LOS on a Freeway Segment

Here is a question for you all.

A 6-lane rural freeway has a posted speed limit of 50 mph and is located within rolling terrain. The freeway has 11-foot lanes with 2-foot shoulders on both the right and median-side of the roadway. There are 2 on-ramps and 2 off-ramps located within three miles of the mid-point of the segment and most users of the roadway are regular users. The volume in the peak direction of the roadway carries 5 percent trucks and buses and 2 percent RVs with the volume on the roadway as follows:

Volume not necessary for this question .....

My question for you guys, if you want to determine LOS on this segment, would you begin with: FFS = 75.4 - fLW - fLC - 3.22TRD^0.84 HCM Equation 11-1

Or would you use: FFS = BFFS - fLW -fLC - fM - fA - HCM Equation 14-1

I am leaning towards Eq. 14-1 because with 11-1 the FFS ends up being almost 20 mph over the posted speed limit, which lowers density, which in turn gives an appearance of an LOS which is slightly better.

Your thoughts?

If this were an exam question and I saw the key term freeway, I'd go straight to Chapt 11, not 14.  Is this roadway signalized anywhere?  What are you using for your BFFS in eqn 14-1? 

This is actually a disagreement I'm having in a class on uninterrupted flow.

No idea about signalization, BFFS I am using 55 (50+5)

Either way you end up with LOS 'C'

But using Chapter 11 you get a density of 18 pc/ph/pl where using Chapter 14 you get 25 pc/ph/pl ..... my argument is that (well the question is jacked), but my approach is a more realistic solution.

What's the issue with 18 vs 25?

The fact that the problem info includes the left lateral clearance (and even bothers to mention there is a median) does lead you to Chapt 14.  Also, giving you the posted SL leads you to Chapt 14 too.  I suspect this is not a PE problem (Masters Degree?  TE Exam?  PTOE?  Or?) but, from what I read, NCEES does not include red herrings on their exams anymore.  From what I understand, if a PE exam question gives you information, you are to use it.   

This is an MS problem, and I think it's jacked to be honest.

The issue with 18 vs 25 is the 18 is almost LOS 'B' and the 25 is almost LOS 'D'

However, to make this relevant to EB, in my EET notes I have written "whenever speed limit it is given use that as BFFS" and go from there.

John QPE said:

This is an MS problem, and I think it's jacked to be honest.

The issue with 18 vs 25 is the 18 is almost LOS 'B' and the 25 is almost LOS 'D'

However, to make this relevant to EB, in my EET notes I have written "whenever speed limit it is given use that as BFFS" and go from there.

Click to expand...

It should be the posted speed limit plus 5 (or 7) mph, right?  Not the posted speed limit.  You can also use the design speed for BFFS, if known.

I would use Chapter 11 eq. 11-1 to solve this problem. I just cannot justify not going with Chapter 11 for this problem. So, if I saw problem like that on exam, I would go with eq. 11-1.

I would expect a tip like street parking/no signals/ no bus stops in order to make me want to use Chapter 14.

What do you mean "jacked"? Like unfair?

"Freeway" makes me want to use Chapter 11. Why the left-clearance and posted speeds would be provided, I have no idea.

My initial thought is that since it says that this is a Freeway, I would without a doubt go to chapter 11 and use that equation. I will run through this problem after work and see what I come up with using my EET binder and HCM. There are speed adjustments that may need to be made based on the volumes but I don't know that for sure. This is an excellent question so thank you, again John QPE for posting it. I will have a response up tomorrow.

I ended up going with Chapter 11, my reasoning is that this is for a class on uninterrupted flow, and the lecture on multi-lane highways is this week (that question came up last week). So, right or wrong, I kind of just backed into what the professor wanted, but I'm still curious as to how speed limit comes into play here?

Exhibit 11-18 in the HCM identifies speed limit as a limiting factor in the methodology, and says to use judgment to improve the results. The more I think about this, I'm thinking I would use the 50 mph (+5 mph) for the S in the density equation. That makes more sense to me than picking the speed from the speed-flow curve.

But you test-taking guys don't listen to this. Your stuff wont be this ambiguous.