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STRIDE Bus Rapid Transit
South Renton Transit Center (SRTC) Site Plan Review, Conditional Use Permit, Master Site
Plan Review, and Lot Line Adjustment Application
Attachment 36a
Traffic Study
June 2024
Prepared by the
I-405 corridor
South Renton Transit Center
Interim Parking Traffic Analysis
Summary Memo
May 2022
Stride program: I-405 corridor
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Consultant Quality Control Form
Version Title Date
Originator/
Drafted by Reviewed by
Approved
by
Notes, as
required
0 South Renton
Transit Center
Interim Parking
Analysis Interim
Traffic Analysis
Memo (Draft)
03/11/2022 A. Lo, T. Ma C. Ell R. Gorman Initial draft
1 South Renton
Transit Center
Interim Parking
Analysis Interim
Traffic Analysis
Memo
4/12/2022 C. Ell A. Lo R. Gorman ST comments
addressed.
Issued as final
2 South Renton
Transit Center
Interim Parking
Analysis Interim
Traffic Analysis
Memo
5/4/2022 C. Ell A. Lo R. Gorman Addressed
comments from
Anchor
Stride program: I-405 corridor
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Table of Contents
1 INTRODUCTION AND BACKGROUND ................................................................................... 1
2 METHODOLOGY ........................................................................................................................ 3
2.1 Scenarios Modeled ................................................................................................ 4
2.2 Background Projects ............................................................................................. 7
2.3 Synchro Model Elements and Assumptions .......................................................... 9
2.3.1 Trip Generation and Distribution ................................................................ 9
2.3.2 Intersection Vehicle Volumes .................................................................... 9
3 ANALYSIS RESULTS ................................................................................................................. 9
3.1 Performance Measures ....................................................................................... 10
3.2 Analysis Results .................................................................................................. 10
4 CONCLUSIONS ........................................................................................................................ 11
Figures
Figure 1-1 I-405 BRT Stride Project Corridor.......................................................................... 2
Figure 1-2 SRTC 30% Design ................................................................................................ 3
Figure 2-1 Interim Parking Analysis Study Area Intersections ................................................ 4
Figure 2-2 Interim Parking Scenario 1 .................................................................................... 5
Figure 2-3 Interim Parking Scenario 2 .................................................................................... 6
Figure 2-4 Full Build Scenario ................................................................................................ 7
Figure 2-5 SW 7th St Widening Project (est. 2024) ................................................................ 8
Figure 2-6 Shattuck Ave S Cycle Track Project (2021) .......................................................... 8
Tables
Table 2-1 Interim Year Parking Trips and Distributions ......................................................... 9
Table 3-1 AM Peak Hour Intersection Performance Results ............................................... 10
Table 3-2 PM Peak Hour Intersection Performance Results ............................................... 10
Stride program: I-405 corridor
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Acronyms and Abbreviations
ADT average daily traffic
API Google Application Programming Interface
ARR access revision report
BRT bus rapid transit
ETL express toll lanes
HCM Highway Capacity Manual
ICE Intersection Control Evaluation
LOS level-of-service
Metro King County Metro
NB northbound
OBS On-Board Systems
PSRC Puget Sound Regional Council
SB southbound
SEPA State Environmental Policy Act
ST Sound Transit
TRAC Washington State Transportation Center
TSP transit signal priority
TTR transportation technical report
WSDOT Washington State Department of Transportation
Stride program: I-405 corridor
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1 INTRODUCTION AND BACKGROUND
This technical memorandum describes the supplemental traffic analysis performed for the South
Renton Transit Center and Roadway Improvements project, which would be used by the I-405
Bus Rapid Transit (BRT) service and other King County Metro bus routes. The project includes
a new transit center facility, which would include a new I-405 BRT station, a park-and-ride
garage, and associated roadway improvements to provide access to the new South Renton
Transit Center (SRTC) and improve speed and reliability to the transit center for the I-405 BRT
service.
In August 2021, Sound Transit’s Board established an updated program schedule for the
completion of ST3 voter-approved projects. For this project, the updated program schedule is to
complete construction of the transit center facilities and roadway improvements in 2026 and
complete construction of the park-and-ride garage in 2034. The Sound Transit Board committed
to reassessing the ST3 program affordability annually. The supplemental traffic analysis
evaluated interim condition scenarios at the transit center site for the years between 2026 and
the start of construction of the park-and-ride garage.
The intent of this analysis is to determine the effects and changes in intersection congestion
levels due to various surface parking supply scenarios for SRTC as they compare to the No
Build scenario and full build out of the park-and-ride garage.
The I-405 BRT Stride project corridor, including SRTC, is shown in Figure 1-1. While the
majority of station areas along the Stride corridor are directly on I-405 with access via the
current and future expanded Express Toll Lane (ETL) system, the off-corridor location of the
SRTC station area would require BRT coaches to leave the freeway facility and navigate along
a segment of SR 167/Rainier Avenue before entering the transit center bus loop. Park-and-ride
capacity is currently provided by two Washington State Department of Transportation (WSDOT)
surface lots (approximately 370 stalls) that would be augmented by the completion of the SRTC
park-and-ride garage and potentially with interim surface parking provided by Sound Transit.
Phase 2 for this project included an initial evaluation of various transit center design layouts that
identified key bus routes into and out of SRTC and investigated a variety of bus priority
treatments. The Phase 2 traffic evaluation summary was documented in the Technical
Memorandum: South Renton Transit Center, BRT Stations, and Park-and-ride Garage Traffic
Report (August 2019). Phase 3 of the project has focused on the modification of a preferred
transit center concept, evaluation of selected improvements outside the transit center, and
feasibility assessments of transit center design refinements including interim parking provisions.
The 30-percent transit center design configuration is illustrated in Figure 1-2.
This memo includes a description of key analysis elements and methodology. Following
sections describe the analysis results for each interim parking scenario at the five (5) targeted
intersection locations. The three (3) interim parking scenarios captured in this traffic analysis
work were based on coordination between Sound Transit’s project design and environmental
review teams. The findings of this memo will establish recommendations on what, if any,
intersection mitigation measures will be needed to accommodate interim surface parking prior to
the construction and operation of the parking garage.
Stride program: I-405 corridor
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SOURCE: Sound Transit 2020
Figure 1-1 I-405 BRT Stride Project Corridor
Stride program: I-405 corridor
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Figure 1-2 SRTC 30% Design
2 METHODOLOGY
The intersection traffic analysis for the interim SRTC parking scenarios was evaluated using
Synchro (version 11) software leveraging traffic models that were previously developed for the
Phase 2 Transportation Technical Report (TTR) and early Phase 3 design feasibility and
validation work. All models used for the purposes of this interim parking analysis reflect an
updated opening year of 2026 (previously 2024).
To investigate the operational effects of the interim parking scenarios on surrounding areas near
the SRTC, a selected number of intersections were included in the model study network. These
study locations are listed below and illustrated in Figure 2-1.
1. Rainier Avenue S & SW Grady Way
2. Lake Ave S & SW Grady Way
3. Shattuck Avenue S & SW Grady Way
4. Shattuck Avenue S & Lake Avenue S
Stride program: I-405 corridor
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5. Shattuck Avenue S & SW 7th Street
Figure 2-1 Interim Parking Analysis Study Area Intersections
2.1 Scenarios Modeled
In addition to the future 2026 No Build scenario, two interim parking scenarios and the Full Build
were captured in the traffic analysis work representing varying levels of park-and-ride capacity.
For each scenario, the AM and PM peak hours were analyzed.
No Build: No Stride S1 service, no new SRTC, existing bus routes and WSDOT surface
retained (approximately 370 parking stalls)
1
2
3
4
5S 7th Street Lake Ave S SRTC site
Stride program: I-405 corridor
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Interim Parking Scenario 1: Stride S1 service begins with MetroConnects bus route
restructuring, new SRTC facility in place (including bus loop and layover), no additional
Sound Transit (ST) park-and-ride capacity beyond WSDOT surface lots (370 parking stalls).
The proposed site configuration is shown in Figure 2-2.
Figure 2-2 Interim Parking Scenario 1
Stride program: I-405 corridor
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Interim Parking Scenario 2: Same as Interim Parking Scenario 1 but with additional onsite
ST surface park-and-ride capacity (350 parking stalls), at the future transit-oriented
development and ST parking garage sites on the west side of the transit center; WSDOT
surface lots retained for a total of 720 parking spaces. The proposed site configuration is
shown in Figure 2-3.
Figure 2-3 Interim Parking Scenario 2
Stride program: I-405 corridor
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Full Build Scenario: Full Build scenario similar to Interim Parking Scenario 2 but with a 752-
stall parking garage replacing 350 stalls of ST surface parking, WSDOT surface lots
retained; a total of 1,102 park-and-ride spaces. This scenario is similar to the Full Build
condition represented in the original National Environmental Policy Act (NEPA)
Transportation TTR. The proposed site configuration is shown in Figure 2-4.
Figure 2-4 Full Build Scenario
2.2 Background Projects
The analysis of interim parking conditions for SRTC included two background projects:
S 7th Street Widening
Expansion of S 7th Street to four lanes between Shattuck Ave S and Talbot Rd S, including the
reconfiguration of the intersection with Smithers Ave S to form a continuation between S 7th
Street and Talbot Rd S. A concept sketch of this background project is shown in Figure 2-5.
Stride program: I-405 corridor
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Figure 2-5 S 7th Street Widening Project (est. 2024)
Shattuck Ave S Cycle Track
Dedicated bicycle lanes on the west side of Shattuck Ave S from S 7th Street to Lake Ave S
creating a separated two-way bicycle facility and reduces the vehicle travel lane count (to one
lane each direction). This project, initially completed in July 2021, was identified and developed
as part of King County Metro’s safe access to transit program through coordination with the City
of Renton. A concept sketch of this cycle track is shown in Figure 2-6.
Figure 2-6 Shattuck Ave S Cycle Track Project (2021)
N
S 7th Street
Shattuck Ave S S 7th Street
Stride program: I-405 corridor
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2.3 Synchro Model Elements and Assumptions
The model elements and assumptions described in the following sections were included for all
interim parking scenarios SRTC design layout of the transit center and access driveways.
2.3.1 Trip Generation and Distribution
For the parking garage trip generation and distribution, information from King County Metro S
Renton P&R License Plate Survey (2016) was used. Table 2-1 below shows trips generated by
the parking garage for each interim year build scenario. Detail descriptions of the scenarios are
included in section 2.1.
Table 2-1 Interim Year Parking Trips and Distributions
2.3.2 Intersection Vehicle Volumes
Intersection peak hour turning movement volumes for year 2024 No Build scenario were
extracted from the previous Synchro models (previously developed for the Phase 2 NEPA DCE
TTR and early Phase 3 design feasibility and validation work). An annual growth rate of 1.2%
was applied to calculate the updated opening year 2026 No Build volumes.
In the build scenarios where the transit center is constructed, the King County Metro buses
taking the east entrance/exit are assumed to take Lake Avenue S. Parking trips from/to each
origin/destination were assigned to intersections along their routes. The bus trips (both King
County Metro buses and Stride S1 buses) and parking trips were then added to the 2026 No
Build volumes to get the volumes for each build scenario.
3 ANALYSIS RESULTS
This section presents the results of the future 2026 year-of-opening traffic analysis for the Full
Build scenario and the No Build scenario, as well as the traffic analysis for the two potential
interim condition scenarios as described in Section 2.1. A brief description of the performance
measures used for reporting and the interim parking scenarios considered is also included. A
significant traffic impact (i.e., an impact that would require mitigation) is based on the following:
Sound Transit would provide proportionate mitigation if the project results in an
additional 15 seconds or more of delay to a given intersection or corridor that is
In Out In Out In Out In Out In Out In Out
Rainier Ave (N of 7th) 9% 5 5 6 6 15 5 6 15 27 7 8 28
Morris Ave (N of 7th) 15% 9 9 10 10 25 8 9 26 46 11 14 47
Rainier Ave (S of Grady) 34% 20 20 22 22 57 18 21 58 104 26 31 106
Grady Way (E of Talbot) 33% 20 20 21 21 55 17 20 57 101 25 30 103
7th St (W of Edwards)1%111121123113
Grady Way (W of Lind)8% 5544144514245 825
Total 100% 60 60 64 64 168 53 62 172 305 75 92 312
AM PM
%
DistributionOrigin/Destination AM PM AM PM
Interim Parking Scenario 1 Interim Parking Scenario 2 Full Build Scenario
Stride program: I-405 corridor
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projected to exceed the established standard under the No Build conditions. The
mitigation contribution would be based on the proportionate amount of delay that would
be added by the project.
3.1 Performance Measures
To evaluate the transit center and the possible improvements outside the transit center,
intersection delay and LOS were reported for each scenario of the Synchro model.
3.2 Analysis Results
The full build out of the SRTC site would be expected to draw additional trips to the transit
center area, due to increased bus activity (Stride and King County Metro restructuring) as well
as additional vehicles using the surface lots or parking garage to connect to the new Stride
service and King County Metro bus routes. Table 3-1 and Table 3-2 show the AM and PM peak
hour overall average intersection delays and LOS. Reflected in these tables are the No Build
and build scenario results for each of the five (5) study intersections.
Table 3-1 AM Peak Hour Intersection Performance Results
Table 3-2 PM Peak Hour Intersection Performance Results
Though the build scenarios include greater volume levels in general, the intersection delay and
LOS results remain similar to the No Build condition. During the AM peak hour, the only location
where LOS degrades (with the proposed transit center in place) was at Lake Ave & Grady Way.
The AM No Build condition shows 14 sec delay and an LOS of B which is maintained for Interim
Parking Scenario 1. However, with an increase in park-and-ride capacity (350 additional parking
stalls) assumed in build scenario 2, the intersection delay increases to 20 and the LOS falls to
C. During the PM peak hour, the intersection Rainier Ave & Grady Way was already reflecting
overcapacity conditions in the No Build condition with average delays greater than two minutes.
For the build scenarios, by optimizing the signal timing splits, the delays could be slightly
Delay LOS Delay LOS Delay LOS Delay LOS Methodology
Rainier Ave & Grady Way 66 E 58 E 59 E 61 E HCM 2000
Lake Ave & Grady Way 14 B 19 B 20 C 24 C HCM 2000
Shattuck Ave & Grady Way 24 C 25 C 25 C 26 C HCM 2000
Shattuck Ave & Lake Ave 4 A 4 A 3 A 3 A HCM 2010
Shattuck Ave & 7th St 9 A 9 A 9 A 9 A HCM 2000
Full Build ScenarioNo Build Interim Parking Scenario 1 Interim Parking Scenario 2Intersection
Delay LOS Delay LOS Delay LOS Delay LOS Methodology
Rainier Ave & Grady Way 138 F 86 F 88 F 92 F HCM 2000
Lake Ave & Grady Way 21 C 27 C 31 C 37 D HCM 2000
Shattuck Ave & Grady Way 20 C 17 B 18 B 20 B HCM 2000
Shattuck Ave & Lake Ave 6 A 6 A 6 A 9 A HCM 2010
Shattuck Ave & 7th St 19 B 19 B 19 B 19 B HCM 2000
Full Build ScenarioNo Build Interim Parking Scenario 1 Interim Parking Scenario 2Intersection
Stride program: I-405 corridor
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reduced though the intersection would still operate at LOS F. The only location during the PM
peak hour where LOS was shown to change was at Lake Ave & Grady Way. Under the Full
Build Parking Scenario (with the parking garage in place and WSDOT surface lots retained) the
resulting delays fall to LOS D compared to LOS C for No Build conditions, which would not
require mitigation by Sound Transit.
4 CONCLUSIONS
The proposed 30-percent SRTC design would incorporate a new signal at the intersection of
Rainier Avenue S and Hardie Avenue SW for Rainier Avenue S transit access and also provide
transit center access along Lake Avenue S near the current WSDOT park-and-ride lots north of
S Grady Way. The interim park-and ride lot (and eventual parking garage) would provide access
on Rainier Avenue S, S Grady Way and Lake Avenue S. The analysis examined the impact of
interim parking levels along S Grady Way and Shattuck Avenue S, and the results indicate that
the transit center design and access provisions would adequately accommodate the full range
of onsite parking capacity scenarios with no notable congestion effects (15 additional seconds
of delay per vehicle from the No Build bringing the LOS to E or F) or mitigation requirements.
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