Problem 4: Okeechobee
Road StopControlled Analysis
Printable Version
This
intersection is located at the north end of the Krome Avenue study area. It
is a T intersection currently stopcontrolled, with the
northbound movement stopped in favor of all eastwest traffic. The
intersection geometrics are shown in Exhibit 324.
This is the only movement
on the entire Krome Avenue route that is stop controlled. It is currently operating
beyond capacity. By 2010, volumes will exceed the capacity by a substantial
amount.
The eastbound right turn is unopposed, because the T intersection
configuration and the geometrics allow it to flow freely. The northbound
right turn merges with the eastbound through movement at a point
approximately 400 ft east of the intersection. Both the northbound and
westbound left turns conflict with the eastbound through traffic. The
critical movement is the northbound left turn.
Exhibit 325. Peak Hour Volumes:
Krome Avenue at Okeechobee Road 
Approach

Left 
Through 
Right 
Northbound 
257 
 
433 
Southbound 
 
 
 
Eastbound 
 
2,010 
389 
Westbound 
120 
358 
 
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Subproblem 3c [
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Exhibit 324. Aerial view of Okeechobee Road
intersection. 


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Problem 4:
Okeechobee Road
StopControlled Analysis
Stopcontrolled intersection analyses
will be treated in four separate subproblems:
Subproblem 4a will
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.
Subproblem 4b will
invoke the full HCM procedure, treating the operation as a conventional
stopcontrolled intersection and ignoring the unusual separation between the conflict
points.
Subproblem 4c will
separate the conflict points for stopcontrol and treat each conflict
point individually.
Subproblem 4d
will consider the question of how best to analyze the capacity of the
northbound right turn, which is well removed from the intersection
operation by channelization.
Questions to consider as you proceed through this problem:

How do you
think the analysis methods compare to the actual performance of the
intersections? 

Do you feel
that this should be modeled as a single intersection? 
 How does the
merge play a role in the intersection capacity? 
Discussion:
Take
a few minutes to consider these questions. When you are ready to
continue, click continue below to proceed.
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Continue ] to Subproblem 4a 
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Subproblem 4a: Potential LeftTurn Capacity
Step 1. Setup
We will now look at the operation of the northbound leftturn movement and consider the potential leftturn capacity of that
movement as it crosses the eastbound movement.
Exhibit 326 shows the northbound leftturn queue at the
Okeechobee Road Intersection. Observe the number of heavy vehicles in the
traffic stream, each of which will require longer gaps in order to pass
through the conflicting traffic stream. Notice also the skid marks in the
foreground, possibly indicating that at least some northbound left turns are
attempted through eastbound vehicle gaps that are too short. This
observation may be further confirmation of the capacity deficiency for the
northbound left turns that seems apparent in the picture.
The heavy northbound congestion evident in the current operation in
Exhibit 326
suggests that the capacity of the northbound left turn should first be
examined by the basic principles set forth in the HCM before the full
procedure is invoked by software. This step will give us a better
understanding of the basic relationships that apply to TWSC control.
Consider:

What volumerelated factors affect the northbound leftturn movement
capacity? 

What geometryrelated factors affect the northbound
leftturn movement capacity? 

How could we take into account the separation of the
roadways? 
Discussion:
Take
a few minutes to consider these questions. Click continue when you are
ready to proceed. [
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Continue ] with
SubProblem 4a 
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Northbound leftturn at Okeechobee Road. 


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Subproblem 4a:
Northbound LeftTurn Capacity
Step 2: Results
What volumerelated factors affect the northbound leftturn movement
capacity?
The basic relationship
between movement capacity is defined by the conflicting flow rate and the
driver characteristics (critical gap and
followup time). Using what are
widely considered default values for critical gap and follow up time as
described in HCM Exhibit 175, one can graphically represent the
relationship as shown below (which is similar to HCM Exhibit 177). This
exhibit shows how the capacity for the stopped movement decreases as the
conflicting volume (flow ratio on the xaxis) increases. At very high levels of conflicting traffic,
the capacity for the stopped movement becomes effectively zero because the
availability of acceptable gaps is eliminated.
What geometryrelated factors affect the northbound
leftturn movement capacity? How could we take into account the separation of the
roadways? A review of the
aerial
shows that the median space provides a potential refuge for vehicles that
use twostage gap acceptance. The rightturning traffic is removed from the
intersection. Thus, consideration of the northbound leftturn movement
must recognize that the eastbound through traffic is the only opposing movement
in the first part of the two stage movement. The analysis
that follows considers the first stage in evaluating the capacity of
the northbound Krome Avenue leftturn movement. It should be noted that this
is a simplification and may not consider the effect of vehicles in the
median, which can prevent northbound leftturn traffic from even initiating the firststage
movement. In this example, it is clear that the
primary conflict under these traffic conditions is the eastbound through
movement, which is significantly higher than the westbound through movement.
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SubProblem 4a 
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Subproblem 4a:
Northbound LeftTurn Capacity
Exhibit 327
shows several plotted lines
representing the different minor movements at an unsignalized intersection. The dashed
horizontal lines near the bottom of the graph identify the estimated
potential capacity for both the minor street leftturn movement (shown as a
yellow dashed line and depicting the northbound leftturn from Krome Avenue
onto Okeechobee Road), and also the major street leftturn movement (shown
as a green dashed line and depicting the westbound left turn from Okeechobee
Road onto Krome Avenue). Both of these capacity estimates are based on the
projection that there will be a conflicting volume of about 2,010 vehicles
during the peak hour for the first stage of the minor street leftturn movement (see
Exhibit 325).

Exhibit
327. Potential capacity of a stopcontrolled movement as a function of
the conflicting traffic volume. (Source: HCM Exhibit
177). 
The minor street (northbound) leftturn's estimated capacity of
65
vph is considerably less than the peak hour demand of 257 vph, confirming
the capacity deficiency we observed in the
photo.
Conversely, the estimated capacity of the major street (westbound) left turn
(220
vph) appears sufficient to accommodate the peak hour demand of 120 vph.
[
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SubProblem 4a 
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Subproblem 4a: Northbound LeftTurn Capacity
The logical conclusion to
draw from the graph on the preceding page is that the minor street conflicting movement volume is
too heavy to permit a viable TWSC operation at this intersection. Without
going into the numbers in any greater detail, the graphical presentation indicates that the demand
volume is considerably higher than the capacity. Keeping in mind that the
potential capacity for a movement does not consider the competition from
other movements at the same priority level, it will generally represent an
optimistic assessment of the capacity. When even this optimistic assessment
fails, you would conclude that there is no point in proceeding any further
with the investigation of TWSC.
Normally you would stop at
this point and look at some other control alternatives. We will, however,
carry the TWSC concept into a couple of other subproblems to illustrate
some features of the HCM analysis procedure and to set the stage for the
consideration of other alternatives.
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SubProblem 4b 
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Subproblem 4b:
Twoway Stop Controlled
Analysis
Step 1.
Setup
Exhibit 328. Conventional
Intersection Conflicts 

The unusual geometrics,
especially the physical distance separating the conflicting movements at the
Krome Avenue/Okeechobee Road intersection, will require some thought
about how to represent the intersection for analysis by the HCM procedures.
The conventional intersection conflict points are shown in Exhibit 328. 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.
In this
subproblem, we will carry out a conventional intersection analysis. Then we
will examine the results to determine if our treatment was appropriate.
Consider:

What movements are considered in the HCM procedures?

 What is the basis for determining LOS in the
unsignalized intersections methodology? 
Discussion:
Take
a few minutes to consider these questions. Click continue when you are
ready to proceed.
[ Back
] to SubProblem 4a [
Continue ] with SubProblem 4b 
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Subproblem 4b:
Twoway Stop Controlled
Analysis
What movements are considered in the HCM procedures?
The HCM procedures compute
the capacity, control delay, and level of service for all movements that must
yield to other movements, including the left turns from the major street.
Through and rightturn movements on the major street are excluded from the
analysis and are assumed to have no delay.
This simplifying
assumption raises a point of interest. Heavy vehicles making right turns
will sometimes cause significant delays to traffic on the major street. This
phenomenon is overlooked by the HCM procedure. If such delays are of concern
to a particular analysis, it will be necessary to apply microscopic
simulation modeling tools to supplement the HCM analysis. For purposes of
this discussion, we will assume that traffic delay to the through movements
on the major street is not an issue.
Exhibit 329. LOS Thresholds for
TWSC Intersections
(HCM Exhibit 172) 
LOS 
Average Control Delay
(sec/veh) 
A 
≤ 10 
B 
> 10–15 
C 
> 1525 
D 
> 2535 
E 
> 3550 
F 
>50 
What is the basis for determining LOS in the unsignalized
intersections methodology?
The level of service is
based on the control delay according to Exhibit 329. HCM Chapter
17 prescribes the full procedure for computing control delay.
[
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Subproblem 4b:
Twoway Stop Controlled
Analysis
Step 2. Results
The results of this
analysis are presented in
Exhibit 330. These results reaffirm the conclusions
drawn from subproblem 4a, specifically that TWSC is not a viable control
alternative. The v/c ratio for the NBL movement is calculated to be 3.72,
which means the demand volume
is equivalent to 372% of the available capacity. The NBR movement v/c ratio,
calculated to be 1.92, is also an obvious problem.
The WBL movement, on the
other hand, appears to be operating within its capacity, with a v/c ratio of
0.71. This presents an interesting contrast with the NBL movement, since
both movements have to contend with the same conflicting volume (i.e., 2,010
vph from the WBT). The difference may be seen in both the graphical
representation of
Exhibit 327
and the numerical discussion presented in
subproblem 4a.
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Subproblem 4b:
Twoway Stop Control with
a Normal Urban Intersection Treatment
Exhibit 330 explains why
the capacity lines shown in
Exhibit 327 are in different places. The formula for computing the capacity
of a movement that must yield to an opposing movement is given in HCM
equation 173. This equation contains two parameters:

The critical gap,
which indicates the length of a gap in the opposing flow required to
accommodate the first queued vehicle trying to cross the opposing flow.

The followup time,
which indicates the additional gap length required to accommodate each
subsequent vehicle entering the same gap in the opposing traffic.
Larger values for each of
these parameters will lower the capacity for the entering movement. The
values shown in Exhibit 330 indicate lower values for the left turn crossing
the opposing traffic than for the minor street entry movements. This
indicates that drivers making left turns from the major street are able
to accept shorter gaps in the opposing traffic than drivers who are
entering the major street from a minor street approach. The result is a
higher capacity for the WBL movement compared to the NBL movement.
Exhibit 330. TWSC Analysis
with a Normal Urban Intersection Treatment 
Assumed
Parameters 
Movement 
Input Data 
EBT 
WBT 
NBL 
WBL 
NBR 
Volume (vph) 
2,010 
358 
257 
120 
433 
Number of lanes 
2 
2 
1 
1 
1 
Median storage
(vehicles) 
N/A 
N/A 
4 
N/A 
N/A 
Percent trucks 


20 
41 
10 
Analysis Results 
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 (sev/veh) 
N/A 
N/A 
??? 
67 
464 
LOS 
N/A 
N/A 
F 
F 
F 
Simplifying Assumptions
Analysis
period=15 min
No pedestrians
No upstream
signals
PHF = 0.93 for
all movements
Level Terrain 
One of the objectives
of this exercise was to judge whether it is appropriate to consider the
intersection in the context of a normal urban intersection with TWSC
control. This task can be accomplished best by comparing the results in
Exhibit 330 with the corresponding results obtained by treating each of the
conflict points separately. We will examine the separation of conflict
points in the next subproblem. [
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Continue ] to SubProblem
4c 
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Subproblem 4c: Separating the
Conflict Points for TWSC Control
Step 1.
Setup
Because of the wide
median, the high speed, and the rural nature of the channelization design
for the right turns at the Krome Avenue/Okeechobee Road intersection (see
photo), it
could be argued that this intersection is unlikely to operate
as the single urban intersection we considered in subproblem 4a but rather as
four separate intersections, with each intersection representing one of the
conflict points identified in the diagram below. As a general rule, the
separate analysis of individual conflict points will usually give a more optimistic assessment of the
operational characteristics of the intersection than will the aggregation of conflict points into a single
intersection.
Consider:

Why would separating conflicts produce a more
optimistic assessment of the intersections? 
 How are the conflict points interrelated? 
Discussion:
Take
a few minutes to consider these questions. Click continue when you are
ready to proceed. [
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Continue ] with SubProblem 4c 
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Subproblem 4c: Separating the
Conflict Points for TWSC Control
Let's consider the questions from the previous page.
Why would separating conflicts produce a more
optimistic assessment of the intersections?
Separating conflicts may produce a more optimistic assessment because, as
conflicting streams of traffic are removed from consideration, there may
appear to be more opportunities for acceptable gaps (and fewer
impedance effects from higherranked minor movements) than is actually
the case. The relationship is exponential and depends on a number of conditions,
resulting in the potential of significant overestimation of
capacity. For this reason, caution must be used when separating the conflict points for an unsignalized intersection.
How are the conflict points interrelated? The
most obvious relationship between the conflict points is how the paths of
vehicles overlap multiple conflict points. For example, the northbound leftturn movement must pass through two points. Thus, if the second conflict
path (northbound left turn and westbound through) is currently blocked by a
queue of vehicles waiting
for access, the analysis may be invalid. To determine whether the conflict
points at an intersection may be separated, it is necessary to estimate the
queue length for the each of the entering movements. If the estimated queue lengths are greater than
the available storage space, then the separation of conflict points may
overestimate or produce an unrealistic assessment of the operation.
Step 2. Results
Exhibit 331
shows the
results of this analysis. In all cases, the movement capacities were
improved in comparison with Subproblem 4b (see
Exhibit 330), which considered all of the
intersection conflicts simultaneously. This would be expected, but the
important question is whether or not the queue backup would exceed the
available storage space, thereby invalidating the analysis. Inspection of
Exhibit 331
indicates that the 95^{th}percentile queue lengths remained well
within the storage boundaries. So, it could be concluded that it is
appropriate to separate the conflict points for this intersection. While the
separation of conflict points improved the operation slightly, some of the
movements remain badly oversaturated—and the earlier conclusion that TWSC
will result in a peak hour deficiency is reaffirmed.
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Subproblem 4c: Separating the
Conflict Points for TWSC Control
One more observation may
be made from Exhibit 331. The NB right turn, even with the conflict points
separated, indicates an oversaturated condition when analyzed as
a TWSC operation. Because of the geometrics, the rightturn entry has more
of the characteristics of a merge than a stopcontrolled approach. This
should raise some question as to whether another analysis procedure might be
more appropriate. The treatment as a freeway entrance ramp will be
considered in the next subproblem.
Exhibit 331. TWSC Analysis
with Conflict Points Separated 
Input Data 
EBT 
WBT 
NBL 
WBL 
NBR 
Volume 
2,010 
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 
 

LOS 
 
 
F 
 

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 
 

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 

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 




288 
LOS 




F 
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4d 
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Subproblem 4d:
Further Consideration of
the Northbound Right Turn
Step 1. Setup
The analyses conducted
at Krome Avenue and Okeechobee Road to this point have treated the
northbound right turn as a stopcontrolled movement. As shown in the
previous problem, this results in an estimated capacity that anticipates a
failing condition. Closer
inspection of the assumption reveals that this may be too conservative and
that it should be considered as a merge. As shown in the aerial photograph of
Exhibit 324 and in Exhibit 332, below, the rightturn channelization is designed more as a merge
than a conventional intersection.
Exhibit 332. Northbound Right Turn Site Photograph
Consider:

What HCM procedure might be employed to consider the
operations of this location? 
 What is the basis for determining LOS in the
unsignalized intersections methodology? 
Discussion:
Take
a few minutes to consider these questions. Click continue when you are
ready to proceed. [
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Continue ] with SubProblem 4d 
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Subproblem 4d:
Further Consideration of the Northbound Right Turn
What
HCM procedure might be employed to consider the operations of this location?
The ramps and ramp junctions procedure (Chapter 25) will be
considered in order to explore an alternative approach to assessing the
performance of the northbound rightturn movement. In using this procedure,
we would evaluate LOS on the basis of the thresholds shown in Exhibit 333.
In order to analyze the
intersection as a merge area,
though,
we must first find a better way to analyze the capacity for this movement.
Ideally, we would like to eliminate the movement from consideration within
the more standard intersection analysis. Before
we do that, we must satisfy ourselves that it will not experience capacity
problems.
Exhibit 333. LOS Thresholds for
Merging
(HCM Exhibit 254) 
LOS 
Density (pc/mi/ln) 
A 
≤ 10 
B 
> 10–20 
C 
> 20–28 
D 
> 28–35 
E 
> 35 
F 
v/c>1.0 
The HCM does not prescribe
an explicit procedure for evaluating atgrade intersections with merge area
characteristics. Therefore, we must develop an approximate estimation of
performance using one of the available procedures. We have already tried the HCM procedure for TWSC analysis and have not been able to justify the
elimination of the movement based on the
v/c ratio. On the other hand, we
must view the TWSC procedure as pessimistic because of the design of the
merge area.
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Subproblem 4d:
Further Consideration of the Northbound Right Turn
What is the basis for determining LOS in the unsignalized
intersections methodology?
HCM Chapter 25 provides a procedure for estimating freeway merge area
performance in terms of the traffic density. Density is used in all HCM
freewayrelated chapters as an indicator of congestion level. The density
thresholds for each LOS are given in
Exhibit 333.
Chapter 25 of the HCM
suggests that
these procedures may be applied, in an approximate manner, to completely
uncontrolled ramp terminals on other types of facilities such as multilane
and twolane highways. Since
an approximation is what we are seeking, we can apply the
procedures with some
confidence.
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Subproblem 4d:
Further Consideration of the Northbound Right Turn
Step 2. Results
Exhibit 334 shows the
assumptions and results for the merging analysis. The estimated merge area
density is 17.7 vehicles per mile per lane, indicating LOS B. Based on this
analysis, we would be fairly safe in concluding that the NB right turn will
operate well within its capacity. We can therefore feel comfortable about
eliminating this movement from the TWSC analysis.
Exhibit 334. Merging
Analysis for NBRT Using HCM Chapter 25 
Input Data 
EBT 
NBR 
Volume 
2,010 
433 
Number of lanes 
2 
1 
Free flow speed 
55 
35 
Analysis Results 
Adjusted flow
rate 
2,010 
433 
Merge area
density 
17.7 pcpmpl 
LOS 
B 
Simplifying
Assumptions
No other ramps
present
Driver pop.
adjustment =1.0
PHF =1
10% Trucks and
RVs
Right side entry
1,200 foot
acceleration lane
Level terrain 
We have performed an
exhaustive analysis of TWSC operation in this problem. Several interesting
situations have been explored with respect to the HCM; however, our basic
conclusion about the infeasibility of TWSC as a control mode has not changed.
We will therefore explore the idea of signalization in the next problem of
this case study.
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