MEMORANDUM
Date:
September 22, 2014
To:
Don Skillingstad, Spokane Transit Authority
From:
Yongliang Zhu, Lochner
Re:
Median Flyer Stop Weave Analysis
West Plains Transit Center IJR
Project No.: 9715
Attachments:
•
•
•
Exhibit 1: HOV Access Ramp Exhibit
Exhibit 2: Base Year Model Link Volume and Actual Traffic Count Comparison
Exhibit 3: Weave Analysis HCS Reports
Summary
On August 8, 2014, at the West Plains Transit Center Kick-Off meeting, Mike Frucci (WSDOT) expressed his concern about
buses being able to accelerate from a median flyer stop located immediately west of the I-90/SR 902 interchange and exit at the
I-90/SR 904 interchange to the west. Mike recommended a weave analysis to confirm that the movement was possible before
continuing to study that option. In response to WSDOT’s request, Lochner conducted a weave analysis for the westbound I-90
segment between the proposed West Plains Transit Center median flyer stop and SR 904 off ramp, with the methodology
defined by the 2010 Highway Capacity Manual, assuming 2040 as the design year. The analysis is summarized as follows:
•
•
During the AM peak hour, the freeway operates at LOS B
During the PM peak hour, the freeway operates at LOS C
Median Flyer Stop Alternative
The Medical Lake interchange is categorized as an urban interchange with a 70 mph design speed on I-90. The on-connection
for the median flyer stop would have a total width of 24’ and a maximum grade of 5% (See WSDOT Design Manual page 142012). For a left-side on-connection, WSDOT recommends a parallel on-connection, which adds a parallel lane long enough for
the merging vehicle to accelerate in the lane and then merge with the through-traffic. This merge is similar to a lane change and
the driver can use side- and rear-view mirrors to his or her advantage.
A parallel on-connection consists of acceleration length, gap acceptance length, and taper, as shown below:
1
The length of the proposed on-connection is 4270’, which includes:
•
•
•
Acceleration length: La = 3045’(WSDOT DM page 1420-14),
Gap Acceptance Length: Lg = 925’ (WSDOT DM page 1420-15), and
300’ taper. (WSDOT DM page 1420-13)
See attached Exhibit 1 for details.
During the design process, the ramp length will be adjusted to account for final profile and alignment to meet current design
standards.
Model Validation and Post Processing
The traffic volumes for the weave analysis are taken from the Spokane Region Transportation Council (SRTC) 2040 model.
Before the model being used to obtain the future traffic volumes, the preliminary checks are performed to ensure that the
forecasting processes are suitable for this project. This check involved comparing the 2010 base year simulated link volumes
with the actual traffic count data. A statistical measure of root-mean-square-error (RMSE) is calculated per NCHRP Report 765
Chapter 4.4. The calculation is summarized as follow:
•
2
For the AM peak period, the average traffic volume for the selected links within the study area is 420 vph with a RMSE
of 159. This results a traffic forecasting error of 38%, which is within the accuracy limit of 42% listed on NCHRP Report
765, page 81, Table 4-6.
September 09, 2014 | hwlochner.com
•
For the PM peak period, the average traffic volume for the selected links within the study area is 468 vph with a RMSE
of 153. This results a traffic forecasting error of 32%, which is close to the accuracy limit of 31% listed on NCHRP
Report 765, page 81, Table 4-6.
This preliminary check indicates that SRTC models are suitable for the future traffic forecasting for the study area and is also
adequate for the weave analysis. The 2010 base year link volumes and actual traffic count data comparison is attached as
Exhibit 2.
To account for localized discrepancies between existing “ground counts” and model-generated volumes, a model “postprocessing” is utilized. This method is described as Factoring Procedure – Difference Method in NCHRP 765 Chapter 6, which
add the “model growth increment” (the difference between the 2010 and the 2040 model volumes) to the existing ground count
traffic volumes. The weave analysis uses the adjusted 2040 peak hour traffic volumes coming out of the post-processing.
Weave Analysis
The weaving area on the westbound I-90 between the proposed West Plains Transit Center median bus lane and SR 904 off
ramp is 4,360 feet long. It is considered a two-sided weave with a one-lane, left-sided on-ramp followed by a one-lane rightsided off-ramp. The traffic volumes are taken from the Spokane Region Transportation Council (SRTC) 2040 model and
adjusted per NCHRP Special Report 765. The weave analyses were conducted for AM and PM peak hours for the design year
2040. They were summarized as follows:
•
•
During the AM peak hour, the freeway weaving operates at LOS B with weaving segment density of 14.1 pc/mi/ln.
During the PM peak hour, the freeway weaving operates at LOS C with weaving segment density of 21.5 pc/mi/ln.
The detailed weaving reports are attached in Exhibit 3.
Conclusion
This analysis shows that a bus exiting the median flyer stop has approximately 4,360 feet after entering the freeway mainline, to
change lanes and exit the freeway at SR 904. The section of I-90 between the proposed median flyer stop and the off-ramp to
SR 904 is in a tangent and on level terrain. Based on these conditions and the volumes presented in the analysis, the weave
distance provided, and shown in Exhibit 1, is adequate to safely make the maneuver in all weather conditions.
3
2010 Base Model Link Volume vs Traffic Count
Comparison
Link No.
AM
Link
PM
V (forecast)
C (count)
V (forecast)
C (count)
1
SR 902 EB On Ramp
503
493
552
426
2
SR 902 EB Off Ramp
311
144
198
196
3
SR 902 WB On Ramp
161
140
388
198
4
SR 902 WB Off Ramp
491
368
477
400
5
SR 902 Between EB and WB Ramps NB
464
283
436
282
6
SR 902 Between EB and WB Ramps SB
642
537
717
555
7
Westbow Blvd East of Aero Rd WB
291
132
316
107
8
Westbow Blvd East of Aero Rd EB
234
85
276
92
9
Aero Rd North of Westbow Blvd NB
487
440
584
417
10
Aero Rd North of Westbow Blvd SB
473
330
511
442
11
Aero Rd South of Westbow Blvd NB
196
121
268
186
12
Aero Rd South of Westbow Blvd SB
239
203
236
201
13
Hayford Rd East of Medical lake Rd EB
380
190
305
219
14
Hayford Rd East of Medical lake Rd WB
190
115
467
287
15
Medical Lake Rd North of Hayford Rd NB
410
291
426
334
16
Medical Lake Rd North of Hayford Rd SB
448
393
456
343
17
Medical Lake Rd South of Hayford Rd NB
675
396
601
445
18
Medical Lake Rd South of Hayford Rd SB
523
427
794
518
19
SR 904 Off Ramp
853
443
886
652
Average Link Volume
420
468
Phone:
E-mail:
HCS 2010: Freeway Weaving Release 6.50
Fax:
______________________________Operational Analysis_____________________________
Analyst:
Agency/Co.:
Date Performed:
Analysis Time Period:
Freeway/Dir of Travel:
Weaving Location:
Analysis Year:
Description:
Yong Zhu
Lochner
8/15/2014
AM peak
I-90 Westbound
Between SR904 and SR902
2040
West Plains Transit Center
___________________________________Inputs______________________________________
Segment Type
Weaving configuration
Number of lanes, N
Weaving segment length, LS
Freeway free-flow speed, FFS
Minimum segment speed, SMIN
Freeway maximum capacity, cIFL
Freeway
Two-Sided
2
4360
70
15
2400
ln
ft
mi/h
mi/h
pc/h/ln
Terrain type
Grade
Length
Level
0.00
0.00
%
mi
___________________Conversion to pc/h Under Base Conditions____________________
Volume Components
VFF
VRF
VFR
VRR
Volume, V
861
0
705
8
veh/h
Peak hour factor, PHF
0.92
0.92
0.92
0.92
Peak 15-min volume, v15
234
0
192
2
Trucks and buses
20
0
10
100
%
Recreational vehicles
0
0
0
0
%
Trucks and buses PCE, ET
1.5
1.5
1.5
1.5
Recreational vehicle PCE, ER
1.2
1.2
1.2
1.2
Heavy vehicle adjustment, fHV
0.909
1.000
0.952
0.667
Driver population adjustment, fP
1.00
1.00
1.00
1.00
Flow rate, v
1029
0
805
13
pc/h
Volume ratio, VR
0.007
_________________________Configuration Characteristics_________________________
Number of maneuver lanes, NWL
0
ln
Interchange density, ID
0.5
int/mi
Minimum RF lane changes, LCRF
0
lc/pc
Minimum FR lane changes, LCFR
0
lc/pc
Minimum RR lane changes, LCRR
2
lc/pc
Minimum weaving lane changes, LCMIN
Weaving lane changes, LCW
Non-weaving vehicle index, INW
Non-weaving lane change, LCNW
Total lane changes, LCALL
26
163
400
2356
2519
lc/h
lc/h
lc/h
lc/h
_______________________Weaving and Non-Weaving Speeds__________________________
Weaving intensity factor, W
0.147
Average weaving speed, SW
Average non-weaving speed, SNW
63.0
65.4
mi/h
mi/h
_________Weaving Segment Speed, Density, Level of Service and Capacity_________
Weaving segment speed, S
65.4
mi/h
Weaving segment density, D
14.1
pc/mi/ln
Level of service, LOS
B
Weaving segment v/c ratio
0.403
Weaving segment flow rate, v
1680
veh/h
Weaving segment capacity, cW
4164
veh/h
_______________________Limitations on Weaving Segments_________________________
If limit reached, see note.
Weaving length (ft)
Density-based capacty,
cIWL (pc/h/ln)
v/c ratio
Minimum
300
Maximum
5793
Maximum
2400
Actual
4360
Analyzed
2290
Maximum
1.00
Analyzed
0.403
Note
a,b
c
d
Notes:
a. In weaving segments shorter than 300 ft, weaving vehicles are assumed to
make only necessary lane changes.
b. Weaving segments longer than the calculated maximum length should be
treated as isolated merge and diverge areas using the procedures of
Chapter 13, "Freeway Merge and Diverge Segments."
c. The density-based capacity exceeds the capacity of a basic freeway segment,
under equivalent ideal conditions.
d. Volumes exceed the weaving segment capacity. The level of service is F.
______________________________________________________________________________
Phone:
E-mail:
HCS 2010: Freeway Weaving Release 6.50
Fax:
______________________________Operational Analysis_____________________________
Analyst:
Agency/Co.:
Date Performed:
Analysis Time Period:
Freeway/Dir of Travel:
Weaving Location:
Analysis Year:
Description:
Yong Zhu
Lochner
8/15/2014
PM peak
I-90 Westbound
Between SR904 and SR902
2040
West Plains Transit Center
___________________________________Inputs______________________________________
Segment Type
Weaving configuration
Number of lanes, N
Weaving segment length, LS
Freeway free-flow speed, FFS
Minimum segment speed, SMIN
Freeway maximum capacity, cIFL
Freeway
Two-Sided
2
4360
70
15
2400
ln
ft
mi/h
mi/h
pc/h/ln
Terrain type
Grade
Length
Level
0.00
0.00
%
mi
___________________Conversion to pc/h Under Base Conditions____________________
Volume Components
VFF
VRF
VFR
VRR
Volume, V
1318
0
996
8
veh/h
Peak hour factor, PHF
0.92
0.92
0.92
0.92
Peak 15-min volume, v15
358
0
271
2
Trucks and buses
20
0
10
100
%
Recreational vehicles
0
0
0
0
%
Trucks and buses PCE, ET
1.5
1.5
1.5
1.5
Recreational vehicle PCE, ER
1.2
1.2
1.2
1.2
Heavy vehicle adjustment, fHV
0.909
1.000
0.952
0.667
Driver population adjustment, fP
1.00
1.00
1.00
1.00
Flow rate, v
1576
0
1137
13
pc/h
Volume ratio, VR
0.005
_________________________Configuration Characteristics_________________________
Number of maneuver lanes, NWL
0
ln
Interchange density, ID
0.5
int/mi
Minimum RF lane changes, LCRF
0
lc/pc
Minimum FR lane changes, LCFR
0
lc/pc
Minimum RR lane changes, LCRR
2
lc/pc
Minimum weaving lane changes, LCMIN
Weaving lane changes, LCW
Non-weaving vehicle index, INW
Non-weaving lane change, LCNW
Total lane changes, LCALL
26
163
591
2537
2700
lc/h
lc/h
lc/h
lc/h
_______________________Weaving and Non-Weaving Speeds__________________________
Weaving intensity factor, W
0.155
Average weaving speed, SW
Average non-weaving speed, SNW
62.6
63.3
mi/h
mi/h
_________Weaving Segment Speed, Density, Level of Service and Capacity_________
Weaving segment speed, S
63.3
mi/h
Weaving segment density, D
21.5
pc/mi/ln
Level of service, LOS
C
Weaving segment v/c ratio
0.595
Weaving segment flow rate, v
2479
veh/h
Weaving segment capacity, cW
4167
veh/h
_______________________Limitations on Weaving Segments_________________________
If limit reached, see note.
Weaving length (ft)
Density-based capacty,
cIWL (pc/h/ln)
v/c ratio
Minimum
300
Maximum
5772
Maximum
2400
Actual
4360
Analyzed
2292
Maximum
1.00
Analyzed
0.595
Note
a,b
c
d
Notes:
a. In weaving segments shorter than 300 ft, weaving vehicles are assumed to
make only necessary lane changes.
b. Weaving segments longer than the calculated maximum length should be
treated as isolated merge and diverge areas using the procedures of
Chapter 13, "Freeway Merge and Diverge Segments."
c. The density-based capacity exceeds the capacity of a basic freeway segment,
under equivalent ideal conditions.
d. Volumes exceed the weaving segment capacity. The level of service is F.
______________________________________________________________________________