N Location and Configuration
Observations? N • T Intersection • Very wide median • Might operate as separate conflict points • Right turns removed
What’s missing and why? What’s critical? How Critical? What do we need to analyze? Left Thru Right NB 257 --- 433 EB --- 2,010 389 WB 120 358 --- Observations? Peak Hour Volumes
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
Conclusion: Volume > Capacity NBLT Conflicting Flow = 2010 vph Volume (257) vph Capacity (< 100 vph) HCM Exhibit 17-7
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 What to do next? Conclusion: Volume > Capacity
Conventional T Intersection Conflict Points 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. Then examine the results to determine if our treatment was appropriate.
Assumptions • Analysis period=15 min • No pedestrians • No upstream signals • PHF = 0.93 for all movements • Level Terrain
Run Input Data
Observations? Results 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 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 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
N 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.
Separated Conflict Points Sub-problem 4c ·Separate the conflict points for TWSC control and treat each conflict point individually. Then compare the results with the treatment of the previous sub-problem.
Conventional T Intersection Conflict Points Separated Conflict Points 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? Because there is no need to adjust the potential capacity of any movement because of impedance from other movements
Conventional T Intersection Conflict Points Separated Conflict Points When is it appropriate to separate the conflict points? Only when the queue from one conflict point does not back up into an upstream conflict point
Run Input Data
Observations? NB Left vs EB Through
Observations? NB Left vs WB Through and Left
Observations? WB Left vs EB Through
Observations? NB Right vs EB Through
Is this really a TWSC operation? 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? Would it be better to consider this operation in the context of freeway merging NB Right vs EB Through
Intersection Merge Area Sub-problem 4d Further Consideration of the Northbound Right Turn
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.
Run 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
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.