Problem 3: Weaving & Ramp Sections

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.

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?

Observations?

Route 7 / I-787 Interchange

G

F

E

D C

B

A

H

AlternateRte. 7

I-787

23rd Street

North

I

J

K

L

Route 7

I-787

Weaving sections of interest include:

A

B

C

E

Observations?

Weaving Sections

What is important to consider when analyzing a weaving section?

1) Type of weave2) Weaving length3) Distribution of flows within the weave4) Speeds of the weaving & non-weaving movements5) PHF6) Percentages of trucks, buses, & recreational vehicles7) Passenger car equivalents

Observations?

Weaving Segments A & B

Characteristics: 4 lanes Length of A = 3100’ Length of B = 2600’ Entering & exiting

facilities each 2 lanes

View of Route 7 looking west at the western end of Location A

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

Weaving Segments A & B

Sw Snw S

mph mph  mph

A AM 1.11 0.18 41.07 61.59 49.03 16.63 B Constrained

A PM 0.66 0.09 48.06 65.38 54.91 8.53 A Constrained

B AM 0.71 0.09 47.25 65.25 55.95 10.02 B Constrained

B PM 0.92 0.14 43.69 63.38 52.07 13.48 B Constrained

Exhibit 4-47. Weave Analysis Results A & B

Weaving Segment

Peak Period Ww Wnw

Density pcpmpl LOS

Type of Operation

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

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?

Sw Snw S

mph mph  mph

A AM 1.11 0.18 41.07 61.59 49.03 16.63 B Constrained

A PM 0.66 0.09 48.06 65.38 54.91 8.53 A Constrained

B AM 0.71 0.09 47.25 65.25 55.95 10.02 B Constrained

B PM 0.92 0.14 43.69 63.38 52.07 13.48 B Constrained

Exhibit 4-47. Weave Analysis Results A & B

Weaving Segment

Peak Period Ww Wnw

Density pcpmpl LOS

Type of Operation

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.

Observations?

Then what length of weaving section is required to have a reasonable LOS?

Weaving Segment C

Characteristics:Inbound facilities:

I-787 NB – 3 lanes23rd St on-ramp – 1 lane

Outbound facilities:I-787 NB – 2 lanesRt 7 EB off-ramp – 2 lanes

View of I-787 North at Location C just before the two right hand lanes leave to go toward Route 7 east

What type of weave is this?

Type C

Observations?

Weaving Segment C

Do we know the distribution of flows for the weaving and non-weaving segments?

How do we get these volumes?

No, collecting this type of data in the field requires significant data collection coordination and significant time.

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

I-787NB 23rd Streeton ramp

I-787NB Rte. 7EBoff ramp

3392 1013

3997 408

Observations?

Weaving Segment C

3 possible scenarios for flow distributions1) 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

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?

Greatest weaving volumes in Scenario 1. The higher the weaving volume the higher the density

Scenario Ww Wnw Sw mphSnw 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

Exhibit 4-52. Flow Distribution Analysis Results

Observations?

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

Observations?

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

Weave Period Ww Wnw Sw mphSnw 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

Exhibit 4-53. AM and PM Peak Hour Analysis Results for Weave E

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?

Sff Sw Snw S

mph mph  mph mph

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

Exhibit 4-54. Location E, PM Peak Hour PHF and Free Flow Speed Sensitivity Analysis

Weave PHF Ww Wnw

Density pcpmpl LOS

Operation of Type

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.

Observations?

Why might the only case of LOS E occur when the PHF = 1 and the FFS = 65 mph?

Weaving Segment E

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

Change geometric constraints (i.e. length of weave and number of lanes in the weave)

Sw Snw S

mph mph mph

790 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

Exhibit 4-56. Effects of Weave Length at Location E

Weave Length Ww Wnw

Density pcpmpl LOS

Operation of Type

Existing

HCM Max

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 unique attributes of this analysis that need to be addressed?

Observations?

Ramps of Interest

Data for Ramps of InterestCapacities from HCM Exhibit 23-3

What factors affect the capacity?

- Number of lanes on the ramp

- Ramp FFS

Observations? What ramps may have problems and at what time of day?

AM PM AM PM

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

Exhibit 4-58. Operational Analysis of the Route 7/I-787 Ramps

Ramp Location # of Lanes

Volume HCM Capacities

v/cSpeed (mph)

Merge sections of interest include:

Location I Characteristics: - single lane on-ramp - 2-lane freeway - Acceleration lane ~ 1/10 of a

mile long - FFS on the ramp = 25 mphLocation J Characteristics: - single lane on-ramp - 2-lane freeway - On-ramp continues as 3rd

lane - FFS on the ramp = 35 mph

G

F

E

D C

B

A

H

AlternateRte. 7

I-787

23rd Street

North

I

J

K

L

Route 7

I-787

J

I

I 216 C

Location J v/c

On-ramp 0.34

Arriving Freeway 0.54

Departing Freeway 0.46

Location LOSDensity (pcpmpl) What can be done to

improve the LOS to A?

Extend acceleration lane

1,000 21.7 C

1,500 18.6 B

3,000 9.2 A

Exhibit 4-60. Location J -Results of Acceleration Lane Length

LOSDensity* pcpmpl

LA (ft)

Observations?

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

Exhibit 4-61. Variation of the Acceleration Lane Lengths at Both Locations I & J

LOS at I LOS at JDensity at I (pcpmpl)

LA at J (ft)

LA at I (ft)

Density at J (pcpmpl)

What would be the effect of changing the acceleration lane length at Location I?

Existing I w/ J = 3000’ La

Max I, Min La @ J

Max I, w/ J = 1650’ La

What are the benefits of each choice?

Observations?

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.

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

Observations?

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?

D = 3.9 pcpmpl

What was the density of where the 4 lane merge starts?

D = 20.8 pcpmpl

Why is there such a difference in the densities?

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

B D

A C

1904

164

100

3123

Weaving Diagram for Weave D AM Peak Hour Volumes

What type of weave is this? Type B

Observations?

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.

Would the weave start early or later under heavier traffic conditions?

Earlier

Conditions Depending on Length of Weave

1,055 47.94 F Unconstrained

1,850 43.51 F Unconstrained

2,000 42.29 E Unconstrained

Exhibit 4-62. LOS Variance with Weave Length

Length of Weave ft

LOS Type of Operation Density (pcpmpl)

What are the effects of having a weave that varies in length?

Although LOS remains poor, as length increases, density decreases!!

What would have to happen to improve the LOS?

Much greater weave length

B D

A C

1904

164

100

3123

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.

Observations?

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 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

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

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

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

Here we will focus on the weaving section that takes place with the C-D Road

Results of the C-D Weave

D = 5.02 pc/mi/lnLOS = AUnconstrained 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.