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Problem 4:
Okeechobee Road
Stopped Control
Analysis
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Location and Configuration
N
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T Intersection Very wide median Might operate as
separate conflict points
Right turns removed
NObservations?
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Peak Hour Volumes
What’s missing and why? What’s critical? How Critical? What do we need to
analyze?
Left Thru Right
NB257 --- 433EB --- 2,010 389WB 120 358 ---
Observations?
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Sub-problem 4a
Examine the capacity of the critical minor street movement (the northbound left turn) using the graphical solution presented in the HCM, without going through the full procedure
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Conflicting Flow = 2010 vph
Volume (257) vph
Capacity (< 100 vph)
NBLT
HCM Exhibit 17-7
Conclusion:
Volume > Capacity
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What to do next?
Normally we would stop at this point and declare that TWSC is not a viable choice
In this case, we will proceed with more problems to illustrate more features of the TWSC procedure
Conclusion:
Volume > Capacity
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Sub-problem 4b
Invoke the full HCM procedure, treating the operation as a conventional TWSC intersection and ignoring the unusual separation between the conflict points.
Conventional T Intersection Conflict
Points
Then examine the results to determine if our treatment was appropriate.
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LOS Thresholds for TWSC Intersections (HCM Exhibit 17-2)
LOS Average Control Delay(sec/veh)
A ≤ 10
B > 10–15
C > 15-25
D > 25-35
E > 35-50
F >50
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Assumptions
Analysis period=15 min No pedestrians No upstream signals PHF = 0.93 for all movements Level Terrain
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Input Data
EBT WBT NBL WBL NBR
Volume 2010 358 257 120 433
Number of lanes
2 2 1 1 1
Median storage
N/A N/A 4 veh N/A N/A
Percent trucks
20 41 10
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Results
EBT WBT NBL WBL NBR
Critical gap (sec) N/A N/A 7.2 4.9 7.1
Follow up time (sec) N/A N/A 3.7 2.6 3.4
Adjusted flow rate (vph)
2010 358 257 120 433
Adjusted capacity (vph)
N/A N/A 69 168 226
v/c ratio N/A N/A 3.72 0.71 1.92
95% queue length (veh)
N/A N/A 27.1 4.4 31.1
Delay (sec/veh) N/A N/A ??? 67 464
LOS N/A N/A F F F
Observations?While the HCM equations do not limit the range of v/c ratios for which delay may be computed, some software products impose limitations as a practical consideration
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Results
EBT WBT NBL WBL NBR
Critical gap (sec) N/A N/A 7.2 4.9 7.1
Follow up time (sec) N/A N/A 3.7 2.6 3.4
Adjusted flow rate (vph)
2010 358 257 120 433
Adjusted capacity (vph)
N/A N/A 69 168 226
v/c ratio N/A N/A 3.72 0.71 1.92
95% queue length (veh)
N/A N/A 27.1 4.4 31.1
Delay (sec/veh) N/A N/A ??? 67 464
LOS N/A N/A F F F
Why does the WBL have a higher capacity than the NBL when both movements have to yield to same conflicting volume of EB through traffic?
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Results
EBT WBT NBL WBL NBR
Critical gap (sec) N/A N/A 7.2 4.9 7.1
Follow up time (sec) N/A N/A 3.7 2.6 3.4
Adjusted flow rate (vph)
2010 358 257 120 433
Adjusted capacity (vph)
N/A N/A 69 168 226
v/c ratio N/A N/A 3.72 0.71 1.92
95% queue length (veh)
N/A N/A 27.1 4.4 31.1
Delay (sec/veh) N/A N/A ??? 67 464
LOS N/A N/A F F F
Because the HCM tells us that the critical gap and follow up times are both lower for a left turn from the major street than from the minor street. In other words drivers on the major street are willing to accept smaller gaps, so more vehicles can get through the same volume of conflicting traffic
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Because of the wide separation of conflicts at this intersection, it should occur to us that we probably shouldn’t treat this situation as a typical urban intersection.
So, we will examine the separation of conflict points in the next subproblem.
N
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Sub-problem 4c
Separate the conflict points for TWSC control and
treat each conflict point individually.
Separated Conflict Points
Then compare the results with the treatment of the previous sub-problem.
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Why will the separation of conflict points usually give a more optimistic assessment of the operation than the aggregation of conflict points into a single intersection?
Separated Conflict Points
Conventional T Intersection Conflict
Points
Because there is no need to adjust the potential capacity of any movement because of impedance from other movements
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When is it appropriate to separate the conflict points?
Separated Conflict Points
Conventional T Intersection Conflict
Points
Only when the queue from one conflict point does not back up into an upstream conflict point
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Input Data
Input Data EBT WBT NBL WBL NBR
Volume 2010 358 257 120 433
Number of lanes 2 2 1 1 1
Percent trucks 20 41 10
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NB Left vs EB Through
Subproblem 4b Capacity 69
Subproblem 4c Capacity 99
95% queue length (veh) 24
Queue storage (veh) N/A
Is storage adequate? N/A
v/c ratio 2.6
Delay 814
LOS F
Observations?
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NB Left vs WB Through and Left
Subproblem 4b Capacity N/A
Subproblem 4c Capacity 559
95% queue length (veh) 2.4
Queue storage (veh) 4
Is storage adequate? Yes
v/c ratio 0.46
Delay 17
LOS C
Observations?
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WB Left vs EB Through
Subproblem 4b Capacity 168
Subproblem 4c Capacity 213
95% queue length (veh) 2.07
Queue storage (veh) 3.06
Is storage adequate? Yes
v/c ratio 0.56
Delay 41.7
LOS E
Observations?
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NB Right vs EB Through
Subproblem 4b Capacity 226
Subproblem 4c Capacity 283
95% queue length (veh) 25
Queue storage (veh) N/A
Is storage adequate? N/A
v/c ratio 1.53
Delay 287
LOS F
Observations?
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NB Right vs EB Through
Have we used the proper procedure for analyzing the operation of the NB right turn?
Is this really a TWSC
operation?
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NB Right vs EB Through
Would it be better to consider this operation in the context of freeway merging
Is this really a TWSC
operation?
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Sub-problem 4d
Further Consideration of the Northbound Right Turn
Intersection
Merge Area
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The HCM does not prescribe an explicit procedure for at-grade intersections with merge area characteristics.
We must view the TWSC procedure as pessimistic because of the design of the merge area.
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The logical next step would be to treat this entrance as a freeway merge, using HCM Chapter 25, which prescribes a procedure for estimating freeway merge area performance in terms of the traffic density.
Density is used in all HCM freeway-related
chapters as an indicator of congestion level. The density thresholds for each LOS are given in HCM Exhibit 25-4.
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LOS Thresholds for Merging(HCM Exhibit 25-4)
LOS Density (pc/mi/ln)
A ≤ 10
B > 10–20
C > 20–28
D > 28–35
E > 35
F V/C>1.0
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Assumptions and Parameters
Right side entry, No other ramps present Driver pop. adjustment =1.0, PHF =1 10% Trucks and RVs Level terrain, 1200 foot acceleration lane
Input Data EBT NBR
Volume 2010 433
Number of lanes 2 1
Free flow speed 55 35
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Observations?
EBT NBR
Adjusted flow rate 2010 433
Merge area density 17.7 pc/mile/lane
LOS B
Results
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Problem 4 Conclusions
HCM TWSC procedure applies to all movements except the channelized right turns, which may be eliminated from the analysis
Conflict points may be separated because queues do not block upstream conflict points
TWSC is not a viable control mode because it will not provide adequate capacity for all movements
Problem 5 will therefore examine signalization of this intersection.
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End of Presentation …