PHF Mansour Practice Test

Hey Guys and Gals,

I did a quick search on the forum looking for a similar question and did not see one, but please forgive me if this exact thing has been hashed out before. I have a good understanding of PHF (peak hour factor), however, I came across a solution to a problem in Mansour's Practice AM book that made me question myself a little.

The problem provides 15 min volumes from 4:00 until 6:00 and asks you to solve the for the PHF for an intersection on a two lane highway. So my method of solving the problem would be finding the peak hour by summing 4 consecutive 15 min intervals until I found the hour with the most traffic in it. Then I would take that hourly volume and divide it by 4 times the peak 15 min flow rate which occurs in that same hour. PHF = VOL / (4xV15). I did this and ended up with the wrong solution.

I figured out that the book is actually calculating a PHF for each possible hour of the interval given and then choosing the highest PHF. So essentially they generate a higher PHF, but it does not occur during the hour of maximum volume. The book is wrong right? The PHF should only apply to the peak hour during an interval, correct?

I have attached an image of the problem in question. I get C. I believe the statement made at the beginning of the solution is incorrect, but I could be wrong and thus my question.

You're using 4:30 to 5:30, right?  That would give your answer C.  I agree with that.  What hour is the book using?  What answer does the book get, D?

Edit:  Meant to say 4:30 to 5:30, not 4:15 to 5:15. 

I have D

PHF = 0.90

4:15 - 5:15

950+1000+750+900 = 3600

3600/(4*1000) = 0.90

I should have taken a picture of the rest of the solution. The book calculates a PHF for all the possible hours. So they get 5 different PHFs and then choose the largest of them all. Which is what their statement says at the end of my picture.

I think the definition of PHF essentially states that it must be calculated using the highest hourly volume of the day which would be 4:30-5:30 --> 3700 veh

4:30-5:30 has the greatest volume: 1000+750+900+1050= 3700 with the peak 15 min being 1050 ; therefore PHF = 3700 / [4x1050] = 0.88 --> C

The book calculates a PHF for 4-5, 4:15-5:15, ect until it  finds the highest PHF, The highest PHF occurs from 4:15 - 5:15, but that hour is not the peak hour, so by definition of PHF I don't think that works. 

Am I just thinking of that wrong?

Sorry, I meant to say 4:30 to 5:30 in post # 2.  But I still get C.  John, why 4:15 to 5:15 and not 4:30 to 5:30? 

ptatohed said:

Sorry, I meant to say 4:30 to 5:30 in post # 2.  But I still get C.  John, why 4:15 to 5:15 and not 4:30 to 5:30? 

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Also used 4:30 to 5:30 = C

Should be using 4:30-5:30 since that yields the highest volume per hour.

Ok, my understanding is sound and the practice book has it wrong. The highest hourly rate governs, not the highest PHF. Thanks!

ptatohed said:

Sorry, I meant to say 4:30 to 5:30 in post # 2.  But I still get C.  John, why 4:15 to 5:15 and not 4:30 to 5:30? 

Click to expand...

Is that the highest volume? Apparently.

I change my answer.