Grade Adjustment Factor

Guys I have a question,

I'm doing problem 2f, chapter 73 Roads and Highways: Capacity Analysis, of the companion Practice Problems from Lindeburg which states the following:

Assuming a FFS of 57mph, what is the average travel speed?

steps: I know the ATS = FFS - .00775 (vd + vo) - np.

When Finding vo (passenger eq. volume for opposing direction) what process do you go about finding the Grade Adjustment Factor fg?

The answer uses the same fg for the analysis direction or vd. Because the problem states that the upgrade is 3% and length is 2mi. which qualifies for to use the fg based on upgrades do we use the fg found on exhibit 20-13 in the HCM. And does this fg apply to both the vd and vo? I suppose since the grade is 3% and more than the 0.6mi requirement we use the this exhibit 20-13 but I'm just not positive why you would apply this to the opposing direction. It seems there should be another way to determine the fg for downgrades or we should use the regular fg found from exhibit 20-7. Thanks.

I have a same question. see page 20-14 paragraph 8 " for most specific...". it says fG is 1 for most specific downgrades. that's what i m gonna use for fG for specific downgrade. what do you think?

maximus808 said:

Guys I have a question,
I'm doing problem 2f, chapter 73 Roads and Highways: Capacity Analysis, of the companion Practice Problems from Lindeburg which states the following:

Assuming a FFS of 57mph, what is the average travel speed?

steps: I know the ATS = FFS - .00775 (vd + vo) - np.

When Finding vo (passenger eq. volume for opposing direction) what process do you go about finding the Grade Adjustment Factor fg?

The answer uses the same fg for the analysis direction or vd. Because the problem states that the upgrade is 3% and length is 2mi. which qualifies for to use the fg based on upgrades do we use the fg found on exhibit 20-13 in the HCM. And does this fg apply to both the vd and vo? I suppose since the grade is 3% and more than the 0.6mi requirement we use the this exhibit 20-13 but I'm just not positive why you would apply this to the opposing direction. It seems there should be another way to determine the fg for downgrades or we should use the regular fg found from exhibit 20-7. Thanks.

Click to expand...

You both bring up really good points. max, I agree with you that (right or wrong) the fG of 0.97 in the Lindeburg problem was obtained from the upgrade Exhibit 20-13 (even though the problem is requiring us to solve the pc flow rate of the opposing/downgrade direction). And yes jig, I agree with you that on page 20-14 of the HCM under 'Analysis of Downgrades", it states that fG is typically 1.0. I would suggest using 1.0 for the fG in the downhill direction.