Problem 3: Weaving

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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

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Sub-Problem 3a

• This sub-problem focuses on four weaving
  sections in the Route 7 / I-787 interchange.

• 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?

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Route 7 / I-787 Interchange
Route 7
A
E
B
I-787
C
Weaving sections of interest include
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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
• 7) Passenger car equivalents

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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?
View of Route 7 looking west at the western end
of Location A
Type A
How can the speeds for weaving (Sw) non-weaving
(Snw) vehicles be computed?
HCM 2000 Eqn 24-2
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Weaving Segments A B
At higher volumes what happens to Ww and Wnw?
They are also higher
Density increases, therefore, LOS decreases
What does this mean?
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Weaving Segments A B

• 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 cant 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?
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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?
View of I-787 North at Location C just before the
two right hand lanes leave to go toward Route 7
east
Type C
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Weaving Segment C

• 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.

• How do we get these volumes?

Estimate the volumes, then conduct a sensitivity
analysis to determine the validity of the
estimates
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Weaving Segment C

• 3 possible scenarios for flow distributions
• All the 23rd St on-ramp traffic goes to I-787 N.
  This maximizes the weaving volumes.
• Inbound flows go to the outbound legs
  proportional to the exiting volumes.
• 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.

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Weaving Segment C

• What does it mean that the LOS C in all cases?

The LOS isnt very sensitive to the distribution
of volumes among the four weaving movements
Why is the density greatest in scenario 1? What
does this mean?
Greatest weaving volumes in Scenario 1. The
higher the weaving volume the higher the density
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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

View of I-787 North at Location E just before the
loop ramp diverges to go toward Route 7 west
What type of weave is at this location?
Type A
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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.

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Weaving Segment E
Based on the results what conclusions can be made?
Much better service in the AM
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Weaving Segment E
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.

• How would changing the PHF or Speed affect the
  results of this weave?

Why might the only case of LOS E occur when the
PHF 1 and the FFS 65 mph?
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Weaving Segment E
Change geometric constraints (i.e. length of
weave and number of lanes in the weave)

• What could be done to this facility to improve
  the LOS?

Existing ?
HCM Max ?
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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

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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 unique attributes of this
  analysis that need to be addressed?

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Ramps of Interest
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Data for Ramps of Interest
Capacities from HCM Exhibit 23-3
What factors affect the capacity?
What ramps may have problems and at what time of
day?
- Number of lanes on the ramp - Ramp FFS
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Merge sections of interest include
Route 7
J

• Location I Characteristics
• - 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
• - FFS on the ramp 35 mph

I
I-787
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What can be done to improve the LOS to A?
Extend acceleration lane
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What would be the effect of changing the
acceleration lane length at Location I?
What are the benefits of each choice?
Existing I w/ J 3000 La
Max I, Min La _at_ J
Max I, w/ J 1650 La
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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.

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Sub-Problem 3c

• This sub-problem focuses on the southwestern
  quadrant of the I-787 interchange, where the
  ramps are all non-standard.

• 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

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The Southwestern Quadrant
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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.)

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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. Well 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.

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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

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• 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 doesnt end (so
  assume a long arbitrary distance)

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Merge Analysis

• Set 1st lane length to 790
• Set 2nd lane length to 4,000

What is the new influence area Density?
D 3.9 pcpmpl
What was the density of where the 4 lane merge
starts?
D 20.8 pcpmpl
The introduction of a long acceleration lane
significantly reduces the density
Why is there such a difference in the densities?
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• With D 3.9 pcpmpl what is the LOS of this
  merge?

A
F
or
?
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
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I-787 SB Weave Analysis
What type of weave is this?
Type B
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.
Weaving Diagram for Weave D AM Peak Hour Volumes
Would the weave start early or later under
heavier traffic conditions?
Earlier
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Conditions Depending on Length of Weave
What are the effects of having a weave that
varies in length?
What would have to happen to improve the LOS?
Although LOS remains poor, as length increases,
density decreases!!
Much greater weave length
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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.

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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
  collector-distributor doesnt 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
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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.

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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?
There is only 1 lane on the freeway
How can that be worked around?
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

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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.

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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.