Appendix D Traffic Impact Analysis - westplainseis.com

through and right turn vehicles on both the minor and major streets. Therefore, in most cases, the minor
street left turn delay is the critical movement at a two-way stop controlled intersection.
The Transportation Research Board has determined what levels of service for unsignalized intersections
should be, by designating level of service A through F, where level of service A represents a facility
where no vehicle in any movement is delayed very long and level of service F which represents a facility
where there is excessive delay for the average vehicle in at least one movement in the intersection.
Corresponding LOS and ranges of delay are shown in Table 1 for signalized intersections and Table 2 for
unsignalized intersections.
Table 1. Level of Service Criteria for Signalized Intersections
Level-of-Service
Control Delay per Vehicle (s/veh)
A ≤ 10.0
B >10.0 and ≤ 20.0
C >20.0 and ≤ 35.0
D >35.0 and ≤ 55.0
E >55.0 and ≤ 80.0
F >80.0
Source: Highway Capacity Manual 2000, Transportation Research Board.
Table 2. Level of Service Criteria for Unsignalized Intersections
Level of Service
Average Control Delay (s/veh)
A ≤ 10
B >10 and ≤ 15
C >15 and ≤ 25
D >25 and ≤ 35
E >35 and ≤ 50
F >50
Source: Highway Capacity Manual 2000, Transportation Research Board.
Additionally, for unsignalized intersections, volume to capacity (V/C) ratio and queue length are
sometimes considered when evaluating congested intersections and when determining if mitigations or
improvements are warranted.
West Plains Development – Spokane Tribe Page | 27
Traffic Impact Analysis April 28, 2011