Appendix D Traffic Impact Analysis - westplainseis.com

2.5 Methodology
The weekday PM peak hour has been identified as the time period when the greatest traffic demands are
placed on the transportation system. This will be the time period utilized by this study for analyzing the
traffic impacts of the proposed project. Mitigation needs, if any will generally arise during the peak
hours. By inspection, if the system operates acceptably under PM peak hour traffic conditions, it will
operate better as lower traffic volumes are observed during other times of the day.
The impact due to the proposed project was measured by comparing the level of service of the existing,
future without project and future with project scenarios. The Without Project scenarios include the
existing traffic conditions and Year 2012, 2015 and 2019 conditions. Background traffic volumes were
derived from inclusion of approved, unbuilt pipeline projects. Refer to the future conditions sections of
this document for additional discussion of the pipeline projects. Trip generation for most elements of the
proposed project were calculated using the data provided by the Institute of Transportation Engineers'
(ITE) Trip Generation Manual, 8th Edition manual. Trip generation for the Casino/Hotel component is
based on information provided in Recalibration of Trip Generation Model for Las Vegas Hotel/Casinos.
May, 2002 ITE Journal pp. 26-33. Rowe, Kaseko, Ackeret. Trip generation for the proposed police/fire
facility was based on data from Police Station PM Peak Hour Trip Generation in the Portland Metro
Area, December 2, 2009, Portland State University, Armans, Edmonds, et al.
The LOS evaluation for this study was conducted using Synchro 7.0 developed by Trafficware. The
software is primarily based on the procedures outlined in the Transportation Research Board’s Highway
Capacity Manual (HCM) 2000. This software uses detailed information on geometry, lane use, signal
timing, peak hour volumes, and vehicle arrival types. The information is then used to calculate delays
and to determine the LOS for each movement/approach. LOS was calculated for each study intersection
under the three analysis scenarios. SIDRA software was used for roundabout analysis as is the standard
practice in the state of Washington. Detailed Synchro analysis output worksheets are included in
Appendix C of this report, while SIDRA analysis output is included in Appendix F.
2.6 Level of Service
Level of service (LOS) is a qualifiable premise developed by the transportation profession to quantify
driver perception for such elements as travel time, number of stops, total amount of stopped delay, and
impediments caused by other vehicles afforded to drivers who utilize the transportation network. It has
been defined by the Transportation Research Board in the 2000 Highway Capacity Manual. This
document has quantified level of service into a range from “A” which indicates little, if any, vehicle
delay, to “F” which indicates significant vehicle delay and traffic congestion which may lead to system
breakdown due to traffic volumes which may exceed capacity.
2.6.1 Signalized Intersections
For signalized intersections, recent research has determined that average control delay per vehicle is the
best available measure of level of service. Control delay can be defined as the difference between the
actual travel time and the free-speed travel time. Control delay includes the initial deceleration of vehicle
from free-speed (posted speed limit) to join the queue, time spent in the queue and the acceleration from
stop bar to the free-speed. HCM methodology estimates the average control delay for each lane group
and aggregated for each approach and intersection as a whole.
2.6.2 Unsignalized Intersections
For unsignalized intersections, level of service is based on the delay experienced by each movement
within the intersection. In the case of a two-way stop controlled intersection, the delay experienced by
the minor street approach movements and the major street left turn movements are considered as the
performance measure. Vehicles passing straight through the intersection on the major (uncontrolled)
street experience no delay at the two-way stop controlled intersection. On the other hand, vehicles which
are turning left from the minor street must stop for all left turning vehicle from the major street and all
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Traffic Impact Analysis April 28, 2011