Transcript Document

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
After Working Through this Case Study You
Should be able to:
 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.
Network to be Studied
US-9
Exit
7
North
Alternate Rte. 7
I-87 / NY 7 Interchange
I-87
Exit
6
I-787
NY-2
NY 7: Basic Freeway Section
23rd Street
Observations?
I-787 / NY 7 Interchange
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
EB
WB
Observations?
AM
PM
AADT
3250
2400
2400
3500
29700
30000
Length of basic freeway section = 3 miles
 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?
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?
Observations?
 How else to account for the
variability between data samples
and typical roadway conditions?
Flow Patterns
Eastbound Volumes in 2001
4000
Hourly Volume (vph)
3500
3000
2500
AM Peak: 7-8
PM Peak: 4-5
EB ~3500 vph
EB ~2900 vph
WB ~ 2500 vph
WB ~ 4000 vph
2000
1500
1000
500
0
0
5
10
15
20
W e s tbound V olum e s in 2 0 0 1
25
Hour of the Day (0-23)
4500
Min flow between 2-3 am
Hour ly V olum e (vph)
4000
3500
3000
2500
2000
1500
1000
500
0
0
Observations?
5
10
15
Ho u r o f t h e Da y ( 0 - 2 3 )
20
25
Peak Hour Factor (PHF)
 What is the relationship
between hourly
volumes and the peak
hour factors?
W e s tbound P HF
We s tbound PHF
1 .2
1
 When is there more
0 .8
variation in the PHF?
0 .6
0 .4
0 .2
0
0
1000
2000
3000
W e s tb o u n d V o lu m e (vp h )
4000
5000
Speed Flow
 What is the typical
Observations?
mean speed?
Speed-Flow, EB-First Lane
the flow
increases?
80
15-Minute Mean Speed (mph)
 What happens as
70
60
50
40
30
20
10
0
0
500
1000
1500
15-Minute Flow (vph)
2000
2500
Flow Occupancy
 Is this what should be
expected?
F l o w -O c c u p a n c y , E B -F i r st L a n e
 What volume should
15-M inute M e an Flow (ve h/hr )
2500
we select as being
“typical” for the peak
period analysis?
2000
1500
1000
500
0
0
20
40
60
1 5 - M in u t e M e a n O c c u p a n c y ( % i)
80
100
Trends in the
Traffic Volume
Which value is the Let’s say 90th
right one to pick? percentile
15-Minute Peak Hour Flow Rate Distribution
1.200
95th Percentile = 3,385 vph
Observations?
Cumulative Probability
1.000
90th Percentile = 3,340 vph
0.800
0.600
50th Percentile = 3,096 vph
0.400
Mean = 2,916 vph
0.200
0.000
0
500
1000
1500
2000
2500
3000
15-Minute Peak Hour Flow Rate (veh/hr)
3500
4000
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
From Sub-problem 1a we have:
Speed-Flow, EB-First Lane
What is a good choice
for the FFS?
15-Minute Mean Speed (mph)
80
70
60
say ~55 MPH
50
40
30
20
10
0
0
500
1000
1500
15-Minute Flow (vph)
2000
2500
Observations?
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
Observations?
 Calculating the average
density:
D = vp / S
D = 1,902 pcphpl / 54.8 mph
D = 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 hour
What is the performance
of this facility like during
a reasonably heavy AM
peak hour?
Do these match the field
observations?
Distribution of AM Eastbound Peak 15-Minute Density
1.200
Exhibit 4-13. Peak Hour LOS
Distribution
Max D
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%
1.000
# Hours Percent
Cumulative Probability
LOS
0.800
0.600
Range between
the bars = LOS D
(~80%)
0.400
0.200
Mainly LOS D
0.000
0.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
80.00
Eastbound AM Peak 15-Minute Density
Yes, field data matches!!!
90.00
Sub-problem 1c
Perform basic freeway analysis of the westbound section of
Alternate Route 7.
 This sub-problem is similar to 1b.
Observations?
 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.
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 pcph
D=16.4 pcpmpl
LOS = B  Good
 Other 2 Lanes (no trucks): 
vp = 1,711 pcphpl, D= 31.1
pcpmpl, & LOS = D
Observations?
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!!!
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?
Westbound PM Peak Level of Service Distribution
Number of Peak Hours
250
200
 Is this reasonable?
LOS = C
150
100
50
Observations?
0
A
B
C
Level of Service
D
E
Level of Service
 What effect would “regular
drivers” vs. “vacationers” have
on the system?
Regular drivers
mainly provide a
LOS = C and
Vacationers
mainly provide a
LOS = D.
LOS
A
B
C
D
E
F
MaxD
11
18
26
35
45
>45
Observations?
RegDriv
NHr
Pct
8
3.1%
7
2.7%
195
76.2%
37
14.5%
4
1.6%
5
2.0%
Vacation
NHr
Pct
7
2.7%
2
0.8%
20
7.8%
210
82.0%
11
4.3%
6
2.3%
 How likely are these situations? Neither exactly describes
the facility, probably
somewhere in between