Generally values of 0.85 for unsignalled junctions have been used extensively and many modelling products pander to this by setting defaults that, of course, encourage it even more.
Although it is understandable why such values are popular, and genuinely have their place, there may be a tendency for these values to become the ONLY goal, at the expense of evaluating situations in a more thorough and useful way.
There are a number of reasons why you should not rely on just one single acceptable maximum value of RFC. For example:
RFC values vary throughout a peak, and can rise and fall sharply or slowly.
The consequences of a high RFC depend on the flow. An RFC value of 1.2 might not matter with a very low flow whereas a value of 0.8 might be disastrous with a high flow.
The important criteria for judging the success of a design (from the point of view of congestion) are the total delay to all vehicles, and the mean delay per vehicle on each of the approaches. The latter is a question of “fairness” and “politics”. Is it acceptable for some drivers to suffer twice as much delay as others? How about ten times as much? That is a matter of opinion.
Revised by Jim Binning (Jan 2011)