Flathead County Transportation Plan – Phase II

Flathead County Transportation Plan – Phase II
2.4.1 SIGNALIZED INTERSECTIONS
The procedures used to evaluate signalized intersections use detailed information on geometry, lane
use, signal timing, peak hour volumes, arrival types and other parameters. This information is then used
to calculate delays and determine the capacity of each intersection. Generally, an intersection is
determined to be functioning adequately if operating at LOS C or better.
For signalized intersections, recent research has determined that “average control delay” per vehicle is
the best available measure of level of service. The amount of control delay that a vehicle experiences
is approximately equal to the time elapsed from when a vehicle joins a queue at the intersection (or
arrives at the stop line when there is no queue) until the vehicle departs from the stopped position at
the head of the queue. Control delay takes into account uniform delay, incremental delay, and initial
queue delay. The control delay is primarily a function of volume, capacity, cycle length, green ratio, and
the pattern of vehicle arrivals. Table 2.3 defines LOS in terms of average control delay per vehicle.
The intersection of Fairmont Road and MT Highway 35 is the only signalized intersection specifically
analyzed as part of this Transportation Plan. This intersection was recently signalized on a temporary
basis and the data collection at this location occurred while the intersection was still unsignalized. Table
2.4 shows the LOS analysis completed for this intersection using an assumed signal timing. Under
unsignalized conditions, this intersection had a failing LOS and met signalization warrants. For more
information on this intersection and other intersections that were evaluated in detail, see Section 2.7.2.
Signalized Intersection
Table 2.4: Existing Level of Service for Signalized Intersections
AM Peak Hour PM Peak Hour
Delay LOS V/C Delay LOS V/C
MT Highway 35 / Fairmont Road 1 11.10 B 0.69 8.70 A 0.59
Eastbound Left / Thru / Right 5.00 A 0.32 8.70 A 0.68
Westbound Left / Thru / Right 12.40 B 0.80 5.20 A 0.36
Northbound Left / Thru / Right 32.00 C 0.21 32.00 C 0.21
Southbound Left / Thru / Right 30.40 C 0.00 30.90 C 0.06
1 Signal timing was assumed and may not reflect actual signal timing.
2.4.2 UNSIGNALIZED INTERSECTIONS
Unsignalized intersections include two‐way stop‐controlled (TWSC) and all‐way stop‐controlled (AWSC)
intersections. The LOS for an AWSC type intersection is computed in the same way as signalized
intersections and is based on the average control delay per vehicle at the intersection. Since there is no
major street, the highest delay could be experienced by any of the approaching streets.
TWSC type intersection LOS values are computed in a manner different than signalized and AWSC type
intersections. The LOS for a TWSC intersection is generally not defined for the intersection as a whole,
but rather is determined by the delay experienced for each individual minor street approach. However,
for the purposes of this Plan, and to gauge the overall intersection performance, the LOS for a TWSC was
based on the average delay experienced along the minor street approaches. This difference from the
method used for signalized intersections is necessary since the operating characteristics of a stop‐
2‐16 | P age Existing Transportation System