2.1. Traffic Flow Characteristics

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CIVL4660TrafficEngineering

2.1:TrafficFlowCharacteristics

Dr.JeffX.Ban

ra c ng neer ng

Rensselaer

Polytechnic

Institute

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Amacroscopic

view

of

vehicular

traffic

as

flowsvs.amicrosco icviewoftraffic

focusingonindividualvehicles

Understandtrafficflowcharacteristics

Investigatethe

traffic

flow

relationships

in

TrafficEngineeringstudies

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traffic

stream

characteristics

are

based

solely

for example:freewaysegmentsmorethan2miles without fixed interru tions

InterruptedFlow

forexample:signalizedintersections

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Volume

or

flow

rate

density

headway

spacing

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Headwayh (s/veh):time

interval

that

two

consecutivevehicles frontbum ertofront

bumper)passonestationarypoint

stationarypointduringaunittimeinterval

aperiodoftime(temporally)

q

h =5


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highwayduring

aspecific

time

interval

(one

lane,

onedirectionetc);unit:veh

Flowrate:numberofvehicles/hour;themeasurement

duration

could

be

less

than

1hour

Dailyvolume:representannualtrendofvolumeinhighway.

Hourlyvolume:peakhourvolume(directional)

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Average

Annual

Daily

Traffic

(AADT) Average24hourvolumeoverayear

Average24hrvolumeperweekday

AverageDaily

Traffic

(ADT)

SameasAADT, utaverageoverperio esst anayear,say,amonth

AverageWeekdayTraffic(AWT)

Same

as

AAWT,

but

average

over

aperiod

less

than

ayear,

say,

amonth

**Generallynotdirectional

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AADTApplications

Estimationofhighwayuse

Estimationoftrends

Economicfeasibility

evaluation

Planning

Maintenance

pp cat ons Planning

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Dailyvolume

is

useful

for

planning

purposes,

butcannotbeusedalonefordesi nand

operationpurposes

Peakhourvolumegenerallyisdirectional

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DDHV=AADT*K*D

K:

proportion

of

daily

traffic

occurring

in

the

peak

our

D:proportionofpeakhourtraffictravelinginthe

peakdirection

KandDgenerallydecreaseinhighdensityarea

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Themax

#veh passing

apoint

on

ahighway

over

60

consecutiveminutes

= PHV

(0.25


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memeanspee spotor nstantaneousspee Individualvehiclesspeedismeasuredatasingle

Arithmetic averageofallvehiclesthataremeasureddurin a time eriod

i

=N

it vN

u1

1

N: number of vehicles that are measured at a pre-specified time interval13


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Spacemeanspeed(harmonicmean) Individualvehiclesspeedisnotdirectlymeasured

n v ua ve c e s rave meacrossaprespec e distanceismeasured

Obtainavera e

travel

time

Obtainaveragespacemeanspeed

=D

=N

i

avg

i

i

v

D

Nt

v

1

1

==N

i

avg

s

vNt

Du

1

11

1

it 14


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Relationshipbetween

timemeanspeedand

spacemeanspeed

Timemeanspeedisgreaterorequalto spacemean

speed Thetwospeedshavethefollowingrelationship:

s

sst

uuu +=

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44,42,

51,

49,

46

mph.

Assume

each

vehicle

is

,

timemeanspeedandspacemeanspeed?

Answer:ut=46.4mph>us =46.2mph

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Spacing s (ft/veh):distance

(front

bumper

to

frontbumper)betweentwoconsecutivevehicles

Densityk(veh/lane/mile):numberofvehiclesper

laneper

mile,

which

is

defined

on

asegment

of

roa spatia y atagiventime tempora y

s1

=

s 17


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aneoccupancy:measureuse n reewaysurveillance. Ratioofthetimethatvehiclesarepresentatadetection

stationinatrafficlanecomparedtotheobservationtime(0


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InductanceLoopDetector

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Laneoccupancy ei

LCLt =+= Time that the vehicle used to travelL+C:

100

occupiedisdetectorvehicletimetotal

==

ti

ss

sv

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Laneoccupancy Assumekvehiclesareevenlyspreadouton1mile

highwayatspeed (mile/hr)sv Density = k vehicle/mile

Totaltimeneededtohaveallvehiclespassthe

1 mile

detectoris:

(hrs)

Therefore:

svT 1=100

/1

5280100

s

si

v

v

T

t

==

(veh/mile)100

5280

)(

5280

eLCLk

=+

=

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Signal Coordination

Length of acceleration andece era on anes o reeway

on-ramps and off-ramps

Estimation of safe passing

si ht distances

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UsingGPSDatatoConstructTime

SpaceDiagram

4.0

4.5

.

Oak Villa Blvd

3.0

3.5

(m

i) Wooddale

Airline Hwy interchange

2.0

2.5

Distanc

Ardenwood

Donmoor

Bon Marche MallLobdell

0.5

1.0

. Posted speed limit

Actual profile

N Foster

Cloud

0.0

0 100 200 300 400 500 600

Travel time (s)

Acadian Thruway

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7000o(feet)

5000

6000

ucktowardsS

anPabl

2000

3000

4000

from

University:Shatt

7000

Pab

lo(feet)

5.54 5.545 5.55 5.555 5.56 5.565 5.57

x 104

0

1000

time (seconds)distancetraveled

4000

5000

hattucktowardsSa

2000

3000

ledfrom

University:

5.4 5.5 5.6 5.7 5.8 5.9 6 6.1 6.2

x 104

0

1000

time (seconds)dis

tancetrav

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Time Latitude Lon itude S eed(UTC) () () (mph)

16:27:39 30.389885 -91.242426 40.016:27:40 30.390090 -91.242276 40.6

16:27:41 30.390159 -91.242225 40.7

16:27:42 30.390298 -91.242122 41.016:27:43 30.390507 -91.241966 41.8

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RelationshipsamongFlow,speed,and

density

v=S*D(textbook)

q=k*v(commonly

used)

Flow veh hr =S eed mile hr *Densit veh mile

Proof:Considerasegmentoffreeway.

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60

70

80

40

50

ed

(mph)

20

30sp

0

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

densit veh/mile/lane

Field data Vo=15 mph & Kj=250 veh/mi/ln

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45

55

25

35

ee

d(mph)

5

15S

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140Density (veh/mi/ln)

alpha =1.0 (Vo =5, Vf =35, Kj =75) alpha =1.25 (Vo =5, Vf =37, Kj =90)

alpha =1.67 (Vo =5, Vf =39, Kj =130)

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Speed,Flow,andDensity

Relationship

Wehave

BkAv =2==

( ) vv

AvAvq

2

=

=

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RelationshipsbetweenSpeed,

Densityand

Flow

A:

almost

zero

density,

free

flow

speed,

very

low

volume B:increaseddensity,reducedspeed,increasedvolume

C:increaseddensity,reducedspeed,maxvolume

D:jamdensity,minspeed(crawling),verylowvolume

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GeneralPropertiesforanyTrafficFlow

Model

Needto

satisfy

four

boundary

conditions

Flowiszeroatmaximumdensity

Flowdensitycurvesareconcave(i.e.,thereisapoint

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TheproductofthexycoordinatesofthepointP istheflowassociatedwithP

Maxflowoccursat:B

kBkAk

q

2020 ===

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Theslope ofthelineconnectinganypointonthecurveandtheoriginistheaveragespeed

Fundamental Diagram

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Theslopeofthelineconnectinganypointonthecurve

and

the

origin

is

the

inverse

of

the

density

Maxflowoccurredat:2

0 vv

q==

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Greenshields model Greenbergs model k

=jam

fk

vv 1

=jamk

v n

Modified Greenshields model Underwoods model

=jam

fk

vvvv 100 )(

=m

fk

vv exp

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. ,

200ft/veh.

Calculate

the

average

speed

of

the

.

Flowq=1/h=0.4veh/s=1440veh/h

.

.

Speedv=q/k=1440/26.4=54.5mi/h

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TextbookChapter

5

38

2.1. Traffic Flow Characteristics

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