SSD Equation Limitations

[NIKE]

Hey All:

So the AASHTO Stopping Sight Distance eqn for us customary units is S = (1.47 * tp * v) + (v^2 / 30 * (f + G))

Something feels wrong about using the same equation to estimate the stopping distance of a Prius that you would for a semi truck. I understand that mass cancels out in the derivation of this equation, but I cant wrap my head around how you can accurately estimate stopping sight distance without considering mass. What am i missing?

BTW, I will never remember this as "Stopping Sight Distance". Ever since college its been "Sight Stopping Distance"... Maybe that means im dyslexic.

 

[MWC PE]

Hey All:

So the AASHTO Stopping Sight Distance eqn for us customary units is S = (1.47 * tp * v) + (v^2 / 30 * (f + G))

Something feels wrong about using the same equation to estimate the stopping distance of a Prius that you would for a semi truck. I understand that mass cancels out in the derivation of this equation, but I cant wrap my head around how you can accurately estimate stopping sight distance without considering mass. What am i missing?

BTW, I will never remember this as "Stopping Sight Distance". Ever since college its been "Sight Stopping Distance"... Maybe that means im dyslexic.

f and G account for all the deceleration and generally the f used is pretty conservative, add in the 2.5 seconds for t and the SSD should be more than enough. You have to remember that roads are designed with much less FS than other civil projects. We leave a lot of the safety up to the driver,I mean gurdrails aren't even tested to stop anything larger or faster than a Dodge Ram at 62 mph

 

[ptatohed]

Hey All:

So the AASHTO Stopping Sight Distance eqn for us customary units is S = (1.47 * tp * v) + (v^2 / 30 * (f + G))

Something feels wrong about using the same equation to estimate the stopping distance of a Prius that you would for a semi truck. I understand that mass cancels out in the derivation of this equation, but I cant wrap my head around how you can accurately estimate stopping sight distance without considering mass. What am i missing?

BTW, I will never remember this as "Stopping Sight Distance". Ever since college its been "Sight Stopping Distance"... Maybe that means im dyslexic.

The formula isn't saying "this is the stopping sight distance all drivers and vehicles need", it's saying "this is a conservative formula in which to design by, to capture the majority of the drivers and vehicles under the majority of conditions". At the same speed, a 25yo in a new sports car will require a lot less driver reaction time and vehicle stopping distance than a 75yo in a 1972 pick-up with drum brakes and worn tires.

 

[NIKE]

Excellent, thanks for the answers fellas! This is what I like about engineering- its so practical.

Well, i think some of the authors/test writers are going a bit too far when they want you to use this eqn for accident investigation type problems e.g. "if skid marks were this long and it took so and so this long 2 seconds to react, how fast was he traveling?" But i suppose it still helps practice the concept, just in a confused way.

 

[Guest]

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