peak hour factor 2

It's actually

phf = highest hour
highest 15 min x 4

It's a measure of how "peaky" the highest hour is. If the phf is high, than traffic is steady for the whole hour. If it is low, than most of the peak is in the peak 15 minutes.

ok, i got that part. but when phf is used to adjust flow rate (as in synchro), why bother with the calc (1 hr. volume divided by phf)? why not simply take the highest 15 min interval X 4 to get the adjusted flow rate?

The phf is used to adjust the hourly volume to reflect the peak flow rate (generally a 15 minute FR). What you are suggesting is essentially correct. The peak 15 minute flow x 4 can be entered as the hourly flow rate. The phf would then be set at 1. The process of utilizing the phf in the HCM methodology helps to ensure the data entry and adjustments are done in an orderly manner. Using a phf does provide additional information to someone reviewing the analysis, that may not be evident if the raw counts are adjusted beforehand.

In the UK we use a factor 1.125 to account for a localised variation in the flow. In the computer junction software that we use (arcady for roundabouts and Picady for priority junctions) it is already accounted for, so for manual cals the flows are alwways factored by 1.125 before you start.

HTH

Ian, how do you calculate this factor? It's not the same as the US PHF, since it's over 1.

In the UK we use a formulae to develop the peak.

Basically the data is in 15 minute chunks, and we analyse for 1 1/2 hours we then multiply as follows

00:00 fLOW IS 0
00:15 INTERPOLATED BETWEEN 00:00 AND 00:30
00:30 0.75 HOURLY FLOW
00:45 1.125 HOURLY FLOW
01:00 0.75 hourly flow
01:15 INTERPOLATE
00:30 0

This approximates to a normal distribution. Most of the software actually has this built in.

However for manual calcs we need to multiply all flows by 1.125 to account for localised variations within the 15 minute periods.