SSD Problem

[ulua808]

[SIZE=11pt]When finding the stopping sight distance using the following equation [/SIZE]

SSD = 1.47 Vt + (V^2 / 30(f ± g))

[SIZE=11pt]how do we know when to use f+g or f-g? (Unable to find any explanation like “with a downslope, use f+g”.)[/SIZE]

 

[ptatohed]

[SIZE=11pt]When finding the stopping sight distance using the following equation [/SIZE]

SSD = 1.47 Vt + (V^2 / 30(f ± g))

[SIZE=11pt]how do we know when to use f+g or f-g? (Unable to find any explanation like “with a downslope, use f+g”.)[/SIZE]

Merry Christmas ulua.  First, you should use G, not g.  g equals gravity as in 'a' as in deceleration (f x g) (friction x gravity) (typically 0.35 x 32.2 = 11.2 ft/s).  [See GDHS eqns 3-2 and 3-3]  G equals longitudinal street grade.  G, in the formula you provided, is positive on an upslope/upgrade and negative on a downslope/downgrade.  This is because SSD (stopping sight distance) is increased on a downslope and decreased on an upslope.  [See GDHS table 3-2]  Does that make sense?  

 

[ulua808]

Merry Christmas ptotohed! Thanks for your reply. I try to maintain the original author's lettering when using an equation, thus the use of "g" (for the grade in decimal) instead of "G". For reference to the problem, I was looking at Practice Exams for the Civil PE Exam by Indranil G., Pg. 129, problem 130. For reference to the equation used, please see the attached image. It makes sense that the SSD will be shorter on an upslope and longer on a downslope, just couldn't nail down where to find this explanation.

 

[ptatohed]

Merry Christmas ptotohed! Thanks for your reply. I try to maintain the original author's lettering when using an equation, thus the use of "g" (for the grade in decimal) instead of "G". For reference to the problem, I was looking at Practice Exams for the Civil PE Exam by Indranil G., Pg. 129, problem 130. For reference to the equation used, please see the attached image. It makes sense that the SSD will be shorter on an upslope and longer on a downslope, just couldn't nail down where to find this explanation.

ulua, you are welcome.  I see what you are saying.  But, really, these SSD formulas (and  variable 'G') ultimately come from AASTO's GDHS, right?  AASHTO uses G.  But, regardless, you are right.  The use of +/- G is not explained very clearly.  Either in the GDHS or, as you mentioned, in Dr. Goswami's book.  But, now you know.  Best of luck on the exam!   

 

[John QPE]

Just think back to your vertical curve profiles...

+G goes uphill

-G goes downhill

This will become second nature as you work more and more transpo stuff. But for these problems, all you need to use are tables 3-1 and 3-2 when it comes test time.

 

[ptatohed]

Hmmmm.... interesting.  Found this in MDOT's Sight Distance Guidelines.  They say they don't generally adjust SSD due to grade, G.  I've never seen that before but their justification seems to have merit. 

 

[John QPE]

A higher effective eye height isn’t always a good thing. There is such a thing as being “too high” and completely looking over or past a signal head, hazard, etc.