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however, for the purposes of this example, these are the agency's desired
alternatives.
OPERATIONAL EVALUATION
Input Data
Exhibit 6-3 illustrates the design-year turning-movement volumes at the
example intersection.
Exhibit 6-3
Turning-Movement Volumes
Exhibit 6-4 provides the specific input parameters to use in the operational
evaluation of the approach alternatives.
Exhibit 6-4 Input Parameter Description
Input Parameters VTH = 425 vph Total approach through demand
STH = 1,800 vph Through-movement saturation flow rate
VRT = 75 vph Total approach right-turn volume
SRT = 1,550 vph Right-turn movement saturation flow rate
VA = 35 mph Approach speed
IW = 110 ft Intersection width, from stop bar to far curb
GE = 25 sec Approach effective green time
C = 110 sec Intersection cycle length
LVEH = 20 ft Average length between vehicles under stop condition
AVEH = 10 ft/sec2 Average vehicle acceleration rate from stop condition
TC = 6 sec Critical gap in neighboring CTL traffic lane
RT = 1 sec Driver reaction time
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The remaining sections describe the analysis and design steps for the sample
application.
Inser t Input Data into the Computational Engine
Exhibit 6-5 illustrates the input data for this example as entered in the one-
CTL computational engine. It is important to understand when using the
computational engine that input parameters should remain consistent through
the evaluation of all alternatives unless site-specific data are collected to
demonstrate otherwise. In this example, all of the input parameters remain
consistent across all four treatment options.
1-CTL ONLY Exhibit 6-5
COMPUTATIONS OF ATL LENGTHS (UPSTREAM AND DOWNSTREAM)-- FOR VARIOUS LANE CHOICES Computational Engine Input Screen
INPUT DATA HERE - CASE I IS THE BASELINE (SINGLE SHARED THRU+RIGHT LANE)
ALL CELLS EXCEPT INPUT CELLS ARE PROTECTED
1 ENTER THE CASE STUDY ID OR TITLE IN YELLOW BOX HYPOTHETICAL CASE STUDY
2 ENHANCEMENT: EXCLUSIVE RIGHT TURN LANE (Y/N)? N Please enter data in CAPS for first two entries
3 ENHANCEMENT: ADDITIONAL EXCLUSIVE ATL (Y/N) ? N This entry cannot be "Y" if previous enry is "N"
4 TOTAL APPROACH THROUGH VOLUME= 425 VPH
5 RIGHT TURN VOLUME= 75 VPH
6 APPROACH SPEED (MPH)= 35 MPH
7 THRU SATFLOW PER LANE= 1800 VPH
8 RIGHT SATFLOW PER LANE= 1550 VPH
9 APPROACH EFFECTIVE GREEN= 25 SEC
10 INTERSECTION CYCLE LENGTH= 110 SEC
11 APPROACH EFFECTIVE GREEN WITH ATL / OTHER ADDS= 25 SEC DEFAULT
12 AVERAGE VEHICLE SPACING AT STOP= 20 FT 20
13 AVERAGE ACCELERATION RATE FROM STOP = 10 FT/SEC/SEC 10
14 INTERSECTION WIDTH (STOPLINE TO FAR CURB)= 110 FT 40
15 CRITICAL GAP IN NEIGHBORING CTL TRAFFIC LANE= 6 SEC 6
16 DRIVER REACTION TIME= 1 SEC 1
BASED ON THE INPUTS- THIS IS ANALYSIS CASE III 1-CTL+SHARED ATL
Evaluate Alternative ATL Configurations
This step involves toggling the input values in Line 2 and Line 3 to match the
desired ATL configuration. To analyze an ATL with an exclusive right-turn lane,
the input parameter in Line 2 is set to "Y", otherwise it is set to "N". If the ATL is
exclusive and a right-turn lane is present, Line 3 is set to "Y", otherwise it is set
to "N". The base case condition of a single CTL with shared right-turn
movements is analyzed automatically.
Exhibit 6-6 displays the results from the evaluation of the four alternatives
for the eastbound approach as reported from the computational engine.
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Exhibit 6-6 95 th %
Traffic Operations Analysis Delay Queue
Results for the Eastbound Condition Lane VTH VRT VTot v/c (sec) LOS (ft)
Approach Base Case CTL+RT 425 75 500 1.25 174 F 800
Add Right- RT 0 75 75 0.21 36 D 100
Turn (RT) CTL 425 0 425
Lane 1.04 97 F 500
Add Shared ATL 138 75 213 0.55 43 D 200
ATL CTL 287 0 287 0.70 49 D 300
Add ATL & RT 0 75 75 0.21 36 D 100
RT Lane
ATL 138 0 138 0.34 38 D 100
CTL 287 0 287 0.70 49 D 300
Results from the analysis indicate the following:
· Under the base-case alternative, the eastbound approach has a volume-to-
capacity ratio of 1.25 and operates at LOS F. These indicators show that
the approach has more demand than the existing lane configuration and
signal timing scheme can discharge through the intersection under
forecast traffic conditions and that vehicles will experience high delays.
This finding reinforces the need for a capacity improvement.
· With the addition of an exclusive right-turn lane, the CTL continues to
operate at LOS F and slightly above capacity at a volume-to-capacity ratio
of 1.04. Removing the right-turn demand from the CTL is not a sufficient
improvement for this approach as the through demand is too high to be
served by a single CTL.
· With the addition of the shared ATL, 138 out of 425 through vehicles (32
percent) are forecast to use the ATL. The result is that both the ATL and
CTL operate below capacity and at LOS D. The 95th percentile back of
queue is estimated to be 300 feet for the CTL.
· With the addition of an exclusive right-turn lane and an ATL, all of the
right-turn volume shifts to the exclusive right-turn lane resulting in
further improved operation of the ATL over the previous alternative.
Given that the total through traffic in the CTL remains the same the
performance of the CTL is identical to the shared ATL alternative.
Evaluate Anticipated Safety Effects
In addition to evaluating intersection operations, the potential safety impacts
of installing an ATL will be investigated. As stated in Chapter 4, ATLs add lane -
changing activity and this may lead to an increase of sideswipe crashes,
especially in the downstream merge area. On the other hand, the forecast
increase in through-movement capacity demonstrated in the above operational
analysis may help prevent some rear-end and other congestion-related crashes
on the ATL approach, especially since the approach is forecast to operate in a
congested condition without the ATL.
The following list presents an assessment of safety considerations:
· Access control. There are no driveways in this example.
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· Sight lines. The subject approach has adequate sight lines in this
example. There are no visual obstructions within the sight lines.
· Queuing downstream of the ATL merge. There are no downstream
bottlenecks causing queue spillback to the ATL.
Evaluate Multimodal Effects
This step evaluates effects on non-auto users to identify additional design
needs for the ATL:
· Pedestrians. Pedestrian volumes in this example are low and the
additional crossing time required to accommodate the ATL can be
accommodated within existing signal timing.
· Bicyclists. Bicycle lanes are not being accommodated in this example.
· Transit Vehicles. No bus stops are included in the ATL in this example.
Select a Preferred Alternative
The selection of the alternative should consider multiple factors, including
user considerations, operational performance, safety performance, cost,
environmental impacts, time to implement, and public perception.
There are multiple ways to evaluate alternatives including but not limited to
criterion rating, benefit-cost analyses, and best-value that meets the desired
operational standard. For the purposes of this example application, the best-
value alternative will be implemented, meaning the alternative with the least
negative impact that satisfies operational performance standards will be selected
as the preferred alternative, provided that it does not significantly compromise
safety or other modes of travel. In this example, it is assumed that all of the
alternatives are deemed feasible from user consideration, safety, and cost
perspectives, and that the local highway agency is therefore focused on
identifying the alternative with the lowest negative impact that meets its
operational standard. The consideration of alternatives was conducted as
follows:
· Alternative 0. Does not meet the local highway agency operational
standard; therefore, this alternative is eliminated from further
consideration.
· Alternative 1. Improves the operational performance of the eastbound
approach, but still falls short of meeting the local highway agency
operational performance standard; therefore, this alternative is also
eliminated from further consideration.
· Alternative 2. Improves the operational performance of the eastbound
approach to a satisfactory condition.
· Alternative 3. Improves the operational performance of the eastbound
approach to a satisfactory condition, but requires additional lane
widening to accommodate an exclusive right-turn lane in comparison to
Alternative 2.
Based on this evaluation, and the earlier statement that each alternative is
adequate from the user needs, safety, and cost perspective, Alternative 2
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