-way roadways, or freeways, where passing (where permitted) occurs within same-direction lanes and is governed by decision sight distance and lane-change sight triangle criteria instead.
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