Case Study 4 New York State Alternate Route 7

Key Issues to Explore:

Capacity of the mainline sections of NYS-7

Adequacy of the weaving sections

Performance of the interchange ramps

Queuing

Speed changes

Determine the appropriate analyses required to address a similar problem.

Understand what input data are required and the assumptions that are commonly made.

Understand when and how to apply the methodologies.

Understand the limitations of the HCM procedures.

Reasonably interpret the results from an HCM analysis.

After Working Through this Case Study You Should be able to:

Network to be Studied

Alternate Rte. 7

I-787

23rd Street

North

US-9

I-87

NY-2

Exit6

Exit7

NY 7: Basic Freeway Section

I-87 / NY 7 Interchange

I-787 / NY 7 Interchange

Observations?

Problem 1: Basic Freeway Sections

1a: Traffic Flow Patterns Variation in volumes Variations in the PHF Speed-flow relationship Flow-Occupancy

1b: Basic Freeway Section Analysis (EB) Selection of Appropriate Data Basic Freeway Analysis

1c: Analysis of WB Freeway Section Number of Travel Lanes Truck Climbing Lanes Effect of Grades on Analyses

Peak Hour Volumes

What time periods should be selected? What are the most important characteristics of this

subarea? Do the defining characteristics differ by direction? How is the configuration of each basic freeway

section likely to affect downstream system elements?

AM PM AADTEB 3250 2400 29700WB 2400 3500 30000

Observations? Length of basic freeway section = 3 miles

Sub-problem 1a

Determining traffic flow patterns using atypical conditions, where traffic data along the study roadway has been monitored for years.

How many volume studies would need to be completed for the same degree of confidence?

How else to account for the variability between data samples and typical roadway conditions?

Observations?

Westbound Volumes in 2001

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Flow PatternsEastbound Volumes in 2001

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Min flow between 2-3 am

AM Peak: 7-8

EB ~3500 vph

WB ~ 2500 vph

PM Peak: 4-5

EB ~2900 vph

WB ~ 4000 vph

Observations?

Peak Hour Factor (PHF)

What is the relationship between hourly volumes and the peak hour factors?

When is there more variation in the PHF?

Westbound PHF

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Westbound Volume (vph)

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Speed Flow

What is the typical mean speed?

What happens as the flow increases?

Speed-Flow, EB-First Lane

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15-Minute Flow (vph)

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Observations?

Flow Occupancy

Is this what should be expected?

What volume should we select as being “typical” for the peak period analysis?

Flow-Occupancy, EB-First Lane

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15-Minute Mean Occupancy (%i)

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Trends in the Traffic Volume

15-Minute Peak Hour Flow Rate Distribution

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15-Minute Peak Hour Flow Rate (veh/hr)

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Observations?

Mean = 2,916 vph

50th Percentile = 3,096 vph

90th Percentile = 3,340 vph

95th Percentile = 3,385 vph

Which value is the right one to pick?

Let’s say 90th percentile

Sub-problem 1b

Perform basic freeway analysis of the eastbound section of Alternate Route 7.

Basic Freeway Section Analysis Methodology

Observations?

What inputs are required? Geometric Data Free-flow Speed

(FFS) Volume

Information

EB Segment Characteristics

The EB section has 2 lanes & is divided into 3 segments: a one-mile segment with a 1-2% upgrade to the

vicinity of Miller Road a one-mile segment with a 1-2% downgrade a final one-mile segment with a 5-7% downgrade

ending at the I-787 interchange.

Which segment should be chosen to do the analysis?

The HCM says: use the section that will produce the most conservative estimate of the LOS. That is, worst case governs.

Obtaining the Free-Flow Speed

FFS can be obtained from: Field measurements Estimate from Chapter 23 of HCM

Obtaining FFS using Field Data

Speed-Flow, EB-First Lane

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15-Minute Flow (vph)

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say ~55 MPH

Observations?

From Sub-problem 1a we have:

What is a good choice for the FFS?

Obtaining FFS Chapter 23 of HCM

The basic free flow speed (BFFS) is how fast vehicles are traveling when the volumes are light.

The HCM assumes the BFFS is 70 / 75 mph in urban / rural settings. (Field data shows that these values are too high)

The HCM allows us to use a local value rather than the

defaults. Therefore use BFFS = 60 mph.

After using the HCM method in Chapter 23 what is the FFS?55.5 MPH

Free Flow Speed

FFS from Field Observations = 55 MPH FFS from HCM Chapter 23 = 55.5 MPH

Conclusion: Both methods provide similar results

Additional Data

V = 3,340 veh/hr (HCM Eqn 23-2)

PHF = 0.90 N = 2 PT = 0.05 (field

observations) PR = 0 (field observations) ET = 1.5 ER = 1.2 fp = 1.0

What is the average 15-minute passenger-car equivalent flow rate?

vp = 1,902 passenger cars / hour / lane

What additional data is needed to compute the LOS of this segment?

Use the HCM to compute the average passenger car speed

HCM Equations for Speed-Flow Relationship

If (55 ≤ FFS ≤ 75 mph) & (vp ≤ 3,400 – 30*FFS), then

(from HCM Exhibit 23-3)  S = FFS

If (55 ≤ FFS ≤ 70 mph) & (3,400 – 30*FFS <vp≤ 1,700 + 10*FFS), then

(from HCM Exhibit 23-3) 

And if (70 < FFS ≤ 75 mph) & (3,400 – 30*FFS) < vp ≤ 2,400, then

(from HCM Exhibit 23-3) 

Then what does S equal?

S = 54.8 MPH

Level of Service

LOS defined by the HCM for passenger cars /mile/lane: A: 0-11 B: 11-18 C: 18-26 D: 26-35 E: 35-45 Above 45 is LOS F

Calculating the average density:D = vp / SD = 1,902 pcphpl / 54.8 mphD = 34.7 pcpmpl

What does this mean using the 90th percentile to evaluate?

- 10% of the time in the peak hour the EB LOS is D or worse- 90% of the time it is better than D during the peak hourObservations?

What is the performance of this facility like during a reasonably heavy AM peak hour?

LOS Max D # Hours Percent

A 11 7 2.70%

B 18 7 2.70%

C 26 17 6.60%

D 35 208 81.30%

E 45 13 5.10%

F - 4 1.60%

Exhibit 4-13. Peak Hour LOS Distribution

Do these match the field observations?

Mainly LOS D

Distribution of AM Eastbound Peak 15-Minute Density

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Eastbound AM Peak 15-Minute Density

Cu

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yRange between the bars = LOS D (~80%)

Yes, field data matches!!!

Sub-problem 1c

Perform basic freeway analysis of the westbound section of Alternate Route 7.

This sub-problem is similar to 1b.

Think about why conditions on the westbound section would be different than those on the eastbound section?

Consider roadway users, physical conditions, and heavy vehicle needs.

Observations?

WB Segment Characteristics

The WB section has 3 lanes and is divided into 3 segments: 6-7% upgrade 1-2% upgrade 1-2% downgrade

Which segment should be chosen to do the analysis?

The HCM says: use the section that will produce the most conservative estimate of the LOS. That is, worst case governs.

Additional Data

V = 3,240 veh/hr PHF = 0.90

N = 3 FFS = 55 MPH (calculated

similar to sub-problem 1b) PT = 0.05 (field

observations) PR = 0 (field observations) ET = 1.5 ER = 1.2 fp = 1.0

What is the average 15-minute passenger-car equivalent flow rate?

vp = 1,440 passenger cars / hour / lane

What is the average passenger car speed?

S = 55 MPH

Level of Service

LOS defined by the HCM for passenger cars per mile per lane (pcpmpl): A: 0-11 B: 11-18 C: 18-26 D: 26-35 E: 35-45 Above 45 is LOS F

Calculating the average density:D = vp / S

D = 1,440 pcphpl / 55.0 mph

D = 26 pcpmpl

LOS = D

Observations?

Is the 3rd Lane Needed?

How would the system perform if only 2 lanes were available? Vp = 2,160 pcphpl D = 42 pcpmpl S = 52 MPH LOS = E

The 3rd lane has a huge impact!!!

Truck Climbing Lane

What is the effect of the climbing lane?

5% trucks = 162 trucks/hr

From HCM Exhibit 23-9:

162 trucks/hr = 810 passenger cars/hr

What does this mean?

~ ½ lane worth of passenger car capacity is devoted to the trucks

Should it be enforced that trucks can only use the climbing lane?

Truck Lane: V=810 pcph * 2 = 1,620

pcphD=16.4 pcpmplLOS = B Good

Other 2 Lanes (no trucks):vp = 1,711 pcphpl, D=

31.1 pcpmpl, & LOS = D

If all lanes used by all the traffic:

D= 26.2 pcpmpl If trucks separated into

climbing lane:Dtruck = 26.2 pcpmpl

Dpass = 31.1 pcpmpl

What does this mean?Enforcing a truck only

lane is not a good idea!!!

Observations?

Questions

What if the truck percentage increased to 10%? ET would drop from 5 to 3.5

Why? When there are more trucks they

begin to fill in the voids other trucks create

The density would increase to 27.3 pcpmpl

LOS for the 256 peak hours of the year (weekdays only)

What is the predominate LOS for the peak hour?

Is this reasonable?

Westbound PM Peak Level of Service Distribution

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A B C D E

Level of Service

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LOS = C

Observations?

Level of Service

What effect would “regular drivers” vs. “vacationers” have on the system?

How likely are these situations?

NHr Pct NHr PctA 11 8 3.1% 7 2.7%B 18 7 2.7% 2 0.8%C 26 195 76.2% 20 7.8%D 35 37 14.5% 210 82.0%E 45 4 1.6% 11 4.3%F >45 5 2.0% 6 2.3%

LOS MaxDRegDriv Vacation

Regular drivers mainly provide a LOS = C and Vacationers mainly provide a LOS = D.

Neither exactly describes the facility, probably somewhere in between

Observations?