Transpo: FFS and Intersection LOS

[GTScott]

I am a Geotech looking at the AM transpo problems and have run into two issues. The first is a general question.

1) When determinig LOS, I go into the HCM charts with the corrected flow rate based on heavy vehicles and such and also with a corrected free flow speed (ffs). When applying the corrections to the FFS, I may end up with a value like 62 mph. In this case, is there a rule that you round up or down (60 vs. 65) or do you try to interpolate between the 60 mph and 65 mph rows in the HCM?

2) This is similar to a sample problem that I saw. I have only had one transpo class but have NEVER done anything like this. I see the description of how to handle this in the HCM but can't decode the info enough to make it through. Can someone work this one out so that a Geo could understand it? Also, is this something that *could* show up in the AM anyway?

Determine the LOS for this lane group:

Isolated Signalized Intersection

Adjusted volume = 120 vph

Adjusted Saturation Flow = 900 vph

Cycle Length = 90 seconds

Effective green time = 20 seconds

Unit Extension = 3 seconds (what is this??)

Negligible residual demand delay

Anyone?

THanks!

GT

 

[Road Guy]

I *think* i normally round up on the speed (should be more conservative)

I left my HCM at the house, but will look at this problem tonight.

 

[GTScott]

The rounding up is what I recalled as well. Thanks for the confirmation.

As for the intersection example, is that something that you think could go in the AM session?

-GT

 

[Road Guy]

IMO thats a little long, but i think its a good type of problem to work because they might ask one piece of the problem on the exam.

 

[Transpo_guy]

I'll take a look at the HCM question tonight...

 

[Transpo_guy]

I'll take a look at the HCM question tonight...

Okay, I promise I'll look at it tonight.

 

[Road Guy]

I forgot as well. but I did bring my HCM to work today encil:

 

[GTScott]

I have not talked to anyone who has seen that kind of thing on the AM Breadth exam, but I am still a bit curious as to how to convert all of the data presented in the problem. No rush on this one guys, I was just curious.

-GT

 

[Ugly Kid Joe]

do you have any of the solution?

I am going through the example LOS for signalized intersection in the HCM (Example 1)

this is as far as I can get:

lane group capacity = S(g/c) = 900 (20/90) = 200vph

V/C Ration = 120/200 ???? = 0.60

Flow Rate of Lane Group = V/s 120 / 900 = 0.1333

Criticil FLow Rate ro capacity = Xc=Yc*C/C-L

* I think its Xc= (0.1333)(90)/90- L

WTF is L ??

ANy help would be appreciated, I am doing PM Trans. MOst people say they dont have time to make you do an LOW for an entire signal, but I would like to see the rest of this one??

Traffi Guys/Gals????

 

[GTScott]

I don't have the solution to that one yet but I may be able to come up with it shortly.

I will check that HCM example problem and see what I can find. I just don't see this being a valid AM session question. Who knows though. Do you guys think there could be anything about weaves and such? What kinds of questions do you ask about that? Seeing as I am not a transpo person, it all seems kind of foreign to me.

-GT