12 Design Speed and Design Traffic

8/19/2019 12 Design Speed and Design Traffic

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Design Speed and DesignTraffic Concepts

CE 453 Lecture 11


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Objectives Get familiar with design

speeds for functional classes Describe traffic demand and

determine for roadwa design

Define !DT" !!DT" D#$" D"DD#$" %&'actor" and T


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(osted speed ) speed limit *perating speed ) free flow +spot

speed, -unning speed ) length of

highwa section . running time Design speed ) selected speed

used to determine geometricdesign features


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Design Speed Design speed is defined b the !!S#T* Green

/oo0 as ...the maximum safe speed thatcan be maintained over a specified

section of highway when conditions are so favorable that the design features of

the highway govern. Design Speed should 1, 2be consistent with

the speed the drier is li0el to expect6 and7, 2 fit the trael desires and habits ofnearl all driers6

8ot posted speed and not operating speed+but !L9!:S higher than both, See first part of

http;;wwwfhwadotgo;enironment;fle


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Design Speed Considerations 'unctional classification of the highwa

Character of the terrain

Densit and character of ad?acent landuses

Traffic olumes e


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Design Speed in Green /oo0 +suggested minimum design speed, -ural Local -oads

Source A Policy on GeometricDesign of Highways and Streets +The Green /oo0, 9ashington" DC

!merican !ssociation of State#ighwa and Transportation*fficials" 7==1 4th Ed


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Design Speed in Green /oo0 +suggested minimum design speed,

-ural Collectors

Source A Policy on GeometricDesign of Highways and Streets +The Green /oo0, 9ashington" DC

!merican !ssociation of State#ighwa and Transportation*fficials" 7==1 4th Ed


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Design Speed in Green /oo0 +suggested minimum design speed,

-ural !rterials @= A 17= 0ph +4=&B5 mph,

Depends on Terrain

Drier e


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$alues represent theminimum acceptabledesign speeds for the

arious conditions ofterrain and trafficolumes associatedwith new orreconstructedhighwa facilities


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Design Traffic


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Traffic Definitions

Volume:

number of ehicles" pedestrians"

etc passing a point during a specificperiod of time

for ehicles" usuall e


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Demand: number of ehicles" pedestrians" etc that desire to

trael between locations during a specific period 'reFuentl higher than olume during certain pea0

times

Trips are dierted or not made when there areconstraints in the sstem difficult to measure actual demand because capacit

constrains the demand Capacity:

ma


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Characteristics of Traffic 'low #ighl ariable

Time of da

Da of wee0

Season

-oad characteristics

Direction


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0

1000

2000

3000

4000

5000

6000

7000

Time of Day

F l o w

i n v e h i c l e s p e r h

o u

Highw ay Capacity

Highly Congested

Traffic Tpicall (ea0s twice per da

Source wwwecnpurdueedu;darc


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$olume Studies !!DT !nnual aerage dail

traffic +counted for 3@5 das,

!DT aerage dail traffic+counted for H 1 da and I 3@5,

(#$ pea0 hour olume

Classification counts fleet mi<


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Estimating !!DT !nnual !erage Dail Traffic

se count station information

E


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!!DT Data #elps to Estimate highwa reenues

Establish oerall olume trends

Establish annual accident rates

!nalJe benefits of road

improements


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Counting (rogram To satisf the traffic olume data

needs for all roads under a

particular ?urisdiction" we establisha Counting Program

A systematic pattern ofcounting at different times andlocations


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Traffic Counts Kapshttp;;wwwiowadotmapscom;msp;traffic;inde


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Traffic Counts Kaps


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Design $olume suall hourl olume 9hich hour

!erage hourl olume A inadeFuatedesign Ka


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Traffic Demand

Design #ourl $olume +D#$, A future hourlolume +both directions, used for design &tpicall 3=th ##$ +highest hourl olume, inthe design ear

9h 3=th ##$ /rea0point of 7&7N Compromise too high is wasteful" too low poor

operation !ppro


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Traffic Demand +cont,3 E


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Design #ourl $olume D#$ is a representation of pea0 hour

traffic" usuall for the future" orhoriJon ear

%&factor represents proportion of !!DTthat occurs in the 3=th ##$

%&factor ) PPD#$ < 1==!!DT

% ) N to 17M urban" 17 to 1NM rural


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Design #ourl $olume +E


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Question 9hat>s the impact of choosing

different % factor for design

f !!DT is 35== pd" how will the design olumediffer for 0&factor ) NM s 17M

D#$ ) %&factor < !!DT

1==

D#$0)NM ) N < 35== ) 7N= ph

1==

D#$0)17M ) 17 < 35== ) 47= ph +diff of 14=

1==eh,


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Traffic Demand +cont,R D ) directional distribution ) one

wa olume in pea0 direction

+e


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Directional Distribution

+e


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Traffic Demand +cont,R T ) percentage of hea ehicles

during design hour +owa interstate

35M plus,R !ffects capacit" abilit to pass on

two&lane rural roads" etc

R Larger" occup more space

R Should determine M during designhour +truc0 patterns ma not be sameas passenger ehicles,


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(#' ) pea0&hour olume

4+pea0 15&min olume,

'low is not uniform throughout an hour

#CK considers operating conditions

during most congestion 15&minuteperiod of the hour to determine sericeleel for the hour as a whole

(ea0 #our 'actor +(#',


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(ea0 #our 'actor


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D#$ ) (ea0our $olume

(#'E

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12 Design Speed and Design Traffic