Problem 4: Okeechobee Road Stopped Control Analysis

Location and Configuration N

T Intersection Very wide median Might operate as separate conflict points Right turns removed N Observations?

Peak Hour Volumes What’s missing and why? What’s critical? How Critical? What do we need to analyze? Left Thru Right NB EB---2, WB 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) NBLT 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) LOSAverage Control Delay (sec/veh) A≤ 10 B> 10–15 C> D> E> F>50

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

Input Data EBTWBTNBLWBLNBR Volume Number of lanes Median storage N/A 4 vehN/A Percent trucks

Results EBTWBTNBLWBLNBR Critical gap (sec)N/A Follow up time (sec)N/A Adjusted flow rate (vph) Adjusted capacity (vph) N/A v/c ratioN/A % queue length (veh) N/A Delay (sec/veh)N/A ???67464 LOSN/A FFF 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 EBTWBTNBLWBLNBR Critical gap (sec)N/A Follow up time (sec)N/A Adjusted flow rate (vph) Adjusted capacity (vph) N/A v/c ratioN/A % queue length (veh) N/A Delay (sec/veh)N/A ???67464 LOSN/A FFF 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 EBTWBTNBLWBLNBR Critical gap (sec)N/A Follow up time (sec)N/A Adjusted flow rate (vph) Adjusted capacity (vph) N/A v/c ratioN/A % queue length (veh) N/A Delay (sec/veh)N/A ???67464 LOSN/A FFF 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. N

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? 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

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

Input Data EBTWBTNBLWBLNBR Volume Number of lanes22111 Percent trucks204110

NB Left vs EB Through Subproblem 4b Capacity69 Subproblem 4c Capacity99 95% queue length (veh)24 Queue storage (veh)N/A Is storage adequate?N/A v/c ratio2.6 Delay814 LOSF Observations?

NB Left vs WB Through and Left Subproblem 4b CapacityN/A Subproblem 4c Capacity559 95% queue length (veh)2.4 Queue storage (veh)4 Is storage adequate?Yes v/c ratio0.46 Delay17 LOSC Observations?

WB Left vs EB Through Subproblem 4b Capacity168 Subproblem 4c Capacity213 95% queue length (veh)2.07 Queue storage (veh)3.06 Is storage adequate?Yes v/c ratio0.56 Delay41.7 LOSE Observations?

NB Right vs EB Through Subproblem 4b Capacity226 Subproblem 4c Capacity283 95% queue length (veh)25 Queue storage (veh)N/A Is storage adequate?N/A v/c ratio1.53 Delay287 LOSF Observations?

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?

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?

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) LOSDensity (pc/mi/ln) A≤ 10 B> 10–20 C> 20–28 D> 28–35 E> 35 FV/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 DataEBTNBR Volume Number of lanes21 Free flow speed5535

Observations? EBTNBR Adjusted flow rate Merge area density17.7 pc/mile/lane LOSB 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 …