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Lec 5: Capacity and LOS (Ch. 2, p.74-88)

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Lec 5: Capacity and LOS (Ch. 2, p.74-88) Understand capacity is the heart of transportation issues. Understand the fundamental flow of diagram Understand how a shock ... – PowerPoint PPT presentation

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Title: Lec 5: Capacity and LOS (Ch. 2, p.74-88)


1
Lec 5 Capacity and LOS (Ch. 2, p.74-88)
  • Understand capacity is the heart of
    transportation issues.
  • Understand the fundamental flow of diagram
  • Understand how a shock wave is caused
  • Understand capacity analysis was set up for ideal
    cases and modifications are made to reflect
    prevailing conditions
  • Understand many factors, geometric, traffic, and
    control characteristics affect the capacity of a
    facility
  • Understand capacity and level of service
    correspond each other directly only for
    uninterrupted flow

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
  • 2000 HCM is available

3
Highway capacity software
4
Flow-density relationships
Flow (density) x (Space mean speed)
Space mean speed (flow) x (Average space
headway)
where
Average space headway SMS/(Average time headway)
where
5
Speed, Density, and Flow Rate
6
A few examples
7
Fundamental diagram of traffic flow (flow vs.
density)
Optimal flow or capacity,qmax
Mean free speed, uf
Optimal speed, uo
Flow (q)
Speed is the slope. u q/k
Uncongested flow
Congested flow
Jam density, kj
Optimal density, ko
Density (k)
8
Fundamental diagram of traffic flow (SMS vs.
density SMS vs. flow)
uf
uf
Uncongested flow
SMS
SMS
Congested flow
0
0
kj
qmax
Density
Flow
SMS vs. density
SMS vs. flow
9
Fundamental diagram of traffic flow and shock wave
For upstream
q1
Slope gives velocity uw of shock wave for q1
Flow (q)
q2
Work zone
For bottleneck
k2
k1
kj
Density (k)
Queue forms upstream of the bottleneck. So we use
the diagram of the upstream section
10
Capacity concept
HCM analyses are usually for the peak (worst)
15-min period.
Capacity as defined by HCM 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

11
Capacity values for ideal conditions
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.
Freeway, uninterrupted flow 12-ft lane width, 6-ft lateral clearance (right side, 2 ft on left side), all vehicles are passenger cars, familiar drivers, level grade, no heavy vehicles, free-flow speeds (70 mph for urban, 75 mph for rural if you have a speed limit, its a different matter. Capacity used is usually average per lane (e.g. 2400 pcphpl in one direction)
12
Prevailing condition types
Geometric conditions Horizontal vertical alignment, lane width and lateral clearance, grades
Traffic conditions Directional distribution, lane distribution, heavy vehicles in the traffic stream, turning movements
Control factors Speed limits, lane use controls, traffic signals, STOP and YIELD signs
13
Factors affecting examples
Trucks occupy more space length and gap
Drivers shy away from concrete barriers
14
From ideal conditions to real, prevailing
conditions
We use adjustment factors to take into account
the effect of prevailing conditions on capacity
and level of service. Typically it is like
Free-flow speed
Passenger car equivalent flow rate
15
Application of the capacity concept
Precision Low Used in transportation planning studies to assess the adequacy or sufficiency of existing highway networks (Traffic volumes are estimates define targeted LOS first then find the number of lanes)
Precision Medium Used as a design control in the selection of highway type and in determining dimensional needs (Traffic volumes are estimates define targeted LOS first then find the number of lanes)
Precision High Used in traffic operational analyses a) Analysis of existing conditions, b) Estimation of operational improvements (For this analysis volume, geometry and control data exist)
16
Level of service
A level of service is a letter designation that
describes a range of operating conditions on a
particular type of facility.
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
17
MOE in 2000 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 upgrade speed
Interrupted Signalized intersections Approach delay (sec/veh)
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)
18
LOS example freeway basic sections
Basic freeway segments Segments of the freeway
that are outside of the influence area of ramps
or weaving areas.
See Exhibit 23-3.
19
LOS density flow rate - speed
20
Level of Service general descriptions
21
LOS for basic freeway segments
A
C
B
D
22
LOS for basic freeway segments (cont.)
E
F
23
LOS examples near SLC
LOS B
LOS C or D
LOS A
Level of service Density range (pc/mi/ln)
A 0 - 10.0
B 10.1 - 16.0
C 16.1 24.0
D 24.1 32.0
E 32.1 45.0
F gt 45.0
LOS E or F
24
Objective of highway design
Create a highway of appropriate type with
dimensional values and alignment characteristics
such that the resulting design service flow rate
is at least as great as the traffic flow rate
during the peak 15-min period of the design hour,
but not greater enough as to represent
extravagance or waste
Why the peak 15-min period? ? Traffic flow
fluctuates, but it is known from previous studies
that it is stable for about 15 minutes.
25
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
C
SFA
SVi SFi PHF
B
A
Density
26
Design service flow rate vs. design volume
Design volume (DHV, vph) Hourly volume of traffic estimated to use a certain type of facility during the design year (peak period)
Design service flow rate (vph) Maximum hour flow rate of traffic that a new facility can serve without the degree of congestion falling below a pre-selected level
27
Acceptable degree of congestion
Rural freeway Motorists expect high speed smooth traffic always
Urban arterial Motorists accept few delays because they know there are physical limits for improvements (and budgets)
Balance need (demand) and resources available
(supply) to determine the degree of congestion
for design.
28
Targeted LOS
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