Transcript Document

Problem 5: Interstate 87 Interchange
We will show how to apply the HCM procedures to study
several different points within the interchange.
 5a: Freeway merges
and diverges
 5b:Route 146
weaving section
 5c: Stop-controlled
intersections
Characteristics of the I-87 Interchange


Partial cloverleaf with 3 on-ramps and 3 offramps
SB there is:




single-lane off-ramp (direct) leading to
Route 146 W
single-lane off-ramp (loop ramp) leading to
Route 146 E
double-lane on-ramp (semi-direct) for both
WB and EB traffic on Rt 146
NB there is:



Double-lane off-ramp. It expands to a
triple-lane ramp then becomes 4 approach
lanes (2 leading WB and 2 leading EB)
single lane on-ramp (loop) from Route 146
EB
single lane on-ramp (direct) from Route 146
WB
Sub-problem 5a: I-87 Interchange - Merges and
Diverges on the Freeway
What parameter defines levelof-service at a ramp junction?
The density of vehicles within
the ramp influence area.
Where is the ramp
influence area?
Observations?
It is the downstream/upstream area
within 1,500 feet of the
merge/diverge point, respectively,
and includes the two right-most
lanes.
Single Lane Ramps
southbound-to-westbound off-ramp
AM Existing condition:
Freeway volume upstream = 3,751 vph
Ramp volume = 437 vph
The downstream off-ramp volume = 314 vph
AM Without condition:
Freeway volume upstream = 3,841 vph
Ramp volume = 455 vph
The downstream off-ramp volume = 326vph
AM With condition:
Freeway volume upstream = 3,851 vph
Ramp volume = 565 vph
The downstream off-ramp volume = 326vph
How does this
ramp perform?
I-87 Interchange Southbound-toWestbound Off-Ramp
Case
Density Speed LOS
AM Existing
23.2
54
C
AM Without
23.9
54
C
AM With
24.6
54
C
LOS = C (all cases)
Multiple Lane Ramps
How would a second lane
affect ramp performance?
Exhibit 2-57. I-87 Interchange Northbound
Off-Ramp
Case
# Lanes Density Speed LOS
Observations?
PM Existing
1
23.2
52
C
PM Existing
2
9.3
52
B
PM Without
1
24.1
52
C
PM Without
2
10.2
52
B
PM With
1
24.1
52
C
PM With
2
10.2
52
B
The densities for a single lane are more than
twice that of a double lane ramp!!!
Sub-problem 5b: I-87 Interchange: Route 146
Weaving Section
In the HCM what are the input
variables for analyzing weaving
sections?
 Type of weave (A, B, or C)
 Length of the weaving section
 Free-flow speed
 Terrain
 The 4 weaving volumes
 Peak hour factor
 Percent trucks
 Percent recreational vehicles
Performance of the Route 146 weaving section under a variety of conditions
Exhibit 2-60. I-87 Interchange P erformance for
the Eastbound Weave on Route 146
Case
Density
Speed
LOS
AM Existing
14.69
43.95
B
AM Without/With
15.35
43.69
B
PM Existing
22.96
43.68
B
PM Without
24.06
43.38
C
PM With
26.43
41.74
C
Prediction of Delays at the Terminus of the
Southbound-to-Eastbound Off-ramp
How realistic is it to assume a
critical gap of 6.2 seconds for
right-turning vehicles?
These results are not consistent
with field conditions, therefore
the critical gap must be much
shorter
Exhibit 2-61. I-87 Interchange P rediction of Delays at the
Terminus of the Southbound-to-Eastbound Off-Ramp
Case
Delay
v/ c
95% Q
LOS
PM With (6.2
sec)
PM With (4.1
sec)
31.7
0.66
4.56
D
15
0.41
1.98
B
How realistic is it to assume a
critical gap of 4.1 seconds for
right-turning vehicles?
These results are closer to
observed field conditions
It is necessary to use an accurate critical gap value!!!
Sub-problem 5: I-87 Interchange – stopcontrolled intersections
Exhibit 2-62. I-87 Interchange P erformance of the
Southbound-to-Westbound Off-Ramp
Note
95% Q LOS
Delay v/ c
Case
AM Existing
29.6
0.78
7.38
D
AM Without
35.2
0.83
8.71
E
AM With
AM With
Modified
PM Existing
74.3
1.04
16.29
F
31.2
0.84
9.52
D
86.4
1.01
11.6
F
PM Without
115.7
1.1
13.88
F
PM With
PM With
Modified
243.4
1.44
24.95
F
66.5
0.98
12.8
F
CG=4.1sec
CG=4.1sec
What might be done to mitigate the LOS F predictions?
What have we learned?
 The interchange is adequate with the site-generated
traffic, except for the SB-to-WB off-ramp and the
WB left onto the SB on-ramp.
 In the context of the HCM procedures, we’ve seen
how a variety of methods can be used to explore
the performance of specific locations within the
interchange, including:



Merge and diverge analyses to the major ramp terminals
Weaving analysis of the EB section of highway between
the two loop ramps
Use of the unsignalized intersection analysis
methodology to assess the performance of two stopcontrolled intersections