Stopping Sight Distance

The stopping sight distance (SSD) is the total distance you travel during the time you (a) react to apply brakes, (b) apply brakes and actually begin to decelerate, and (c) vehicle comes to a stop.

Reaction time from AASHTO (\( t_R\)) is 2.5 s.

Default deceleration rate from AASHTO is 11.2 \(ft/s^2\)

The total distance traveled using these values on a straight path is:

$$ SSD = Vt_R + \frac{V^2}{2(a\mp gG)}$$

V is the Design Speed (ft/s)

\(t_R\) is the reaction time (2.5s)

a is the deceleration rate (11.2 ft/s^2)

g is the acceleration due to gravity (32.2 ft/s^2)

G is the longitudinal grade in decimal (e.g. 0.05)

There is usually an equation setup to use MPH instead of fps so make sure to be aware of where that is in your reference for quick problem solving!

Also as a speed not ether is a table in most references for some common SSDs at certain design speeds and grades, become familiar with it to save even more time.

There are also equations and methods to calculate SSDs on Vertical Crest Curves, Vertical Sag Curves, Horizontal Curves, and also Passing Sight Distance. I am not sure if I should study those in-depth yet as they may not be on the breadth exam.

About Conrad

I am a Civil Engineer. I work in San Diego and am preparing to take the PE Exam. I am interested in surfing, business, travelling, and spending time with my wife. Thanks!

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