Stopping Sight Distance Problem

[maximus808]

This problem is from the Practice Problems Book from PPI.

Problem 1j)

At 65mph, the SSD for the longer curve (682') is?

Answer is 650. This is for a sag vertical curve. The answer explains to refer the the Green Book Exh. 3-1 which is a basic table which gives a SSD for a given Design Speed. Sorry guys, as I was typing the out the solution I realized that the Greenbook has the same table and values for basic SSD on Exh. 3-1 and for Crest and Sag Vertical Curves on Exhibit 3-72 and 3-75. All tables point to the value of 645' which is correct.

Is there another way to check this? Why would the problem refer to the curve length if all you need is the design speed? Thanks.

 

[sac_engineer]

This problem is from the Practice Problems Book from PPI.
Problem 1j)

At 65mph, the SSD for the longer curve (682') is?

Answer is 650. This is for a sag vertical curve. The answer explains to refer the the Green Book Exh. 3-1 which is a basic table which gives a SSD for a given Design Speed. Sorry guys, as I was typing the out the solution I realized that the Greenbook has the same table and values for basic SSD on Exh. 3-1 and for Crest and Sag Vertical Curves on Exhibit 3-72 and 3-75. All tables point to the value of 645' which is correct.

Is there another way to check this? Why would the problem refer to the curve length if all you need is the design speed? Thanks.

I don't know the problem, but in most cases the length of curve, whether it's crest or sag, is longer than the stop sight distance on a horizontal surface. The curve length calculation always includes the SSD at a given speed and the grade difference along the vertical curve.

 

[maximus808]

Thanks. That's right I was getting the curve length mixed up with the SSD