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Lec 5: Capacity and Level-of-Service Analysis for Freeways, Multilane Highways

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Lec 5: Capacity and Level-of-Service Analysis for Freeways, Multilane Highways & 2-Lane 2-Way Highways (p.2-60 to 2-70) Chapter objectives: By the end of these ... – PowerPoint PPT presentation

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Title: Lec 5: Capacity and Level-of-Service Analysis for Freeways, Multilane Highways


1
Lec 5 Capacity and Level-of-Service Analysis for
Freeways, Multilane Highways 2-Lane 2-Way
Highways (p.2-60 to 2-70)
Chapter objectives By the end of these chapters
the student will be able to
  • Explain why capacity is the heart of
    transportation issues.
  • Define capacity and level-of-service concept and
    explain why capacity is not a fixed value
  • Explain the relationship between the v/c ratio
    and level of service
  • Estimate (determine) the free-flow speed of a
    freeway or a multilane
  • Obtain proper passenger-car equivalents for
    trucks, buses, and RVs (Grade affects the
    performance of these vehicles)
  • Conduct design and planning analyses for the
    basic freeway and multilane highway segments
    (apply the knowledge of capacity and LOS to the
    redesign of Moark Junction.

2
Issues of traffic capacity analysis
  • How much traffic a given facility can
    accommodate?
  • Under what operating conditions can it
    accommodate that much traffic?

Highway Capacity Manual (HCM)
  • 1950 HCM by the Bureau of Public Roads
  • 1965 HCM by the TRB
  • 1985 HCM by the TRB (Highway Capacity Software
    published)
  • 1994 updates to 1985 HCM
  • 1997 updates to 1994 HCM
  • 2001 updates to 2000 HCM
  • 2010 HCM

3
Highway Capacity Software
4
2.4.4 The capacity concept
HCM analyses are usually for the peak (worst)
15-min period.
The capacity of a facility is the maximum
hourly rate at which persons or vehicles can be
reasonably expected to traverse a point or
uniform segment of a lane or roadway during a
given time period under prevailing conditions.
Sometimes using persons makes more sense, like
transit
With different prevailing conditions, different
capacity results.
Some regularity expected (capacity is not a fixed
value)
  • Traffic
  • Roadway
  • Control

5
2.4.5 Level of service
Level of service (LOS) is a quality measure
describing operational conditions within a
traffic stream, generally in terms of such
service measures as speed and travel time,
freedom to maneuver, traffic interruptions, and
comfort and convenience.
LOS A (best)
LOS F (worst or system breakdown)
A Free flow
B Reasonably free flow
C Stable flow
D Approaching unstable flow
E Unstable flow
F Forced flow or breakdown flow
SFA
SFB
SFC
SFD
SFE
Table 2-4, p. 2-66
6
MOE in 2010 HCM
Uninterrupted Fwy Basic sections Density (pc/mi/ln)
Uninterrupted Fwy Weaving areas Density (pc/mi/ln)
Uninterrupted Fwy Ramp junctions Density (pc/mi/ln)
Uninterrupted Multilane highways Density (pc/mi/ln)
Uninterrupted Two-lane highways Percent-time spent following, Average travel speed, and Percent free-flow speed
Interrupted Signalized intersections Approach delay (sec/veh), and v/c
Interrupted Unsignalized intersections Average total delay (sec/veh)
Interrupted Arterials Average travel speed
Interrupted Transit Load factor (pers/seat)
Interrupted Pedestrians Space (sq ft/ped)
7
The v/c ratio and its use in capacity analysis
The comparison of true demand flows to capacity
is a principal objective of capacity and LOS
analysis.
The volume capacity ratio indicates the
proportion of the facilitys capacity being
utilized by current or projected traffic. ? Used
as a measure of the sufficiency of existing or
proposed capacity.
v/c is usually less than or equal to 1.0.
However, if a projected rate of flow is used, it
may become greater than 1.0. The actual v/c
cannot be greater than 1.0 if departure volume is
used for v.
A v/c ratio above 1.0 predicts that the planned
design facility will fail! Queue will form.
8
Freeways and multilane highways
Basic freeway segments Segments of the freeway
that are outside of the influence area of ramps
or weaving areas.
9
Basic freeway and multilane highway
characteristics
(This is Figure 14.2 for basic freeway segments,
Roess, Prassas, and McShane).
10
Equations for curves in Fig. 14.1
Table 14.1 of Roess, Prassas, McShane
11
Base Speed-Flow Curves for Multilane Highways
(For multilane highways)
Fig 14.3 of Roess, Prassas, McShane
12
Base Speed-Flow Curves for Multilane Highways
Fig 14.3 of Roess, Prassas, McShane
13
2.4.5 Level of Service
LOS B
LOS C or D
LOS A
LOS E or F
Table 14.2 of Roess, Prassas, McShane
14
Service flow rates and capacity
Table 14.3 and Table 14.4 of Roess, Prassas,
McShane
15
Capacity and LOS 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 gt 60 mph. Divided. Zero access points. Capacity used is usually average per lane (see slide 9)
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 and familiar users
16
Prevailing condition types considered(p.291 of
Roess, Prassas, and McShane)
  • 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

17
Factors affecting examples
Trucks occupy more space length and gap
Drivers shy away from concrete barriers
18
Types of analysis
  • Operational analysis (Determine speed and flow
    rate, then density and LOS)
  • Service flow rate and service volume analysis
    (for desired LOS) MSF Max service flow rate
  • Design analysis (Find the number of lanes needed
    to serve desired MSF)

19
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
E
F
Flow
D
SVi SFi PHF
C
SFA
Congested
B
A
Uncongested
Density
20
Determining the free-flow speed (1)
Free-flow speed (read carefully definitions of
variables)
Basic freeway segments, eq. 14-5 Multilane
highway sections, eq. 14-6
Passenger car equivalent flow rate
  • BFFS
  • 60 mph without any data
  • Speed limit 40-45 mph, add 7 mph
  • Speed limit 50-55 mph, add 5 mph

Use either the graph or compute
Then Table 14.2 for LOS.
See Figure 14.4 for multilane highway sections.
21
Determining the free-flow speed (2)
Adjustment to free-flow speed on a freeway
TRD Total number of on- and off-ramps within 3
miles of the midpoint of the study segment,
divided by 6 miles.
22
Determining the free-flow speed (3)
Adjustment to free-flow speed on a multilane
highway
fLW use Table 14.5
23
Choosing a free-flow speed curve
Not recommended to interpolate. So, this table
was given. This table is for both freeways and
multilane highways.
24
Determining the heavy-vehicle factor
PP percent passenger cars PT percent trucks
buses PR percent recreational vehicles (RVs) ET
PCE for trucks and buses ER PCE for RVs
Grade and slope length affects the values of ET
and ER.
25
How we deal with long, sustaining grades
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.
Extended segments Type of Terrain Type of Terrain Type of Terrain
Extended segments Level Rolling Mountains
ET (trucks buses) 1.5 2.5 4.5
ER (RVs) 1.2 2.0 4.0
26
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 298 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.

27
Determining the driver population factor
  • 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

28
Planning 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.
29
Determining the driver population factor
  • 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

30
Lec 6 Two-Lane Highway Classes
  • Class I highways generally arterial highways
    that serve long-distance trips and on which
    motorists expect to travel at high speeds.
  • Class II highways highways that serve shorter
    trips and on which motorists do not expect to
    travel at high speeds.
  • Class III highwqays serve more developed areas

31
Two-Lane Highway Design Standards LOS
ATS Average Travel Speed Average speed of all
vehicles traversing the defined analysis segment
for the specified time period (peak 15
minutes) PTSF Percent Time Spent Following
Aggregate percentage of time that all drivers
spend in queues, unable to pass, with the speed
restricted by the queue leader. A surrogate
measure for PTSF is the percentage of vehicles
following others at headways of 3.0 seconds or
less. PFFS Percent Free-Flow Speed is based
on the cmparisonof the prevailing speed to the
free-flow speed, expressed in percentage.
Table 16-1 and Table 16-4 from Roess, Prassas,
and McShane 4th edition.
32
Design Level of Service
Table 2-5 of GB2011, page 2-67.
33
AADT at the Moark Junction (2011)
34
ATR locations
http//www.udot.utah.gov/main/uconowner.gf?n20030
9160954472
35
Hour Begin US 6 EB US 6 WB US 6 Total EB/Total US 6 of ADT US89 SB US 89 of ADT
000 68.0 63.7 131.7 51.6 1.11 6.3 0.60
100 52.3 52.3 104.6 50.0 0.88 1 0.10
200 49.0 38.7 87.7 55.9 0.74 0.7 0.07
300 48.7 33.3 82.0 59.4 0.69 0.7 0.07
400 53.3 41.0 94.3 56.5 0.79 4.3 0.41
500 142.3 75.3 217.6 65.4 1.83 26.7 2.55
600 329.0 156.3 485.3 67.8 4.09 29.3 2.79
700 407.3 176.0 583.3 69.8 4.91 43 4.10
800 510.0 231.4 741.4 68.8 6.24 61 5.82
900 505.7 310.3 816.0 62.0 6.87 64 6.10
1000 494.0 299.5 793.5 62.3 6.68 57.2 5.45
1100 479.3 244.0 723.3 66.3 6.09 64 6.10
1200 480.3 220.3 700.6 68.6 5.90 61.5 5.86
1300 444.0 259.4 703.4 63.1 5.92 66.5 6.34
1400 486.0 272.7 758.7 64.1 6.39 68 6.48
1500 462.7 330.7 793.4 58.3 6.68 91 8.68
1600 465.7 283.7 749.4 62.1 6.31 78.7 7.50
1700 425.7 253.3 679.0 62.7 5.72 86.3 8.23
1800 403.7 275.0 678.7 59.5 5.72 70.3 6.70
1900 330.7 218.3 549.0 60.2 4.62 56.3 5.37
2000 259.7 219.3 479.0 54.2 4.03 50.3 4.80
2100 216.0 181.3 397.3 54.4 3.35 31 2.96
2200 159.7 147.3 307.0 52.0 2.59 17.3 1.65
2300 107.0 111.7 218.7 48.9 1.84 13.3 1.27
Total 7380.1 4494.8 11874.9   1 1048.7 1
2009 data
36
Moark Junction Traffic Distribution
Hourly volumes at ATRs of UDOT can be found
at http//www.udot.utah.gov/main/f?p100pg0
T,V3776,60913
How many lanes do these routes need?
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