What maximum RFC is acceptable?

In fact, it makes no sense to quote a maximum value because:

• RFC varies 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 for opinion.