There are a number of sources to help with this question.
1. the PCU approach yields results that vary depending on
circumstances. For example left or right turning traffic at a busy
intersection can produce interesting usable and nonobvious PCU
results in measurement.
The effects of speed differentials and stops can affect so called
free flow PCU substantially- thus as Colin says, bus pull offs- but
buses still accellerate and decellerate at a rate different to the
rest of the traffic in most cases and thus still create a capacity
effect..
while there are some values you can use, this type of tradeoff is why
microsimulation is coming back into favour after about 30 years- BUT
the behavioural rules required for microsimulation are also costly to
test and the validation process is but rarely faced up to in most of
the recent (l21century) applications I have seen..
BUT it might help you in the case you are addressing.
Marc Wigan
Oxford Systematics
Melbourne
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