PHF vs K-factor explained: the two peaking numbers teams keep mixing up

Every few months someone on a capacity memo writes "K-factor" when they mean peak hour factor, or plugs a PHF into a design hour volume calc where a K-factor should go. They're both peaking metrics, they both show up in the same HCM chapters, and they both get abbreviated to a single letter in spreadsheets, so the confusion is understandable. They're not interchangeable, and using one where the other belongs will throw off a signal timing plan or a lane-sizing recommendation.

What PHF measures

Peak hour factor describes how volume is distributed within the peak hour itself. Take the highest 15-minute count inside that hour, multiply it by four, and divide the full hourly volume by that number. A PHF of 0.85 means the 15-minute peaks and the hourly average sit close together. A PHF of 0.70 means one 15-minute window carries a much larger share of the hour's volume than the other three, and a signal cycle sized for the hourly average will undersize the phase that has to clear that window.

PHF comes from sub-hourly counts. You need 15-minute bins for the peak hour, at minimum. If your count data is only hourly totals, you cannot compute a PHF directly. You can estimate one from a default table instead, and reviewers at most DOTs will want to know which table and which default you picked.

What K-factor measures

K-factor sits one level up. It describes what share of the annual average daily traffic occurs during the design hour, usually expressed as K30 (the 30th highest hourly volume of the year, as a percentage of AADT) or sometimes K100. A corridor with a K30 of 10% means the design hour carries 10% of a typical day's total volume. Rural recreational routes with sharp seasonal spikes run K-factors in the mid-teens or higher. Stable urban arterials with a flat daily pattern often sit closer to 8-9%.

K-factor's job is to convert an annual volume, AADT or a growth-factored AADT, into a single design hour volume for a capacity analysis in a future year. It's a planning-level number, built from a full year of continuous counts at a permanent count station or a long-duration tube count on a similar facility, not from the link under study.

Where they fit together

In a typical design hour volume workflow, K-factor converts AADT into DHV, and PHF converts that DHV into the sub-hourly flow rate the HCM capacity equations use. One answers "what's the design hour," the other answers "how peaked is that hour internally." You need both numbers, and they're not substitutes for each other. A link with a high K-factor and a low PHF is a corridor with a dramatic daily peak that's also spiky inside the hour, which is a different problem than a link with a high K-factor and a smooth PHF.

The practical headache is that both numbers assume you have count data to begin with, and on a lot of secondary arterials and emerging corridors, nobody has ever run a tube or installed a count station. You end up borrowing K-factors from a similar facility type in your region's traffic monitoring report and hoping the analogy holds, while PHF gets defaulted entirely because there's no 15-minute data at all.

That's the gap a single scheduled image pass over the link closes, at least for the count side. Vehicle Counting turns one VHR satellite or aerial image into a count by link and direction, with the marked-up frame included, so you have an actual observed volume for the link in question instead of a borrowed K-factor applied to someone else's AADT. It won't give you a full year of continuous data or replace a permanent count station, but for sizing a study or sanity-checking an assumed peaking factor on a link nobody has ever instrumented, a real count beats another layer of borrowed defaults.

If a link on your study area has never had a counter on it and you need a number before the next field season, that's worth a look.