Transcript Document

Problem 3: Weaving & Ramp Sections
 3a: Analysis of a
Weaving Section
 3b: Freeway Ramp
Analysis
 3c: Non-standard
Ramp and Weave
Analysis
 3d: Analysis of a
Collector/Distributor
Road
Sub-Problem 3a
This sub-problem focuses on
four weaving sections in the
Route 7 / I-787 interchange.
Observations?
Questions to consider:
 What are some of the
elements to consider
when studying a weave
section?
 How do we determine
the LOS for weave
sections?
Route 7 / I-787
Route 7
Interchange
Alternate
Rte. 7
North
J
F
G
I
K
E
A
E L
A
B
B
H
I-787
D
C
C
I-787
Weaving sections of
interest include:
23rd Street
Observations?
Weaving Sections
What is important to consider when
analyzing a weaving section?
1) Type of weave
2) Weaving length
3) Distribution of flows within the weave
4) Speeds of the weaving & non-weaving
movements
5) PHF
6) Percentages of trucks, buses, &
recreational vehicles
Observations?
7) Passenger car equivalents
Weaving Segments A & B
 Characteristics:




4 lanes
Length of A = 3100’
Length of B = 2600’
Entering & exiting facilities
each 2 lanes
What type of weave is at each of
these locations?
Type A
How can the speeds for weaving
(Sw) & non-weaving (Snw) vehicles
be computed?
HCM 2000 Eqn 24-2
View of Route 7 looking west at the
western end of Location A
Weaving Segments A & B
Exhibit 4-47. Weave Analysis Results A & B
Sw
Snw
S
Wnw
mph
mph
mph
Density
pcpmpl
LOS
Type of
Operation
1.11
0.18
41.07
61.59
49.03
16.63
B
Constrained
0.66
0.09
48.06
65.38
54.91
8.53
A
Constrained
AM
0.71
0.09
47.25
65.25
55.95
10.02
B
Constrained
PM
0.92
0.14
43.69
63.38
52.07
13.48
B
Constrained
Weaving
Segment
Peak
Period
Ww
A
AM
A
PM
B
B
At higher volumes what
happens to Ww and Wnw?
They are also higher
What does this mean?
Density increases,
therefore, LOS decreases
Weaving Segments A & B
Exhibit 4-47. Weave Analysis Results A & B
Sw
Snw
S
Wnw
mph
mph
mph
Density
pcpmpl
1.11
0.18
41.07
61.59
49.03
16.63
PM
0.66
0.09
48.06
65.38
54.91
AM
0.71
0.09
47.25
65.25
55.95
10.02
B
Constrained
0.92
0.14
43.69
63.38
52.07
13.48
B
Constrained
Weaving
Segment
Peak
Period
Ww
A
AM
A
B
B
PM
LOS
Type of
Operation
B
Constrained
Observations?
8.53
A
Constrained
There is a signal at the end of weaving section B, where
the PM peak traffic is heavy enough that the length of the
double-lane queue often extends across the bridge. WHAT
DOES THIS MEAN?
Weaving vehicles can’t take advantage of the length of the bridge
to make their lane changes. Motorists must make a lane change
before the end of queue if they want to go north on I-787.
Then what length of weaving section is required to have
a reasonable LOS?
Weaving Segment C
 Characteristics:
Inbound facilities:
I-787 NB – 3 lanes
23rd St on-ramp – 1 lane
Outbound facilities:
I-787 NB – 2 lanes
Rt 7 EB off-ramp – 2 lanes
What type of weave is this?
Type C
Observations?
View of I-787 North at Location C
just before the two right hand
lanes leave to go toward Route 7
east
Weaving Segment C
3392
I-787NB
1013
Rte. 7EB
off ramp
 Do we know the
distribution of flows for the
weaving and non-weaving
segments?
No, collecting this type of data in the field
requires significant data collection coordination
and significant time.
3997
I-787NB
408
23rd Street
on ramp
 How do we get
these volumes?
Observations?
Estimate the volumes, then conduct a
sensitivity analysis to determine the validity
of the estimates
Weaving Segment C
 3 possible scenarios for flow distributions
1) All the 23rd St on-ramp traffic goes to I-787 N.
This maximizes the weaving volumes.
2) Inbound flows go to the outbound legs
proportional to the exiting volumes.
3) A larger percentage going to D from B (40%).
This reduces the amount of traffic from A going
to D. Thus, the weaving traffic decreases and
the non-weaving traffic increases.
Weaving Segment C
Exhibit 4-52. Flow Distribution Analysis Results
Scenario
Ww
Wnw
Sw mph
Snw
mph
S mph
Density
pcpmpl
LOS
Operation
of Type
1
0.7
0.42
47.35
53.79
51.53
24.21
C
Unconstrained
2
0.65
0.34
48.35
55.94
53.58
23.29
C
Unconstrained
3
0.61
0.3
49.1
57.46
55.13
22.62
C
Unconstrained
What does it mean that the LOS = C
in all cases?
The LOS isn’t very sensitive to the
distribution of volumes among the
four weaving movements
Why is the density greatest in scenario 1?
What does this mean?
Observations?
Greatest weaving volumes in Scenario 1. The
higher the weaving volume the higher the density
Weaving Segment E
 Characteristics:




3 lanes
Located on I-787 north
between Route 7 east on-ramp
& Route 7 west off-ramp
Length = 790’
Heavy PM volumes
Observations?
View of I-787 North at Location E
just before the loop ramp
What type of weave is at this location?
diverges to go toward Route 7
west
Type A
Weaving Segment E
Assumptions to be made with the input data
1) The FFS on the freeway = 55 mph
2) The speed on the on- and off-ramps = 25 mph
3) The peak hour factor = 1.0.
Weaving Segment E
Exhibit 4-53. AM and P M P eak Hour Analysis Results for Weave E
Weave
P eriod
Ww
Wnw
Sw mph
Snw
mph
S mph
Density
pcpmpl
LOS
Operation
of Type
E
AM
2.15
0.32
29.31
49.16
35.15
15.85
B
Constrained
E
PM
4.61
0.88
23.02
38.97
27.87
44.99
F
Constrained
Based on the results what conclusions
can be made?
Much better service in the AM
Observations?
Weaving Segment E
 How would changing the
PHF or Speed affect the
results of this weave?
As the PHF increases, the density
of traffic in the weaving segment
decreases and the speeds
increase
As the free flow speed increases,
the densities decrease and the
speeds increase.
Why might the only case of LOS E occur when
the PHF = 1 and the FFS = 65 mph?
Observations?
Exhibit 4-54. Location E, PM Peak Hour PHF and Free Flow Speed Sensitivity Analysis
Sw
Snw
S
Weave
PHF
mph
Ww
Wnw
mph
mph
mph
Density
pcpmpl
LOS
Operation
of Type
E
0.8
55
5.73
1.17
21.69
35.71
26.04
60.21
F
Constrained
E
0.85
55
5.4
1.08
22.03
36.59
26.52
55.64
F
Constrained
E
0.9
55
5.11
1.01
22.36
37.3
26.99
51.63
F
Constrained
E
0.95
55
4.85
0.94
22.69
38.21
27.43
48.14
F
Constrained
E
1
55
4.61
0.88
23.02
38.97
27.87
44.99
F
Constrained
E
0.8
65
5.73
1.17
23.17
40.31
28.29
55.42
F
Constrained
E
0.85
65
5.4
1.08
23.59
41.39
28.87
51.1
F
Constrained
E
0.9
65
5.11
1.01
24
42.41
29.44
47.33
F
Constrained
E
0.95
65
4.85
0.94
24.4
43.37
29.98
44.05
F
Constrained
E
1
65
4.61
0.88
24.8
44.29
30.51
41.1
E
Constrained
Sff
Weaving Segment E
What could be done to Change geometric constraints
this facility to improve (i.e. length of weave and
number of lanes in the weave)
the LOS?
Exhibit 4-56. Effects of Weave Length at Location E
Existing 
HCM Max 
Sw
Snw
S
Wnw
mph
mph
mph
Density
pcpmpl
LOS
Operation
of Type
4.61
0.88
23.02
38.97
27.87
44.99
F
Constrained
1,000
3.83
0.74
24.32
40.91
29.39
42.67
E
Constrained
1,500
2.77
0.54
26.94
44.15
32.3
38.82
E
Constrained
2,000
2.2
0.44
29.07
46.29
34.54
36.31
E
Constrained
2,500
1.84
0.37
30.85
47.83
36.34
34.51
D
Constrained
Weave
Length
Ww
790
Observations?
Conclusions from the Weave Analysis
 Although there are only several types of weaves,
each may have unique characteristics that need to
be considered
 Changing geometric constraints may improve the
operation of a particular weaving section
 HCM guidelines need to be checked versus field
observations to accurately depict what is going
Sub-Problem 3b
This sub-problem
focuses on some basic
issues in ramp analysis.
Questions to consider:
 What analyses might
be applied in this
problem?
 What are some of the
Observations?
unique attributes of
this analysis that need
to be addressed?
Ramps of Interest
Data for Ramps of Interest
Capacities from
HCM Exhibit 23-3
Exhibit 4-58. Operational Analysis of the Route 7/ I-787 Ramps
Volume
v/ c
Ramp Location
# of Lanes
Speed
(mph)
Route 7 EB to I-787 SB
1
30
1,865
785
1,900
0.982
0.414
Route 7 EB to I-787 NB
1
25
155
300
1,900
0.082
0.157
Route 7 WB to I-787 SB
1
40
1,420
745
2,000
0.711
0.371
Route 7 WB to I-787 NB
1
40
125
225
2,000
0.063
0.114
I-787 SB to Route 7 WB
1
35
340
290
2,000
0.171
0.146
I-787 SB to Route 7 EB
1
25
180
150
1,900
0.095
0.079
I-787 NB to Route 7 WB
1
25
715
1,870
1,900
0.376
0.984
I-787 NB to Route 7 EB
2
45
550
1,015
4,100
0.134
0.247
Observations?
AM
PM
HCM
Capacities
AM
PM
What factors affect the
capacity?
What ramps may have
problems and at what
time of day?
- Number of lanes on the ramp
- Ramp FFS
Merge sections of interest
Route 7 Alternate
Rte. 7
include:
North
J
J
F
Location I Characteristics:
G
 - single lane on-ramp
 - 2-lane freeway
 - Acceleration lane ~ 1/10 of a

mile long
 - FFS on the ramp = 25 mph
Location J Characteristics:
 - single lane on-ramp
 - 2-lane freeway
 - On-ramp continues as 3rd lane
23rd Street
 - FFS on the ramp = 35 mph
I
K
I
E L
A
B
H
I-787
D
C
I-787
Location
Density
(pcpmpl)
LOS
I
216
C
Location J
v/ c
On-ramp
Arriving
Freeway
0.34
Departing
Freeway
0.54
What can be done to
improve the LOS to A?
Extend acceleration lane
Exhibit 4-60. Location J-Results of
Acceleration Lane Length
0.46
Observations?
LA (ft)
Density*
pcpmpl
LOS
1,000
21.7
C
1,500
18.6
B
3,000
9.2
A
What would be the
effect of changing
the acceleration lane
length at Location I?
Observations?
What are the benefits of each
choice?
Exhibit 4-61. Variation of the Acceleration Lane Lengths at Both
Locations I & J
Existing I w/ J = 3000’ La
Max I, Min La @ J
Max I, w/ J = 1650’ La
LA at I
(ft)
LA at J
(ft)
Density at
I (pcpmpl)
LOS at I
Density at
J (pcpmpl)
LOS at J
500
3,000
21.8
C
9.2
A
1,500
3,000
15.5
B
9.2
A
2,500
250
9.2
A
18
B
2,500
1,650
9.2
A
10
A
What did we learn from this sub-problem?
 We learned what is involved and that the HCM methodologies are
distributed among 4 chapters: ramps, weaving sections, unsignalized
intersections, and signalized intersections.
 We used the v/c ratio analysis technique in the ramps chapter of the
HCM and determined that 2 of the ramps in the interchange are at or
near capacity. Ideally, their curve radii should be larger or more lanes
should be present.
 We studied the 2 merges that occur on Route 7 going WB. We noticed
that one ramp is difficult to analyze because the acceleration lane
never ends, it continues on as a 3rd lane on the freeway. We
determined how to analyze the level of service of this. We
lengthened the acceleration lane on the 1st ramp to determine how to
achieve LOS A. We found that the pair of ramps could be made to
work well, and the length of the ramp had an impact on performance.
Sub-Problem 3c
This sub-problem focuses
on the southwestern
quadrant of the I-787
interchange, where the
ramps are all non-standard.





Observations?
What is non-standard?
The diverge from Route 7 EB
The split into a
collector/distributor road
The right-hand ramp from
Route 7 EB to I-787 SB
The semi-direct ramp from
Route 7 WB to I-787 SB
The merger of these two ramps
with each other and with I-787
SB
The Southwestern Quadrant
Route 7 EB Diverge
 No deceleration lane. (The exit ramp leaves
Route 7 as soon as it appears. So we need
to set the length of the deceleration lane at
zero.)
 Exit ramp leads to the collector/distributor
road (we need to include the traffic taking
the loop ramp to I-787 north as well as the
traffic taking the right-hand ramp to I-787
south.)
The Short Connector
 150-foot long single-lane ramp between Route 7
east and the beginning of the lane for the
collector/distributor road.
 An analysis of this roadway segment will tell us if
this might be the bottleneck. We’ll compare the
volume it handles with the capacity it ought to have
per the HCM.
 The exiting volume is 2,020 veh/hr, while the
suggested capacity for a single-lane ramp is 2,000
veh/hr; so the v/c ratio is 1.01.
The Right-Hand Ramp
 AM Peak volume on the right-hand ramp =
1,865 veh/hr
 The HCM says capacity for a single lane
ramp with a free flow speed of 30 mph
should be 1,900 veh/hr; so the v/c ratio is
0.98
 4-lane basic freeway section at the merge point
 FFS= 65 mph
 Volume = 5,290 veh/hr
 Density = 20.8pc/mi/ln, which is LOS C.
 The HCM ramp procedure asks us to specify lengths
for both the 1st and 2nd acceleration lane.
 The 1st ramp ends 790 feet downstream of the initial
merge, but the 2nd lane doesn’t end (so assume a
long arbitrary distance)
Merge Analysis
 Set 1st lane length to 790’
 Set 2nd lane length to 4,000’
What is the new
influence area Density?
What was the density
of where the 4 lane
merge starts?
Why is there such a
difference in the densities?
D = 3.9 pcpmpl
D = 20.8 pcpmpl
The introduction of a long
acceleration lane significantly
reduces the density
With D = 3.9 pcpmpl what
is the LOS of this merge?
A or F ?
Why such a poor LOS
with a low density?
The combined volume from the
ramps and the freeway (5,400
veh/hr) produce an influx into the
influence area, which is more than
the 4,600 veh/hr allowed
I-787 SB Weave Analysis
1904
B
What type of
weave is this?
D
100
Type B
3123
A
C
164
Note: The starting point of the weave is
ambiguous. The striping at the north end
of the weave tries to keep the weave from
starting until the lane drop occurs.
Observations?
Weaving Diagram for
Weave D AM Peak Hour
Volumes
Would the weave start early or
later under heavier traffic
conditions?
Earlier
Conditions Depending on Length of Weave
What are the effects of
having a weave that varies
in length?
1904
B
D
100
3123
A
Exhibit 4-62. LOS Variance with Weave Length
Length of
Weave ft
Density
(pcpmpl)
LOS
Type of Operation
1,055
47.94
F
Unconstrained
1,850
43.51
F
Unconstrained
2,000
42.29
E
Unconstrained
Although LOS remains poor, as
length increases, density decreases!!
C
164
What would have to happen
to improve the LOS?
Much greater weave
length
What did we learn from this sub-problem?
 This sub-problem shows that we can use engineering
judgment in combination with the HCM capacities for single
and multi-lane ramp sections to determine where problem
spots might exist in the interchange.
 We also see the attention to detail that is required to identify
bottlenecks.
 In summary, there is more than one way to view a given
situation. Different views are possible, producing different
results. Our responsibility as traffic engineers is to identify
these views, study the system from each, and portray the
results clearly and concisely to decide what recommendations
to make regarding facility enhancements.
Sub-Problem 3d
This sub-problem deals with
the short, single-lane
collector/distributor road
that connects to two ramps:
the I-787 SB to Route 7 EB
loop ramp at its end and the
Route 7 EB to I-787 NB loop
ramp at its beginning.
The focus of this sub-problem
is not on the high volumes or
congested conditions but on
the complexities of performing
the analysis. The collectordistributor doesn’t fit any
standard facility type, yet it
needs to be analyzed.
Consider how you might analyze
this collector/distributor roadway
using the methodologies
presented in the HCM 2000
Observations?
Layout of the Collector-Distributor







The collector-distributor (C-D) connects to Route 7 EB as a single lane exit
without a deceleration lane.
It continues as a single lane for approximately 250’ and a new lane is added
on the left-hand side. The new left-hand lane becomes the continuation of the
C-D road, which means the C-D traffic has to jog left one lane, while the
original lane continues ahead to become the start of the right-hand ramp
leading to I-787 SB.
These two lanes parallel each other for about 1,000’ until the right-hand lane
turns toward I-787 SB. The left-hand lane, the C-D road, continues on for
1,800’ until it joins with the I-787 SB/Route 7 EB loop ramp.
The 1 lane C-D road and the 1 lane loop ramp now become a 2-lane facility.
These 2 lanes continue across a bridge for about 260’ until the right-hand lane
becomes the beginning of the loop ramp to I-787 NB.
We have a small weaving section that starts with the end of the loop ramp
from I-787 SB and ends with the beginning of the loop ramp to I-787 NB.
After the loop ramp to I-787 NB turns off to the right, the C-D road continues
on another 300’ where it rejoins Route 7 EB.
Collector-Distributor Weaving Section
Here we will focus on the weaving section
that takes place with the C-D Road
When doing the weaving
analysis for the C-D Road
what makes it difficult?
How can that be
worked around?
There is only 1 lane on the freeway
Assume 2 lanes on the C-D Road
Characteristics of the Weave:





2 lanes on the C-D Road
Length of Weave = 264’
FFS = 40 MPH
Type A Weave
B-D volume = 0
Results of the C-D Weave
D = 5.02 pc/mi/ln
LOS = A
Unconstrained operation
Weaving & non-weaving speeds are about 33-35 mph
The number of lanes required (Nw = 1.18) is less than the
number needed for unconstrained operation (1.4).
What does this mean?
Although we assumed the C-D road was 2 lanes
wide, and the weaving section 3 lanes wide, only
1.18 lanes were required for the weaving
movements to be unconstrained. The remaining
0.82 lanes were available for any non-weaving
traffic using the C-D road as an alternative to the
mainline lanes for Route 7 EB.
What did we learn from this sub-problem?
 We encountered another situation where the highway
geometrics are non-standard from the perspective of the
HCM; so we need to determine how the situation should be
analyzed.
 Second, a weaving analysis is possible and appropriate
between the loop ramps, provided more-than-normal care is
taken in examining and interpreting the results of the
analysis.
 Third, the computed number of lanes required for an
unconstrained weave needs to be compared with the number
of lanes available, realizing that the non-weaving movements
are effectively zero. This means that if the weaving
movements are acceptable, the entire weaving section is also
acceptable.