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Chapter 5 1
Chapter 5: Traffic Stream CharacteristicsChapter 5: Traffic Stream Characteristics
Explain the difference between Explain the difference between uninterrupted flow and interrupted flowuninterrupted flow and interrupted flow
Explain the three principal traffic-stream Explain the three principal traffic-stream parameters and how to obtain themparameters and how to obtain them
Explain the relationship among the three Explain the relationship among the three macroscopic principal traffic-stream macroscopic principal traffic-stream parametersparameters
Chapter objectives: By the end of these chapters the student will be able to:
Chapter 5 2
5.1 Types of traffic facilities5.1 Types of traffic facilities
Uninterrupted Uninterrupted flow facilityflow facility
No external factors to cause periodic No external factors to cause periodic interruption of flow.interruption of flow. Traffic flow is a product of interaction Traffic flow is a product of interaction between vehicles and geometric and between vehicles and geometric and environmentenvironment
E.g.E.g. Basic sections of a freewayBasic sections of a freeway
Interrupted flow Interrupted flow facilityfacility
Have external devices that Have external devices that periodically interrupt traffic flowperiodically interrupt traffic flow Constant stopping and restarting Constant stopping and restarting neededneeded
E.g. Urban streets and arterialsE.g. Urban streets and arterials
Remember it does not mean the quality of operation.
Chapter 5 3
5.2 Traffic stream parameters5.2 Traffic stream parameters
Macroscopic parametersMacroscopic parameters Microscopic parametersMicroscopic parameters
Volume or flow rate, Volume or flow rate, vv or or qq = 1/ = 1/hh
Headway (or time Headway (or time headway), headway), hh
Speed, S or vSpeed, S or v Speeds of individual Speeds of individual vehicles, svehicles, s
Density, k or D = 1/Density, k or D = 1/dd Spacing (or distance Spacing (or distance headway), headway), dd
Chapter 5 4
5.2.1 Volume and flow rate5.2.1 Volume and flow rate
Can you define these?
AADT
AAWT
ADT
AWT
DDHV = AADT * K * D
What’s the difference between “Volume” and “Flow (or Flow rate)”?
Sub-hourly volume and flow rate
Define PHF = (peak hourly volume) / (max. rate of flow for that hour)
PHF = V/(4 * V15)
What does this tell you?
v = V/PHF
= peak flow rate for the 15-minute peak period
(Review Tables 5.1 and 5.2 & 5.3 queuing)
Illustration of Daily Volume ParametersIllustration of Daily Volume Parameters
Chapter 5 5
Volume, Flow Rate, and QueuingVolume, Flow Rate, and Queuing
Chapter 5 6
If capacity is 4,200 vph, then the 15-min capacity volume is 4,200/4 = 1,050.
Chapter 5
5.2.2 Speed and travel time5.2.2 Speed and travel time
Time mean and space mean speed: Know the difference?
Time mean Time mean speed (TMS)speed (TMS)
Average speed of all vehicles Average speed of all vehicles passing a pointpassing a point over some specified time periodover some specified time period
TMS = ∑ (d/tTMS = ∑ (d/tii)/n)/n
Space mean Space mean speed (SMS)speed (SMS)
Average speed of all vehicles Average speed of all vehicles occupying a given occupying a given sectionsection over some specified time period or over some specified time period or harmonic mean of individual speeds.harmonic mean of individual speeds.
SMS = d/(∑(tSMS = d/(∑(tii)/n) = nd/∑(t)/n) = nd/∑(tii))
(See page 112 and Table 5.5)
sft
n
nnSMS
sftn
nnTMS
/7.583
)2*0.440.88
/0.662
)0.440.88
Illustrative Computation of TMS and SMSIllustrative Computation of TMS and SMS
Chapter 5 8
Chapter 5 9
Speed typesSpeed types
Do you know the difference among these speeds?
Average travel speed
Average running speed
Operating speed = Max safe speed , without exceeding the design speed of the highway segment
Percentile speed = a speed below which the stated percent of vehicles in the traffic stream travel
Chapter 5 10
5.2.3 Density and occupancy5.2.3 Density and occupancy
Definition: the number of vehicles occupying a given length of highway or lane (vpm, vpmpl, v/km, v/km/lane)
Relationship among v, S, D:
v = S * D
Flow rate = Speed * Density
Unit length (1 mile or 1 km)
Chapter 5 11
Speed
ELpt
Speed
Lpt v )()('
Occupancy as a surrogate parameter for density
Density is difficult to measure. So, we use “occupancy” as a surrogate measure for density. This can be obtained by traffic detectors of any kind.
Occupancy: the percent of the roadway (in terms of time) that is covered (occupied) by vehicles.
Apparent occupancy
Actual occupancy
Speed
ELLpt v
)(
This is the occupancy measured at a point.
Chapter 5 12
Flow rate, speed and occupancy are given; Flow rate, speed and occupancy are given; estimate densityestimate density
ELL
miftOD
v
app
5280
mi
veh
hourmi
hourveh
S
q
Speed
rateFlowD
_
Typically occupancies given by the detectors are apparent occupancies.
dv
app
LL
miftOD
5280
But if average flow rate and average speed for a certain time period are given, density can be computed as:
(Eq.5-7)
Chapter 5 13
Derivation of the Density-Occupancy Derivation of the Density-Occupancy RelationshipRelationship
ELLOD
ELLD
ELL
S
q
S
ELL
T
N
T
TO
S
ELLN
S
ELLNT
S
ELLt
tNhrT
t
ELL
t
ELLmphS
Vapp
V
V
SMS
SMS
Voapp
SMS
V
SMS
Vo
SMS
Vp
po
p
V
p
VSMS
52805280
5280
5280
1
5280
5280
3600
3600
1
5280
3600
3600
5280
3600
3600/
5280/)(
)(
)()(
Estimate SMS using detector data
Compute total time occupied (not occupancy) by N vehicles detected in time period T
Solve the first equation for average time occupied by each vehicle
Plug in the 3rd eq into 2nd eq
Compute the occupancy Oapp. N/T turned out to be flow rate, q. Also q/SMS is density by definition. Now the relation between occupancy, Oapp, and density, D, was established.
Solve for D. Voila, you get Eq. 5.7)
Chapter 5 14
5.2.4 Connection between macroscopic and 5.2.4 Connection between macroscopic and microscopic measuresmicroscopic measures
These are in English units.
D (Density) = 5280 / da where da is average spacing
v (Flow rate) = 3600 / ha where ha is average headway
S (Average speed) = da / ha
Spacing or Space headway
Headway or Time headway
15
5.3 Relationships among flow rate, 5.3 Relationships among flow rate, speed, and densityspeed, and density
Flo
w (
v)
Density (D)
Do you remember whose flow model is used for this?
S = Sf –(Sf/Dj)*D
v = S*D
16
5.3 Relationships among flow rate, 5.3 Relationships among flow rate, speed, and densityspeed, and density
Flo
w (
v)
Density (D)
Optimal flow or capacity
Optimal (critical) density
Jam density
Mean free speed
Optimal speed
Speed is the slope. S = v/D
Do you remember whose flow model is used for this?
S = Sf –(Sf/Dj)*D
Uncongested flow Congested
flow
Unstable flow area