TRN-02

AASHTO stopping sight distance

SSD = v tr + v² /(2 a) with v in m/s.

Reading speed

Geometric designAASHTO Green Book

Governing equation

\mathrm{SSD}=v t_r+\dfrac{v^2}{2a}

where

v: Design speed (km/h)
t_r: Reaction time (s)
a: Deceleration (m/s²)
SSD: Stopping sight distance (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-02 — AASHTO stopping sight distance) is the form associated with AASHTO Green Book. Working symbols: $v$ , $t_r$ , $a$ $\rightarrow$ $SSD$ . AASHTO splits perception–reaction (v tr) from braking under a constant deceleration a ≈ 3.4 m/s² on a wet pavement.

Purpose

Purpose: compute

SSD

from

v

t_r

a

in Transport via

\mathrm{SSD}=v t_r+\dfrac{v^2}{2a}

SSD = v tr + v² /(2 a) with v in m/s. 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 = 80.000\,\mathrm{km/h}

t_r = 2.500\,\mathrm{s}

a = 3.400\,\mathrm{m/s^{2}}

, the governing relation

\mathrm{SSD}=v t_r+\dfrac{v^2}{2a}

yields

SSD = 128.177\,\mathrm{m}

. Level grade. Enter speed in km/h. Move a slider: the numbers are this situation, not a canned story.

Calculator

Inputs

Design speed v80 km/hReaction time t_r2.5 sDeceleration a3.4 m/s²

Outputs

Stopping sight distance SSD128.177 m

Reading speed

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Narration of this film

Level grade. Enter speed in km/h.

AASHTO splits perception–reaction (v tr) from braking under a constant deceleration a ≈ 3.4 m/s² on a wet pavement.

Reading speed

Watch on YouTube