Chapter 14: Basic Freeway Segments and Multilane
Highways
Chapter objectives: By the end of these chapters the student will
be able to:
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Estimate (determine) the speed of a basic freeway or a
multilane under prevailing conditions
Determine flow rate and density
Obtain proper passenger-car equivalents for trucks, buses, and
RVs (Grade affects the performance of these vehicles)
Conduct operational and planning analyses for the basic
freeway and multilane highway segments
Chapter 14
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14.1 Facility Types
Chapter 14
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14.2 Basic freeway and multilane
highway characteristics
Basic freeway segments:
Segments of the freeway
that are outside of the
influence area of ramps or
weaving areas.
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14.2.1 Basic freeway and multilane highway
characteristics
(This is Figure 14.2 for basic freeway segments)
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14.2.1 continued
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Fig 14.3 Base Speed-Flow Curves for Multilane Highways
(For multilane highways)
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Fig 14.3 Base Speed-Flow Curves for Multilane Highways
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14.2.2 Level of Service
www.utahcommuterlink.com
LOS B
LOS C or D
LOS A
LOS E or F
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14.2.3 Service flow rates and capacity
These tables show service flow rates. Then,
how do you get service volumes per hour?
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Service flow rates vs. service volumes
What is used for 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
Congested
B
Uncongested
SVi = SFi * PHF
PHF 
Density
Chapter 14
Peak _ hourly _ volume
4 V15 _ peak
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14.3 Analysis methodologies
Most capacity analysis models include the determination of capacity
under ideal roadway, traffic, and control conditions, that is, after
having taken into account adjustments for prevailing conditions.
Multilane
highways
12-ft lane width, 6-ft lateral clearance, all vehicles are
passenger cars, familiar drivers, free-flow speeds = 60
mph. Divided. Zero access points. Capacity used is
usually average per lane (see slide 9). The signal
spacing is more than 2 miles.
Basic freeway segments
Min. lane widths of 12 feet
Min. right-shoulder lateral clearance of 6 feet (median  2 ft)
Traffic stream consisting of passenger cars only
Ten or more lanes (in urban areas only)
Interchanges spaced every 2 miles or more
Level terrain, with grades no greater than 2%, length affects
Driver population dominated by regular
Chapter and
14 familiar users
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Prevailing condition types considered (p.291):
Lane width
Lateral clearances
Type of median (multilane highways)
Frequency of interchanges (freeways) or access points (multilane
highways)
Presence of heavy vehicles in the traffic stream
Driver populations dominated by occasional or unfamiliar users
of a facility
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Factors affecting: examples
Trucks occupy
more space: length
and gap
Drivers shy away from
concrete barriers
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14.3.1 Types of analysis
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Operational analysis
(Determine speed and flow
rate, then density and LOS)
vp 
V
PHF * N * f HV * f p
D
vp
S
Service flow rate and service
volume analysis (for desired
LOS) MSF = Max service flow rate
SFi  MSFi * N * f HV * f p
Design analysis (Find the
number of lanes needed to
serve desired MSFi)
DDHV
Ni 
PHF * MSFi * f HV * f p
SVi  SFi * PHF
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Sample operational analysis
1800 pc/h/ln
FFS=65 mph
S = 65 - 0.00001418 (1800 – 1400)2 = 62.7 mph
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14.3.2 (cont.)
Adjustment to free-flow
speed on a freeway
FFS  75.4  f LW  f LC  3.22TRD 0.84
3.0
TRD = Total number of on- and
off-ramps within ±3 miles of the
midpoint of the study segment,
divided by 6 miles.
*** Why only the right shoulder lateral clearance is
considered but not the median side lateral clearance?
Read p.293 right above Table 14.6.
(Eq. 14-5, p. 293)
Chapter 14
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Read p.294 right column carefully.
14.3.2 (cont.)
Adjustment to freeflow speed on a
multilane highway
FFS  BFFSi  f LW  f LC  f M  f A
BFFSi: Read p.294 left column, bottom (and
slide #16 for fLW).
fLW: use Table 14.5
Chapter 14
(Driveways or other entrances with little traffic, or
that for other reasons, do not affect driver behavior,
should NOT be included in the access-point density.
P. 294 right column)
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Choosing a free-flow speed curve (p.296)
Not recommended to interpolate. So, this table was given.
This table is for both freeways and multilane highways.
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Examples for determining FFSs
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14.3.3 Determining the heavy-vehicle factor
f HV
1

1  PT ( ET  1)  PR ( ER  1)
vp 
V
PHF * N * f HV * f p
1

PP 1  PT ET  PR ER
1

1  PT  PR 1  PT ET  PR ER
PP = percent passenger cars
PT = percent trucks & buses
PR = percent recreational vehicles (RVs)
ET = PCE for trucks and buses
Grade and slope
length affects
the values of ET
and ER.
ER = PCE for RVs
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How we deal with long, sustaining
grades… (p.298)
There are 3 ways to deal with long, sustaining 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.
(p.298, left column)
Table 14.11
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, sustaining
grades…(cont)
(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, refer to page 299 right
column.) 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, ET =
1.5.
 Otherwise, use the table for downgrade ET
 For RVs, downgrades may be treated as level terrain, ER =
1.2.
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Tables for PCE for specific grades
When using these look-up
tables, care must be taken to
observe the boundary
condition.
Upper boundaries are “Closed”
in these tables.
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Chapter 14
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Example for using specific grades
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Average grade or composite grade?
(p.299)
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In a basic freeway segment analysis, an overall average grade
can be substituted for a series of grades if no single portion of
the grade is steeper than 4% or the total length of the grade is
less than 4,000 ft. (See the example in p.300 right column.)
For grades outside these limits, the composite grade procedure
is recommended. The composite grade procedure is used to
determine an equivalent grade that will result in the same final
truck speed as used to determine an equivalent grade that will
result in the same final truck speed as would a series of varying
grades. (page 301-302: read these pages carefully for strength
and weakness of this method)
For analysis purposes, the impact of a grade is worst at the end
of its steepest (uphill) section. (e.g. if 4000 ft of 4% grade were
followed by 1000 ft of 3% grade, passenger-car equivalents
would be found for a 4000 ft, 4%. Chapter-end Problem 14-3(c)
is this case.) But, what if 1000 ft of 4% and 1000 ft of 3%?
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Composite grade (Read p.302 for explanation
of the steps.)
3% grade 3000 ft followed by 5% grade 5000 ft  Find the
equivalent (composite) grade. Chapter 14
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14.3.4 Determining the driver population
factor
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Not well established
Between a value of 1.00 for commuters to
0.85 as a lower limit for other driver
populations
Usually 1.00
If there are many unfamiliar drivers use a
value between 1.00 and 0.85
For a future situation 0.85 is suggested
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Planning analysis (or design analysis)
You want to find out how many lanes are needed for the
targeted level of service.
Step 1: Find fHV using for ET and ER.
Step 2: Try 2 lanes in each direction, unless it is obvious that
more lanes will be needed.
Step 3: Convert volume (vph) to flow rate (pcphpl), vp, for the
current number of lanes in each direction.
Step 4: If vp exceeds capacity, add one lane in each direction
and return to Step 2.
Step 5: Compute FFS.
Step 6: Determine the LOS for the freeway with the current
number of lanes being considered. If the LOS is not good
enough, add another lane and return to Step 3.
See Example 14-7
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14.4 Sample Problems
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We will review the manual method and
HCS2010 on Wednesday.
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