2. Traffic Streams
2
Traffic streams are made up of individual drivers and
vehicles, interacting in unique ways with each other and
with elements of the roadway and general environment.
Because the judgments and abilities of individual drivers
come into play, vehicles in the traffic stream do not and
cannot behave uniformly.
Further, no two similar traffic streams will behave alike,
even under equivalent circumstances, as driver behavior
varies with local characteristics and driving habits.
Traffic streams, however, can be described in quantitative
terms with the use of some key parameters like volume,
speed and density.
3. Traffic Streams
Traffic facilities are broadly separated into two principal
categories:
1. Uninterrupted flow facilities, and
2. Interrupted flow facilities
Uninterrupted Flow Facilities are those on which no
external factors cause periodic interruption to the traffic
stream. Example: Freeway
Interrupted Flow Facilities are those having external
devices that periodically interrupt traffic flow. Example:
Urban Roadways.
The principal devices creating interrupted flow are
primarily traffic signal and also STOPand YIELD signs, etc.
4. Traffic Streams
4
Uninterrupted and Interrupted Flow are terms
that describe the facility and not the quality of
flow.
A congested freeway where traffic is almost
coming to a halt is still classified as
uninterrupted 単ow facility, because the reason
for congestion is internal to the traPic stream.
Awell-timed signaling system on an arterial may
result in almost uninterrupted traffic flow, but
still is classified as interrupted flow facility.
5. Traffic Stream Parameters
5
TraPic Stream Parameters fall into two
broad categories:
Nacrozcopic parameters, and
Nicrozcopicparameterz.
Macroscopicparameters characterizethe
traPic stream as a whole
Microscopic parameterscharacterizethe
behavior of individual vehicles in the
traPic stream with respect to each other.
6. Traffic Stream Parameters
6
Macroscopicflow
traffic in the aggregate.
overall speeds, traffic flows, densities,
etc.
Microscopic flow
traffic at the level of the individual vehicle
observe the paiticular behaviors of drivers,
and of individual vehicles in the traffic
stream.
vehicle headways & lane-chan9ing behavior
(merging, diverging, weaving, passing, etc.)
critical 0 microscopic level.
7. Traffic Strea Pa meters
7
A traPic stream may be described
macroscopically by three parameters:
1. Volume or Rate of Flow
2. Speed
3. Density
8. Traffic Strea
8
Pa meters
Volume and Flow
Pa/wme is defined as the number of vehicles
that pass a point on a highway, or a given lane
or direction of a highway, during a specified time
interval.
Usually expressed as vehicles per unit time, for
example, vehicles per hour or vph.
Aate a///owis the equivalent hourly rate at
which vehicles pass a point on a highway lane
during a time period less than 1 hour.
9. Traffic Stream Parameters
9
Volume and Rate of Flow are two
diPerent measures.
Volume is the actual number of vehicles
observed or predicted to be passing a
point during a given time interval.
Rate of flow represents the number of
vehicles passing a point during a time
interval less than 1 hour, but expressed
as an equivalent hourly rate.
10. Traffic Stream Parameters
10
Avolume of 200 vehicles observed
in a 10-minute period implies a rate
of flow of (200 x 60)/10 = 1200
veh/hr.
Note that 1200 vehicles do not pass
the point of observation during the
study hour, but they do pass the
point at that rate for 10 minutes.
11. Traffic Strea Pa meters
11
. The maximum number of vehicles per
unit time that a particular
transportation facility may accommodate
(veh/h).
12. Traffic Strea
12
Pa meters
Acommon time interval for volumes is a
day.
Daily volumes are frequently used as the
basis for highway planningand general
observations of trends.
TraPic volume projections are often
based on measured daily volumes.
13. Traffic Stream Parameters
13
Dai"ly Volumes and Their Uae (ontd..)
There are four commonly used daily volume parameters:
Average AnnualDaily Traffic (AADT): is the average 24-hr
traffic volume at a given location over a full 365-day year.
Average Annual Weekday Tracce {AAWT): is the average
24-hr traffic volume occurring on weekdays over afull
365-day year.
3
. AverageDai/y Traffic {NDT): is an average 24-hr volume at
a given location for some period of time less than a year,
but more than one day.
Average Weekday TraTlic AWT): is an average 24-hr
traffic volume occurring on weekdays for some period less
than oneyear.
15. Traffic Stream Parameters
15
Hourl Volumes and Their Use
While daily volumes are useful in highway planning, they
cannot be used alone for design or operational analysis
purposes.
Traffic volume varies considerably during the course of a
24-hr day.
The single hour of the day that has the highest hourly
volume is referred to as the peak hour.
Traffic volume within this hour is of greatest interest to
traffic engineers in design or operational analysis.
16. Traffic Strea Pa meters
16
Sub-hourl Volumes and Rates of Flow
The variation within a given hour is also of considerable
interest for traffic design and analysis.
The quality of traffic flow is often related to short-term
fluctuations in traffic demand.
A facility may have capacity adequate to serve the peak-
hour demand, but short-term peaks of flow within the
peak hour may exceed capacity, thereby creating a
breakdown.
17. Traffic Stream Parametem
17
Sub-hourl Volumes and Rates of Flow
Table 3-9 Illustration of Hourly Volume and
Rate of Flow
2
Volume fdr Time
Cot 2/O.25
eate of Flow for
1 interval Time interval
Time Interval (velt) {yp}s)
S:OOS: l5 P.M.
5:155.30 P.M.
1000
4I O
O
4OOO
too
S:3O5:45 r.ter. I2oo 4eoo
5:456:00 P.M. 900 3600
5:OO6:00 r.M. 4200 Vph hourly volume
18. Traffic Stream Parameters
18
Sub-hourl Volumes and Rates of Flow
contd..
The relationship between hourly volume and the
maximum rate of flow within the hour is defined
by the Peak Hour Factor (PHF).
PHF
hourly volume U
maximum rateof flow 4xVJ
5
where, V = hourly volume
minute volume within the h::t
'高 = maximum 15-
19. Traffic Strea Pa meters
19
Sub-hourly Volumes and Rates of Flow [contd..)
The maximum value of PHF is 1.00, which occurs when
the volume in each 15-min period is equal.
The minimum value is 0.25, which occurs when the entire
hourly volume occurs in one 15-min interval.
The normal range of values is between 0.70 and 0.98,
with lower values signifying a greater degree of variation
in flow during the peak hour.
20. Traffic Strea Pa meters
20
eed
Speed is the second principal parameter describing the
state of a given traffic stream.
In a moving traffic stream, each vehicle travels at a
diPerent speed.
Thus, the traffic stream doesnot have asingle
characteristic speed but rather a distribution of individual
vehicle speeds.
From the distribution of vehicle speeds, a number of
average or typical values may be used to characterize
the traffic stream asa whole.
21. Traffic Strea
21
Pa meters
eed contd..
Average or mean speeds can be computed in two different ways:
Time Mean Speed (TLS) is defined as the average speed of all
vehicles passing a point on a highway over some specified time
period.
:fipace Mean Speed(ENE) is defined as the average speed of all
vehicles occupying a given section of a highway over some
specified time period.
Time mean speed is a point measure, while space mean speed is
a measure relating to a length of highway or lane.
22. Traffic Strea
52
Pa meters
eed contd..
SIWS
where, d is the distance traversed, n is the
number of travel times observed and 7,is the
travel time for /-th vehicle.
23. 1 JOOO 18.0 7OOO/18 US.6
2 1OOO 20.O 1OOOWO SO.0
3 OOO z2.O 1OOO22 - 45.S
4 ]OOO ]9.O 1 OOO/19 : 52.6
S
6
IOOO
]ODO 2O.
OOCL"20
1OOOWO
- 5 0. O
SO.O
Toto贈s
Aer8ee
6000
i I 9z6
119.O
1s.a 3O3.T/6
303.z
= 50.6
Traffic Stream Parametem
23
eed contd..
Table S-IO Comyutatiarj of Time M e a n Spee
anc4 Space M e a n Spee<l
D i s t a n c e T r a v e l T i m e Speed
or 6OOO119 - TO.4 lps
24. Traffic Strea Pa meters
24
Relationship between time mean speed
and space mean speed
Time mean speed is greater to equal to space
mean speed
The two speeds have the following
relationship:
Vs
25. Traffic Strea
25
Pa meters
eed contd..
They are two forms of space mean speed.
The Average Travel Speed computation uses
total average travel time while Average Running
Speed computation uses the average running
time.
Running time is defined as the time during
which the vehicle is in motion while traversing a
given highway segment.
26. Traffic Strea
26
Pa meters
eed contd..
Example
Consider the case of a 1-mile section of a roadway. On the
average, it takes a vehicle 3 minutes to traverse the section, 1
minute of which is stopped time experienced at signalized
intersections.
AverageTrave 1Speed
' mph
2()mph
60
Avera geEtinning
27. Traffic Strea
27
Pa meters
eed contd..
Operating Speed is defined as the maximum safe speed at
which a vehicle can be conducted in a given traffic stream,
without exceeding the design speed of the highway
segment.
Operating speed is difficult to measure. It requires that a
test car be driven through the traPic stream in a manner
consistent with the definition.
Asmaximum safe speed" is a judgmental matter,
consistent measurements among test-car drivers are not
often achieved.
28. Traffic Strea Pa
28
meters
Densi
Density, the third measure of traPic
stream conditions, is defined as the
number of vehicles occupying a
given length of highway or lane.
Usually expressed as vehicles per
mile (vpm) or vehicles per mile per
lane (vpmpl).
Density is difficult to measure
directly, as an elevated point is
required.
29. Traffic Strea
29
Pa meters
Densi coctd...
It can, however, be computed from speed
and flow rate using the relationship as
follows:
Where, q = flow rate (vph)
v, = space mean speed (mph), and
k = density (vpm).
30. Traffic Strea Pa meters
ac" a and Time Headwa
Spacing and Time Headway are microsco ic measures,
because they apply to individual pairs of vehicles within
the traffic stream.
/pac7ny is defined as the distance between successive
vehicles in a traffic lane, measured from some common
reference point on the vehicles, such as the front bumpers
or front wheels.
Spacing 0 given point ,o
31. Traffic Strea Pa meters
31
ac" a and Time Headwa
Fime 6ea#rrayis the time between successive vehicles
as they pass a point along the lane, also measured
between common reference points on the vehicles.
Particular
location
32. Traffic Strea
32
Pa meters
Clearance (ft) =
(spac財ng)(average
vehic/e /ength)
Gap (sec) =
(headwdfi)(time
eqciva/ence of the
averape vehicfe
fenpth)
33. Traffic Strea Pa meters
contd..
ac!n and Time Headwa
Average values of Spacing and Time Headway are
related to the macroscoic arameters as follows:
da
k=5280, 3600
9'
where, k = density (vpmpl), vs = 多Iverage speed
(ft/sec), q = rate of 単ow (vphpl), dv' average spacing
(ft), and ha = average time headway (sec).
34. Traffic Strea Pa meters
34
Lane occupancy: measure used in freeway
surveillance.
Ratio of the time that vehicles are present at a
detection station in a traPic lane compared to the
observation time.
dirt cti高n .f travel
I.
Lengih ot vehicle
C. I9istancc between lotps tif detector
35. Traflic Strea Pa
35
meters
Lane occupancy
LO
total time vehicle detectoris occupied
totalobservation time T
Time that the vehicle used to travel 1+6: t0 + C
f. - Length of vehicle
C - Distance between uops of detector
36. TraWic Strea
36
Pa meters
ane occupancy
Assume k vehicles are evenly spread out on 1 mile
highway at speed v, mile/hr
Density
= kvehicle/mile
is: (Br束)/v,
Therefore:
LO
X
(L +C) k
k = LO z6280
m i l e s
Total time needed to have all vehicles pass the detector
(vehicle/mile)
38. Properties of speed-density
curve
38
The product of the x-y coordinates of
the point Pis the 単ow associated with
Max flow occurs at: 属 - A2Bk
0 -
3k
k,
k
represenls the
, and densitv k
k' k
A
2//
39. Properties of speed-flow
curve
39
The slope of the line connecting any point on the
curve and the origin is the inverse of the density
Nax flow occurred at: as 0
t A
41. General properties for a
traffic flow model
41
Need to satisfy four boundary
conditions
Flow is zero at zero density
Flow is zero at maximum density
Mean free-flow speed occurs at zero density
Flow-density curves are convex (i.e. there is a
point of max flow)
42. Connections beMeen speed,
42
density and flow
A: almost zero density, free-flow speed, vey low
volume
B: increased density, reduced speed, increased volume
C: increased density, reduced speed, max volume
D: jam density, min speed (crawling), vey low volume
Density. k D Flow. q
0
Density, k
43. Traffic Strea Pa meters
43
Since a given flow may occur under two
completely diPerent operating conditions (stable
and unstable), volume or rate of flow cannot be
used as a measure describing the operational
quality of the traffic stream.
Speed and density, however, are good measures
of the quality of operations, as both uniquely
describe the state of the traPic stream.
44. Macroscopic Models of Traffic Flow
44
The two most commonly used macroscopic
models are:
The Greenshields Model
The Greenberg Model
45. Macroscopic Models of Traffic Flow
t単 !tn.! liretds Mot≒n.1
45
The general model connecting speed,
flow, and density discussed so far is a
linear model proposed by Greenshield in
1935.
He suggested that the speed and
density were linearly related as follows:
46. Macroscopic Models of Traffic Flow
!e'n.' liie ldc Mc高≒o.I
46
It can be shown that:
The maximum fiow (i.e., capacity) occurs when the speed
of the traffic stream is half of the free-flow speed:
* f
" 2
The maximum flow (i.e., capacit:y) occurs when the density
is half of the jam density:
k
The maximum flow, q a,:
47. Macroscopic Models of Traffic Flow
!e'損beroz I i I座
47
Greenbergdeveloped a model in 1959,
taking speed, flow and density
measurements in the Lincoln Tunnel.
Used a fluid-flow analogy concept.
The model is of the following form:
48. Macroscopic Models of Traffic Flow
!e'nberoz I i f損
48
It can be shown that:
The maximum flow occurs when speed
The maximum flow occurs when density (k) is
related with jam density (ki) as follows:
Then, the maximum flow (q ax) iS the product of
the density (k) and speed (vs) at maximum flow.
49. Macroscopic Models of Traffic Flow
贈1 f,r I" / Modelx
49
The Greeshields Model can be used for light or
heavy traffic conditions.
The Greenberg Nodel is useful only for heavy
traPic conditions.