TRN-22

Passing sight distance

PSD ≈ v tp + v²/(2 a) + d3. Perception, acceleration past the oncoming, and clearance.

Reading speed

Geometric designAASHTO PSD

Governing equation

\mathrm{PSD}=v t_p+\dfrac{v^2}{2a}+d_3

where

v: Speed (m/s)
t_p: Pass time (s)
a: Acceleration (m/s²)
d_3: Clearance (m)
PSD: Passing sight (m)

Lecture brief

Historical brief

Highway and traffic theory crystallized with Greenshields’ 1935 speed–density line, then AASHTO stopping sight and equivalent axles. The lab is that operational arithmetic of flow, braking and pavement load. This sheet (TRN-22 — Passing sight distance) is the form associated with AASHTO PSD. Working symbols: $v$ , $t_p$ , $a$ , $d_3$ $\rightarrow$ $PSD$ . AASHTO splits d1 (reaction), d2 (occupation of the left lane), d3 (clearance) and d4 (oncoming travel).

Purpose

Purpose: compute

PSD

from

v

t_p

a

d_3

in Transport via

\mathrm{PSD}=v t_p+\dfrac{v^2}{2a}+d_3

PSD ≈ v tp + v²/(2 a) + d3. Perception, acceleration past the oncoming, and clearance. Use it when a real transport question must be answered in SI before a code check.

Live realistic example

In symbols

Live case. Given

v = 25.000\,\mathrm{m/s}

t_p = 7.000\,\mathrm{s}

a = 0.700\,\mathrm{m/s^{2}}

d_3 = 30.000\,\mathrm{m}

, the governing relation

\mathrm{PSD}=v t_p+\dfrac{v^2}{2a}+d_3

yields

PSD = 651.4\,\mathrm{m}

. Two cars, an opposing car, a long sight line. Move a slider: the numbers are this situation, not a canned story.

Calculator

Inputs

Speed v25 m/sPass time t_p7 sAcceleration a0.7 m/s²Clearance d_330 m

Outputs

Passing sight PSD651.4 m

Reading speed

Watch on YouTube

Free library

Full library

Free PDF / open book

YouTube channels

00:0 / 00:08

Narration of this film

Two cars, an opposing car, a long sight line.

AASHTO splits d1 (reaction), d2 (occupation of the left lane), d3 (clearance) and d4 (oncoming travel).

Reading speed

Watch on YouTube