Quiz #30

John QPE · Oct 17, 2016

A six-lane freeway in a rural location with rolling terrain has a posted speed limit of 65 mph. The freeway has 11-foot lanes with a 3-foot right shoulder and 11-foot left shoulder. An on and off ramp is located every 2 miles. 10% trucks and buses, 5% RV, and PHF=0.85.

What is the maximum flow rate n(vph) that the freeway can handle and still operate at LOS C?

cogborn84 · Oct 17, 2016

Great twist on a classic problem. I will check it out tonight.

ngmakhlo_PE · Oct 17, 2016

I like it!! will look at solving it tonight! Thanks @John QPE !!

JGipe1 · Oct 17, 2016

I'm getting 3,866, treating this as a freeway. My steps:

- Calculate FFS using FFS = 75.4 -Flw - Flc - 3.22TRD^(0.84)

^ I ended up using FFS = 70 MPH

- Calculate Vp using Max Density for LOS C (26 pc/mi/ln)

^ Vp = 1820 pc/mi/ln

- Calculate Fhv

^ Fhv = 0.833

Solve for V = (Vp)

(PHF)(Fhv)(Fp)

Please tell me what I am doing wrong.

ngmakhlo_PE · Oct 17, 2016

JGipe1 said:
I'm getting 3,866, treating this as a freeway. My steps:

- Calculate FFS using FFS = 75.4 -Flw - Flc - 3.22TRD^(0.84)

^ I ended up using FFS = 70 MPH

- Calculate Vp using Max Density for LOS C (26 pc/mi/ln)

^ Vp = 1820 pc/mi/ln

- Calculate Fhv

^ Fhv = 0.833

Solve for V = (Vp)(PHF)(Fhv)(Fp)

Please tell me what I am doing wrong.

Shouldn't the FFS just be taken as the posted speed limit?

John QPE · Oct 17, 2016

JGipe1 said:
I'm getting 3,866, treating this as a freeway. My steps:

- Calculate FFS using FFS = 75.4 -Flw - Flc - 3.22TRD^(0.84)

^ I ended up using FFS = 70 MPH

- Calculate Vp using Max Density for LOS C (26 pc/mi/ln)

^ Vp = 1820 pc/mi/ln

- Calculate Fhv

^ Fhv = 0.833

Solve for V = (Vp)(PHF)(Fhv)(Fp)

Please tell me what I am doing wrong.

I'm not saying you are wrong.....

This is a problem I'm reviewing, and your approach is different than the way I did it.

I did just notice that my ET and ER were incorrect. So my fHV was jacked.

Keep talking, post discussion in the forum.

JGipe1 · Oct 17, 2016

ngmakhlo said:
Shouldn't the FFS just be taken as the posted speed limit?

No. Do people always go the speed limit??? You can't use that for the free flow speed.

If the Field Measured Speed was 65 MPH, you could that as your FFS.

John QPE · Oct 17, 2016

JGipe1 said:
No. Do people always go the speed limit??? You can't use that for the free flow speed.

If the Field Measured Speed was 65 MPH, you could that as your FFS.

Yes, if you were talking multilane highways, you would use the speed limit (plus some factor which I'm too lazy to look up).

Problem specifically calls this a "freeway" on/off ramps spaced 2 miles (definition of uninterrupted flow).

So yeah ..... calc the FFS

ngmakhlo_PE · Oct 17, 2016

JGipe1 said:
No. Do people always go the speed limit??? You can't use that for the free flow speed.

If the Field Measured Speed was 65 MPH, you could that as your FFS.

OK, sir, just was curious if that would be why you got it wrong.

ngmakhlo_PE · Oct 17, 2016

ngmakhlo said:
OK, sir, just was curious if that would be why you got it wrong.

@JGipe1 or mam.

zzzvvvzzz · Oct 17, 2016

V=2728

cogborn84 · Oct 17, 2016

I get a solution of V = 3685 vph

cogborn84 · Oct 17, 2016

JGipe1 said:
I'm getting 3,866, treating this as a freeway. My steps:

- Calculate FFS using FFS = 75.4 -Flw - Flc - 3.22TRD^(0.84)

^ I ended up using FFS = 70 MPH

- Calculate Vp using Max Density for LOS C (26 pc/mi/ln)

^ Vp = 1820 pc/mi/ln

- Calculate Fhv

^ Fhv = 0.833

Solve for V = (Vp)(PHF)(Fhv)(Fp)

Please tell me what I am doing wrong.

I am not 100% positive, but I believe that the adjusted flow rate vp will not simply be 70*26 = 1820 since that would put vp over the breakpoint, so I think you have to use the breakpoint equation (exhibit 11-3). But I could be mistaken.

cogborn84 · Oct 17, 2016

I just realized that I thought of that wrong, but I think the result is the same. I believe this is the correct thinking: you can say that D = vp/S and if S = 70 then check the breakpoint of vp in the density equation. vp breakpoint for 70 mph = 1000 --> 1000/70 = 14.29. We know that D = 26, therefore vp has to be greater than 1000 thus forcing you into the breakpoint equation in exhibit 11-3 HCM 2010.

geomane · Oct 17, 2016

Sum of the forces = 0

John QPE · Oct 18, 2016

OK so I did this a little differently, which is why I wanted to review this with you guys.

I didn't bring density into this problem when I solved, and as you can see when you guys did, you get a flow rate of 1800+ which exceeds the MSF for LOS 'C'

fHV = 0.8333

MSF LOS 'C' = 1690 pc/ph/pl

SF = (MSF)(N)(fHV)(fP_

SF = (1690)(3)(0.833)(1.0)

SF = 4225 vph

What do you guys think?

cogborn84 · Oct 18, 2016

Does PHF need to be multiplied in the last equation? My gut feeling is that it should. I'll crack a couple books try looking at it your way. I didn't get vp >1800 since I used the breakpoint equation for 's' to solve vp. I didn't really look into a msf.

John QPE · Oct 18, 2016

cogborn84 said:
Does PHF need to be multiplied in the last equation? My gut feeling is that it should. I'll crack a couple books try looking at it your way. I didn't get vp >1800 since I used the breakpoint equation for 's' to solve vp. I didn't really look into a msf.

I was curious about this as well. But the HCM says multiplying by PHF gives service "volume" not flow rate.

11-23 and 11-24 of HCM is what I'm looking at

ngmakhlo_PE · Oct 18, 2016

Okay so I gave this problem a go last night and I solved it using the same method as most of you but for my Vp I used 2,400. If you look at the Speed v. Volume, LOS curve table for freeways, the footer of that table says that the max volume given a FFS of 70 mph (which is what I used as my S since my calculated FFS was about 71) is 2,400, so when you use 2400 as your Vp and solve for Vdemand you get 5,097 so I would have chosen 5070. what do you guys/gals think?

ngmakhlo_PE · Oct 18, 2016

ngmakhlo said:
Okay so I gave this problem a go last night and I solved it using the same method as most of you but for my Vp I used 2,400. If you look at the Speed v. Volume, LOS curve table for freeways, the footer of that table says that the max volume given a FFS of 70 mph (which is what I used as my S since my calculated FFS was about 71) is 2,400, so when you use 2400 as your Vp and solve for Vdemand you get 5,097 so I would have chosen 5070. what do you guys/gals think?

@John QPE now that I look at your answer I'd agree with you, the Vp I used is greater than the required Vp for a LOS C, I was solving for max volume. OOPS!

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