NCEES Practice Exam Question 506 - Vertical Curve Sight Distance

[Danny9Mo]

In question 506 of the practice exam, it asks for the minimum length of curve required for adequate stopping sight distance for a design speed of 60MPH. This is a sag curve with G1= -1.35% and G2= 2.40% The solution uses L=K*A = 136*(3.75%)=510'. What they did not show in the solution is if they checked to see if S<L or S>L utilizing equations (3-48) and (3-50), respectively, in the GDHS-6 Edition. My questions is, when calculating passing/stopping sight distances, is using L=K*A a "failsafe" approach to solving equations like this, or is this the method they choose to solve this particular problem. The reason I ask this is becasue other practice problems show solutions verifying S<L or S>L using the appropriate equations (i.e. sag curve vs. crest curve). Thanks. in adavance.

 

[NJmike PE]

what version of the practice exam?

 

[Danny9Mo]

NCEES Practice Exam: Transportation Depth. On the inside of the front page it has "April 2013 Second Printing". I bought this exam last September, if that helps. Also, to expand on the first post, generally speaking, will L=K*A alwaybe correct?

 

[Predgw]

I believe the table on page 3-161 essentialy does the math for you. The table uses the stopping site distance on flat ground for a given speed So, I believe the answer to your question is yes.

Just be mindful of what they are asking for. For example, if the ask for decision site distance on a sag curve, the table on page 3-161 will give to large a distance.

 

[ptatohed]

I'd solve this problem by going straight to the sag VC figure - Figure 3-44. Quick and easy. No need for any math. No need to worry about K. No need to worry about S > or < L. As a time saver, I'd recommend you guys be familiar with the figures.

 

[Predgw]

I was just following up on this. As I drove back from the bridge site, I was thinking I didn't like how I was not solid on my understanding. I just spent more time reading.

So, yes the tables can be used. I just read the area right of the curved dash section indicates s<l. The lower left is s>L. Looking at the figures now I see that is actaully labled as well. So, the charts definately can and should be used.