The following excerpt regarding LOS F was
taken from Chapter 3, page 3-10, of the 1997 revision of the Highway Capacity Manual
published by the Transportation Research Board.
LOS Criteria and Capacity
LOS F describes breakdowns in vehicular flow. Such conditions generally exist within
queues forming behind breakdown points. Such breakdowns occur for a number of reasons:
- Traffic incidents cause a temporary reduction in the capacity of a short segment, so
that the number of vehicles arriving at the point is greater than the number of vehicles
that can move through it.
- Points of recurring congestion exist, such as merge or weaving areas and lane drops
where the number of vehicles arriving is greater than the number of vehicles discharged.
- In forecasting situations, any location where the projected peak-hour (or other) flow
rate exceeds the estimated capacity of the location presents a problem.
Note that in all cases, breakdown occurs when the ratio of demand to actual capacity or
the ratio of forecast demand to estimated capacity exceeds 1.00. Operations immediately
downstream of such a point, however, are generally at or near capacity, and downstream
operations improve (assuming that there are no additional downstream bottlenecks) as
discharging vehicles move away from the bottleneck.