Chapter 151 Chapter 15: Weaving, Merging, and Diverging Movements on Freeways and Multilane Highways Describe weaving configurations Describe merge and.

Chapter 15 1

Chapter 15: Weaving, Merging, and Diverging Movements on Freeways and

Multilane Highways

Describe weaving configurations Describe merge and diverge configurations Become familiar with the steps of conducting a weaving area

analysis Become familiar with the steps of conducting a merge/diverge

area analysis Conduct these analyses manually and by HCS2010 Discuss the limitations of these models

Chapter objectives: By the end of this chapter the student will be able to:

Chapter 15 2

I-215 and 21st South, SLC

Chapter 15 3

15.1 Turbulence areas on freeways and multilane highways

Turbulence as characterized by the additional lane-changing these maneuvers cause: i.e. one movement must make at least one lane change.

Other elements: the need for greater vigilance on the part of the driver, more frequent changes in speed, and average speeds that may be somewhat lower than on similar basic sections.

The maximum length over which weaving movements are defined varies (2,500ft in HCM 2010). Beyond that, analyze as separate ramps.

The maximum length over which merging and diverging movements are defined is 1,500 ft.

Chapter 15 4

15.2 Level-of-service criteria

Just like the basic section, density is used as MOE.

For merge and diverge areas, densities reflect the “merge/diverge influence area which consists of lanes 1 and 2 (right & next-to-right lane) and the acceleration or deceleration lane 1,500 ft upstream of a diverge or downstream of a merge.

For weaving areas, the density reflects an average for all vehicles across all lanes of the segment between the entry and exit points of the segment.

Chapter 15 5LOS Basic Fwy Segment

Compare the upper boundaries of the LOSs between Basic Freeway Segments and Weaving/Merging/Diverging Segments.

C-D Roadways = Collector-Distributor Roadways

Chapter 15 6

15.3 A common point: converting demand volumes to flow rates

Both procedures rely on algorithms stated in terms of demand flow rates in passenger car units for base conditions. This time as a group, not by lane.

pHV

ii ffPHF

Vv

**

We have seen the definitions of these terms in Chapter 14.

Heavy vehicle and driver population factors are the same ones used for basic freeway and multilane highway segments in Chapter 14.

Make sure you read the introductory section of 15.4 for the history of the weaving analysis method.

Chapter 15 7

15.4 Weaving Segments: Basic characteristics and Variables

15.4.1 Flows in a weaving area

Weaving flows (pc/hr) vs. Non-weaving flows (pc/hr)

ww

w

nww

oonw

www

vvR

vvVR

vvv

vvv

vvv

2

21

21

VR = Volume Ratio

R = Weaving Ratio

Subscript 1 means the larger flow; subscript 2 means the smaller flow of the two.

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15.4.2 Critical geometric variables

Three influential variables Lane configuration

Length of the weaving area, ft

Width (number of lanes) in the weaving area

Lane configuration classifications One-sided vs. two-sided weaving

segments

Ramp-weave vs. major weaving segments

One-sided vs. two-sided? A one-sided weaving segment is one in which no weaving maneuver requires more than two

lane changes.

A two-sided weaving segment is one in which a one-lane on-ramp on one side of the facility is closely followed by a one-lane off-ramp on the other side of the facility.

Chapter 15 9

Numerical characteristics of two-sided weaving configurations (p.322)

• In two-sided configurations, ramp-to-facility and facility-to-ramp movements are NOT the weaving flows (they are merge and diverge flows).

• Ramp-to-ramp vehicles weave across facility-to-facility vehicles. But facility-to-facility (meaning freeway to freeway) vehicles do not have to make lanes changes.

• So, only the ramp-to-ramp vehicles are considered to be “weaving.”

• In (d), LCRR = 3 and NWV = 0; in (c). LCRR = 2 and NWV = 0.

Chapter 1510

Numerical characteristics of one-sided weaving configurations

LCRF = Min # of lane changes that a R-to-F weaving vehicle must make to complete the movement.

LCFR = Min # of lane changes that a F-to-R weaving vehicle must make to complete the movement.

NWV = # of lanes from which a weaving maneuver may be completed with one lane change, or no lane change.

LCRF = LCFR = 1, NWV = 2

LCRF = 1, LCFR = 0, NWV = 2

LCRF = 0 or 1, LCFR = 1, NWV = 3

“Lane balance”: When # of exit lanes = # of entry lanes + 1

(p.323)

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Length of the weaving area (p.324)

Short length (ft)

Base length (ft)

LS = 0.77*LB

Width of a weaving area (p.325)

The total width of the weaving area is measured as the total number of lanes available for all flows, N. In the above figure, N = 4.

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15.5 Computational procedures for weaving area analysis

See Fig 15.7. p.326

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Chapter 15 14

Fig. 15.8: Weaving Variables Defined for One-Sided Weaving Segments

Chapter 15 15

Fig. 15.8: Weaving Variables Defined for One-Sided Weaving Segments (continued)

Nwv

Chapter 15 16

Fig. 15.8: Weaving Variables Defined for One-Sided Weaving Segments (continued)

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Equations (There are 23 of them! We will learn as we work through examples.)

SN

vD

S

v

S

v

S

vv

S

v

S

v

vvS

ffPHF

Vv

w

w

nw

nwwnw

w

w

nw

nw

wnw

pHV

ii

Eq. 15-1

Eq. 15-22

Eq. 15-23

In between 20 equations exist in HCM2010.

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Example 15.1 Analysis of a ramp-weave area(p.342) We will walk through this example to learn how the

weaving analysis is done. (15.8)

Example 15.2 Analysis of a major weaving area

Chapter 15 19

15.6 Basic Characteristics of Merge and Diverge Segment Analysis

The analysis procedures focus on the merge or diverge influence area that encompasses lanes 1 and 2 (shoulder and adjacent) freeway lanes and the acceleration lane for a distance of 1,500 ft upstream of a diverge or 1,500 downstream of a merge area.

Merge area:

Diverge area:

FMF Pvv 12

FDRFR Pvvvv 12

Chapter 15 20

Measuring the Length of Acceleration and Deceleration Lanes

Chapter 15 21

15.7 Computational Procedures for Merge and Diverge Segments

1. Specify all traffic and roadway data for the junction to be analyzed: peak-hour demands, PHF, traffic composition, driver population, and geometric details of the site, including the free-flow speed for the facility and for the ramp. Convert all demand volumes to flow rates in pc/h under equivalent base conditions using Eq. 15-1.

2. Determine the demand flow in lanes 1 and 2 of the facility immediately upstream of the merge (V12) or diverge junction (V12) using the appropriate algorithm as specified. Table 15.3 (PFM) & Table 15.4 (PFD). Need to check whether the subject ramp is isolated or not first.

3. Determine whether the demand flow exceeds the capacity of any critical element of the junction. (Table 15.5) Where demand exceeds capacity, level of service F is assigned and the analysis is complete. (See the right column of page 339+.)

4. If operation is determined to be stable, determine the density (Eq. 15-40 for merge and Eq. 15-41 for diverge areas) in the ramp influence area.

5. Determine the speed of all vehicles within the ramp influence area and across all facility lanes as secondary measures of performance. Table 15.6 and 15.7.

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Figure 15.12 Flow Chart for Analysis of Ramp-Facility Junctions

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Figure 15.12 Flow Chart for Analysis of Ramp-Facility Junctions (continued)

Chapter 15 24FMF Pvv 12

Eq. 15-25 is for an isolated ramp.

Equations for Merge

Chapter 15 25

Isolated on-ramp or not?

403,232.52444.0214.0 RFFSLvvL aRFEQ

for upstream off ramp (eq. 15-30)

for downstream off ramp (eq. 15-31)

a

dEQ L

vL

000107.01096.0

If Lup ≥ LEQ, the subject ramp may be considered to be isolated. Use eq. 15-25. Otherwise, use eq. 15-26.

If Ldn ≥ LEQ, the subject ramp may be considered to be isolated. Use eq. 15-25. Otherwise, use eq. 15-27.

Chapter 15 26 FDRFR Pvvvv 12

Eq. 15-33 is for an isolated ramp.Equations for Diverge

Chapter 15 27

Isolated off-ramp or not?

RF

uEQ vv

vL

000076.0000023.0071.0

for upstream on ramp (eq. 15-36)

for downstream off ramp (eq. 15-37)

RF

dEQ vv

vL

000369.0000032.015.1

If Lup ≥ LEQ, the subject ramp may be considered to be isolated. Use eq. 15-33. Otherwise, use eq. 15-34.

If Ldn ≥ LEQ, the subject ramp may be considered to be isolated. Use eq. 15-33. Otherwise, use eq. 15-35.

Checking the reasonableness of lane distribution predictions, p.339

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The estimated lane distribution must meet these two conditions (to be within the data availability for the regression models):

1. Average flow rate in the outer lanes may not exceed 2,700 pc/h/ln. If not met, adjust as shown below. No = # of outer lanes.

2. Average flow rate in the outer lanes may not be more than 1.5 times the average flow rate in lanes 1 and 2. If not met, adjust as shown below.

oF Nvv 270012

25.12:2_

50.2:2_

75.1:1_

012

12

12

NvvNFor

vvNFor

vvNFor

Fo

Fo

Fo

2

25.1:2_

50.22

225.1:2_

75.12

215.1:1_

0

NNFor

NFor

NFor

o

o

o

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15.7.3 Capacity Considerations

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15.7.4 Determining Density and LOS in the Ramp Influence Area

dR

aRR

LvD

LvvD

009.00086.0252.4

00627.00078.000734.0475.5

12

12

Merge influence area

Diverge influence area

Once Density is calculated, average speeds on ramp influence area, outer lanes, and all lanes will be estimated (section 15.7.5).

15.7.5 Determining Expected Speed Measures

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(Make changes to the typos.)

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Example 15-3 Analysis of an isolated on-ramp

We will walk through this example to learn how an LOS analysis is done step by step.

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Example 15-4 Analysis of a sequence of freeway ramps

We will walk through this example to learn how a sequence of freeway ramps (mix of merging and diverging areas).

Read Appendix II for Special Cases in Merge and diverge Analysis. We do not cover this in this class but keep in mind this topic is discussed in this book as well as in HCM so that you know what to do when you encounter these cases.