  Existing UAS Traffic Management (UTM) frameworks designate preplanned flight
paths to uncrewed aircraft systems (UAS), enabling the UAS to deliver payloads.
However, with increasing delivery demand between the source-destination pairs
in the urban airspace, UAS will likely experience considerable congestion on
the nominal paths. We propose a rule-based congestion mitigation strategy that
improves UAS safety and airspace utilization in congested traffic streams. The
strategy relies on nominal path information from the UTM and positional
information of other UAS in the vicinity. Following the strategy, UAS opts for
alternative local paths in the unoccupied airspace surrounding the nominal path
and avoids congested regions. The strategy results in UAS traffic exploring and
spreading to alternative adjacent routes on encountering congestion. The paper
presents queuing models to estimate the expected traffic spread for varying
stochastic delivery demand at the source, thus helping to reserve the airspace
around the nominal path beforehand to accommodate any foreseen congestion.
Simulations are presented to validate the queuing results in the presence of
static obstacles and intersecting UAS streams.
