CHAPTER 13
FREEWAY CONCEPTS
CONTENTS
13-1
13-1
II.BASIC
Freeway
13-2
13-2
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13-3
Speed-Flow and
Queue Discharge
13-5
13-5
海陆热力性质差异Factors .
13-5
Lane Width and
13-6
Number .
13-6
13-6
13-7
13-7
13-8
<.
13-11
古代字体大全查找Required Input Data and .
13-11
Lane Width and .
13-11
raw是什么意思Specific Grade or .
13-11
13-12
Ba FFS
13-12
Length of
语篇分析
13-12
13-12
13-12
13-12
Service
13-13
III.
13-14
Type A
13-14
13-15
Type B
13-16
Type C
Effects of
13-17
13-17
.
13-18
Type
13-18
13-20
Service
13-20
IV.RAMPS AND .
13-20
13-21
13-22
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Capacity of Merge and
13-22
13-23
<
13-24
Required Input Data and .
13-24
13-24
Length of Acceleration/
13-24
13-24
Length of
13-25
<
Percentage of
13-25
13-i Chapter 13 - Freeway Concepts
Driver
13-25
13-25
Service
13-26
V.
Traffic
13-27
13-27
Freeway Traffic .
13-27
Freeway
13-28
Capacity .
13-28
Demand
13-29
13-30
EXHIBITS
13-2
Exhibit 13-1.Example of Basic
Exhibit 13-2.Speed-Flow Relationships for Basic
13-3
13-4
Exhibit 13-3.Density-Flow Relationships for Basic
13-5
Exhibit 13-4.Queue Discharge
Exhibit 13-5.Required Input Data and Default Values for Basic
13-11
13-13
Exhibit 13-6.Example Service Volumes for Basic
13-14
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Exhibit 13-7.Formation of a
13-15
Exhibit 13-8.Type A
13-16
Exhibit 13-9.Type B
13-17
Exhibit 13-10.Type C
Exhibit 13-11.Measuring the Length of a
13-18
13-19
Exhibit 13-12.Maximum U of Lanes by
13-20
Exhibit 13-13.Example Service Volumes for Freeway
13-21
Exhibit 13-14.On- and Off-Ramp
13-23
Exhibit 13-15.Capacity of
13-23
Exhibit 13-16.Capacity of
Exhibit 13-17.Required Input Data and .
13-24
13-25
Exhibit 13-18.Acceleration Lane
13-25
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Exhibit 13-19.Deceleration Lane
13-26
Exhibit 13-20.Example Service Volumes for Single-Lane On-
13-26
Exhibit 13-21.Freeway .
Chapter 13 - Freeway Concepts13-ii
13-1Chapter 13 - Freeway Concepts
Introduction
I.INTRODUCTION
In this chapter capacity and quality-of-rvice concepts for freeways are introduced.This chapter can be ud in conjunction with the methodologies of Chapter 22 (Freeway Facilities), Chapter 23 (Basic Freeway Segments), Chapter 24 (Freeway Weaving), and Chapter 25 (Ramps and Ramp Junctions).
Freeway defined A freeway is defined as a divided highway with full control of access and two or more lanes for the exclusive u of traffic in each direction. Freeways provide
uninterrupted flow. There are no signalized or stop-controlled at-grade interctions, and direct access to and from adjacent property is not permitted. Access to and from the freeway is limited to ramp locations. Opposing directions of flow are continuously parated by a raid barrier, an at-grade median, or a continuous raid median.
Operating conditions on a freeway primarily result from interactions among vehicles and drivers in the traffic stream and among vehicles, drivers, and the geometric characteristics of the freeway. Operations can also be affected by environmental
conditions, such as weather or lighting, by pavement conditions, and by the occurrence of traffic incidents.
Toll road as freeway
A tollway or toll road is similar to a freeway, except that tolls are collected at
designated points along the facility. Although the collection of tolls usually involves interruptions of traffic, the facilities may generally be treated as freeways. However,special attention should be given to the unique characteristics, constraints, and delays caud by toll collection facilities.
The freeway system is the sum total of all freeway facilities in a given area. The analyst must realize that freeway facilities may have interactions with other freeway facilities as well as local streets and take care to consider interactions with the other facilities. The performance of the freeway may be affected when demand exceeds
capacity on nearby parts of the local street or freeway system or when the capacity of the street or ramp metering system limits demand approaching the freeway.
If the street system cannot accommodate the demand exiting the freeway, the
oversaturation of the street system may result in queues backing onto the freeway, which adverly affects freeway performance. In effect, the limited capacity of the street system reduces the effective capacity of the exit ramp. Therefore, whether the downstream street system capacity can accommodate the exiting freeway demand is an important factor in whether the freeway facility analysis reflects freeway performance. Likewi, the
prence of ramp metering affects freeway demand and must be taken into consideration in analyzing a freeway facility.
Freeway facilities are also assumed to have no interaction with adjacent freeways. In reality, freeway facilities may have interactions with other freeway facilities, as they do with surface streets. Free-flow conditions must therefore exist upstream and downstream of the facility being analyzed. In other words, the analysis of a freeway facility can only address local oversaturation within its time-space domain, not systemwide effects outside of its time-space domain.II.BASIC FREEWAY SEGMENTS
Basic freeway gments are outside of the influence area of ramps or weaving areas of the freeway. Exhibit 13-1 illustrates a basic freeway gment.
E XHIBIT 13-1. E XAMPLE O
F B ASIC F REEWAY S EGMENT
FREEWAY CAPACITY TERMS
•Freeway capacity: the maximum sustained 15-min flow rate, expresd in
pasnger cars per hour per lane, that can be accommodated by a uniform freeway
gment under prevailing traffic and roadway conditions in one direction of flow.
•Traffic characteristics: any characteristic of the traffic stream that may affect
capacity, free-flow speed, or operations, including the percentage composition of the
traffic stream by vehicle type and the familiarity of drivers with the freeway.
•Roadway characteristics: the geometric characteristics of the freeway gment
under study, including the number and width of lanes, right-shoulder lateral clearance,
interchange spacing, vertical alignment, and lane configurations.
•Free-flow speed (FFS): the mean speed of pasnger cars that can be
accommodated under low to moderate flow rates on a uniform freeway gment under
prevailing roadway and traffic conditions.
•Ba conditions: an assumed t of geometric and traffic conditions ud as a
starting point for computations of capacity and level of rvice (LOS).
Capacity analysis is bad on freeway gments with uniform traffic and roadway
conditions. If any of the prevailing conditions change significantly, the capacity of the
gment and its operating conditions change as well. Therefore, each uniform gment
should be analyzed parately.
FLOW CHARACTERISTICS
Traffic flow within basic freeway gments can be highly varied depending on the
conditions constricting flow at upstream and downstream bottleneck locations.
Bottlenecks can be created by ramp merge and weaving gments, lane drops,
maintenance and construction activities, accidents, and objects in the roadway. An
incident does not have to block a travel lane to create a bottleneck. For example, disabled
vehicles in the median or on the shoulder can influence traffic flow within the freeway
lanes.
Chapter 13 - Freeway Concepts13-2
Basic Freeway Segments
13-3Chapter 13 - Freeway Concepts
Basic Freeway Segments The three flow types on basic freeway gments are undersaturated, queue discharge, and oversaturated Freeway rearch has resulted in a better understanding of the characteristics of freeway flow relative to the influence of upstream and downstream bottlenecks. Traffic flow within a basic freeway gment can be categorized into three flow types:
undersaturated, queue discharge, and oversaturated. Each flow type is defined within general speed-flow-density ranges, and each reprents different conditions on the freeway.
Undersaturated flow defined •Undersaturated flow reprents traffic flow that is unaffected by upstream or downstream conditions. This regime is generally defined within a speed range of 90 to 120 km/h at low to moderate flow rates and a range of 70 to 100 km/h at high flow rates.
Queue discharge flow defined •Queue discharge flow reprents traffic flow that has just pasd through a
bottleneck and is accelerating back to the FFS of the freeway. Queue discharge flow is characterized by relatively stable flow as long as the effects of another bottleneck
downstream are not prent. This flow type is generally defined within a narrow range of 2,000 to 2,300 pc/h/ln, with speeds typically ranging from 55 km/h up to the FFS of the freeway gment. Lower speeds are typically obrved just downstream of the bottleneck. Depending on horizontal and vertical alignments, queue discharge flow usually accelerates back to the FFS of the facility within 1 to 2 km downstream from the bottleneck. Studies suggest that the queue discharge flow rate from the bottleneck is
lower than the maximum flows obrved before breakdown. A typical value for this drop in flow rate is approximately 5 percent.
Oversaturated flow defined •Oversaturated flow reprents traffic flow that is influenced by the effects of a downstream bottleneck. Traffic flow in the congested regime can vary over a broad range of flows and speeds depending on the verity of the bottleneck. Queues may extend veral kilometers upstream from the bottleneck. Freeway queues differ from queues at interctions in that they are not static or “standing.” On freeways, vehicles move slowly through a queue, with periods of stopping and movement. Oversaturated flow is discusd further in the freeway facilities ction of this chapter and in Chapter 22.
Speed-Flow and Density-Flow Relationships
Speed-flow and density-flow relationships for a typical basic freeway gment under either ba conditions or non–ba conditions in which FFS is known are shown in Exhibits 13-2 and 13-3 (1).
E XHIBIT 13-2. S PEED -
F LOW R ELATIONSHIPS FOR B ASIC F REEWAY S EGMENTS
130
120
110
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90
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70
60
50
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20
10
004008001200
160020002400A v e r a g e P a s s e n g e r -C a r S p e e d (k m /h )
Flow Rate (pc/h/ln)
