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"At-Capacity" Operations

The capacity of a roundabout entry (also for signals) can only be definitively determined by measuring entering volumes during heaviest use. Genuine "At-Capacity" operation is characterized by:

  • Large sustained queues over the whole survey period. (Not random short peak queues.)
  • Entering traffic 'nosing' a little across the yield line so that circulating drivers voluntarily modify their headway to create gaps for them to enter.
  • Occasional brief periods of "Priority Reversal" where circulating drivers allow entering drivers to proceed.

These symptoms indicate a saturated intersection where traffic demand has reached capacity, and represent an opportunity to observe flows at peak saturation. These observations are then analyzed statistically to determine constraints of the geometric parameters, and how those parameters can be used to refine and improve operations.