Transcript Chapter 3. Highway Design for Performance

Chapter 3. Highway Design for
Performance
Only section 1 is covered in CE361 (We study more
about this topic in CEEn 562 Traffic Engineering): By
the end of this chapter the student will be able to (p.3.1):
1. Determine the level of service and
capacity for a basic freeway segment
under specified traffic characteristics.
Lesson objectives of Section 3.1:
 Define and calculate the capacity of a basic freeway segment.
 Determine the level of service with various traffic mixes and lane
configurations.
Basic freeway segment capacity analysis shows you
the essence of capacity analysis of other facilities
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3.1.1 Operations on Freeways
Basic freeway segments:
Segments of the freeway that
are outside of the influence
area of ramps or weaving
areas and have uniform traffic
and roadway conditions.
I-15 under
construction
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3.1.1 Continued.
LOS B
LOS C or D
LOS A
Level of
service
Density range (pc/mi/ln)
A
0 - 11.0
B
>11.0 - 18.0
C
>18.0 – 26.0
D
>26.0 – 35.0
E
>35.0 – 45.0
F
> 45.0
LOS E or F
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3.1.1 Continued.
Performance measures can be: Density, speed, and
volume-to-capacity (v/c ratio)
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3.1.2 The Highway Capacity Manual Method for BFS
Base conditions for freeway capacity: Good weather,
good visibility, no incidents
Start with an ideal situation and then convert specified nonideal traffic and roadway conditions into adjustment factors
that reduce the ideal values of speed or flow to a value that
can be compared against a table of LOS standards.
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Four types of analysis
Type
Operational
Planning/Design
Input
Output
I
vp, FFS
LOS
II
vp, LOS, FFS
S (Speed)
III
FFS, LOS
Vp (service volume = Vp * PHF)
IV
vp, LOS
N (No. of lanes)
For planning analysis, you need to estimate the
directional design hourly volume (DDHV):
DDHV = AADT x K x D
besides all other potential prevailing conditions. Then,
V = DDHV/(no. of lanes)
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3.1.2 Operational analysis, find LOS
You want to find out LOS, Speed, and Density.
Step 1: Input data - Define and segment the freeway
sections as appropriate
Step 2: Determine the free-flow speed (FFS) on the
freeway segment (for each segment for analysis)
Step 3: Select FFS curve
Step 4: Adjust demand volume (Determine vp)
Step 5: Estimate speed (S, not FFS) and density, D = vp /S
Step 6: Determine LOS
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Operational analysis (Step 1: Input data)
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Operational analysis (Step 2: Determine FFS)
You need to determine these two values.
Determination of free-flow speed (FFS), mph:
FFS  75.4  f LW  f LC  3.22TRD
0.84
TRD = Total number of onand off-ramps within ±3 miles
of the midpoint of the study
segment, divided by 6 miles.
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Operational analysis (Step 3: Select FFS curve)
Do not interpolate; Find the
closest curve
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Operational analysis (Step 4: Adjust demand volume)
V
vp 
PHF * N * f HV * f p
f HV
1

1  PT ET  1  PR ER  1
PP = percent passenger cars
PT = percent trucks & buses
PR = percent recreational vehicles (RVs)
ET = PCE for trucks and buses
ER = PCE for RVs
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Heavy-vehicle adjustment factor
f HV
1

1  PT ( ET  1)  PR ( ER  1)

1
PP 1  PT ET  PR ER

1
1  PT  PR 1  PT ET  PR ER
Extended
segments
Type of Terrain
Level
Rolling
Mountains
ET (trucks & buses)
1.5
2.5
4.5
ER (RVs)
1.2
2.0
4.0
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Grade and slope
length affects
the values of ET
and ER.
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Operational analysis (Step 5: Estimate speed and density)
Speed:
Density:
D
vp
S
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Operational analysis (Step 6: Determine LOS)
Level of
service
Density range (pc/mi/ln)
A
0 - 11.0
B
>11.0 - 18.0
C
>18.0 – 26.0
D
>26.0 – 35.0
E
>35.0 – 45.0
F
> 45.0
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How doe we deal with long,
sustained grades…
There are 3 ways to deal with long, sustained grades:
extended general freeway segments, specific upgrades,
and specific downgrades.
(1) Extended segments: where no one grade of 3% or greater is
longer than ¼ mi or where no one grade of less than 3% is longer
than ½ mi. And for planning analysis. (we only deal with extended
segment cases in this class. The rest is covered by CE562)
Extended
segments
Type of Terrain
Level
Rolling
Mountains
ET (trucks & buses)
1.5
2.5
4.5
ER (RVs)
1.2
2.0
4.0
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How we deal with long, sustained
grades…(cont.) (This will be covered in CE562 in detail.)
(2) Specific upgrades: Any freeway grade of more than ½
mi for grades less than 3% or ¼ mi for grades of 3% or
more. (For a composite grade, see the next slide.) Use the
tables for ET and ER for specific grades.
(3) Specific downgrades:
 If the downgrade is not severe enough to cause trucks to
shift into low gear, treat it as a level terrain segment.
 Otherwise, use the table for downgrade ET
 For RVs, downgrades may be treated as level terrain.
(PC equivalents for these two cases are not covered by the text. Just remember these adjustments
may need to be done for long sustained grades.)
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Planning/Design analysis to determine the number of lanes
needed for future demand
You want to find out how many lanes are needed for the
targeted level of service to meet the design year traffic.
Step 1: Collect input data (with estimated demand)
Step 2: Determine FFS
Step 3: Select FFS curve
Step 4: Estimate the number of lanes needed, N, for targeted LOS
Step 5: Estimate speed and density (to confirm if N is adequate)
Step 6: Determine LOS (to confirm if N is adequate)
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Service flow rates vs. service volumes
What is used for capacity analysis is service flow rate.
The actual number of vehicles that can be served during
one peak hour is service volume. This reflects the peaking
characteristic of traffic flow.
Stable flow
SFE
Unstable
flow
Flow
E
F
D
C
SFA
A
SVi = SFi * PHF
B
Density
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