problem 4: okeechobee road stopped control analysis

Problem 4:

Okeechobee Road

Stopped Control

Analysis

Location and Configuration

T Intersection Very wide median Might operate as

separate conflict points

Right turns removed

NObservations?

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?

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

Conflicting Flow = 2010 vph

Volume (257) vph

Capacity (< 100 vph)

HCM Exhibit 17-7

Conclusion:

Volume > Capacity

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

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.

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

Assumptions

Analysis period=15 min No pedestrians No upstream signals PHF = 0.93 for all movements Level Terrain

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

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

Results

EBT WBT NBL WBL NBR

2010 358 257 120 433

N/A N/A 69 168 226

v/c ratio N/A N/A 3.72 0.71 1.92

N/A N/A 27.1 4.4 31.1

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?

Results

EBT WBT NBL WBL NBR

2010 358 257 120 433

N/A N/A 69 168 226

v/c ratio N/A N/A 3.72 0.71 1.92

N/A N/A 27.1 4.4 31.1

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

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.

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.

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?

Points

Because there is no need to adjust the potential capacity of any movement because of impedance from other movements

When is it appropriate to separate the conflict points?

Points

Only when the queue from one conflict point does not back up into an upstream conflict point

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

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

Observations?

NB Left vs WB Through and Left

Subproblem 4b Capacity N/A

95% queue length (veh) 2.4

Queue storage (veh) 4

Is storage adequate? Yes

v/c ratio 0.46

Delay 17

Observations?

WB Left vs EB Through

95% queue length (veh) 2.07

Queue storage (veh) 3.06

Is storage adequate? Yes

v/c ratio 0.56

Delay 41.7

Observations?

NB Right vs EB Through

95% queue length (veh) 25

Queue storage (veh) N/A

Is storage adequate? N/A

v/c ratio 1.53

Delay 287

Observations?

Have we used the proper procedure for analyzing the operation of the NB right turn?

Is this really a TWSC

operation?

Would it be better to consider this operation in the context of freeway merging

Is this really a TWSC

operation?

Sub-problem 4d

Further Consideration of the Northbound Right Turn

Intersection

Merge Area

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.

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.

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

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

Observations?

EBT NBR

Adjusted flow rate 2010 433

Merge area density 17.7 pc/mile/lane

Results

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.

End of Presentation …

problem 4: okeechobee road stopped control analysis

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