fhwa ttr_brochure
TRANSCRIPT
T R AV E L T I M E R E L I A B I L I T Y
M A K I N G I T T H E R E O N T I M E , A L L T H E T I M E
Traffic congesTion is a daily realiTy in
mosT of The large urban areas in The
uniTed sTaTes.
It’stobeexpected—largenumbersofpeoplealltryingtoreach
theirdestinationsatthesametime,usuallyduringpeakhours.
Driversareusedtotheeverydaycongestionandtheyplanfor
it.Theydon’tlikeit,buttheyleavehomeearlyenoughtoget
toworkontime.It’stheunexpectedcongestionthattroubles
travelersthemostfromdayto
day.Atripthatusuallytakes
ahalf-hour,withlittleorno
warning,takesanhour.
Nowthemotoristislatefor
work,hasmissedadoctor’s
appointment,orisfacinghefty
childcarepenaltiesforpickingup
thekidslate.Maybeatruckergetsheldupinunexpectedtraffic,
makingshipmentslatetothemanufacturer,disruptingjust-in-
timedelivery,andlosingthecompetitiveedgeonothershippers.
Travelerswanttraveltimereliability—aconsistencyor
dependabilityintraveltimes,asmeasuredfromdaytodayor
acrossdifferenttimesofday.Driverswanttoknowthatatrip
willtakeahalf-hourtoday,ahalf-hourtomorrow,andsoon.
wHy is travel time reliability imPortant?
Mosttravelersarelesstolerantofunexpecteddelaysbecausesuchdelayshavelargerconsequencesthandriversfacewitheverydaycongestion.Travelersalsotendtorememberthefewbaddaystheyspentintraffic,ratherthananaveragetimefortravelthroughouttheyear(seeFigure1).
Inordertoimprovetraveltimereliability,thefirststepistomeasureit.Measuresoftraveltimereliabilitybetterrepresentacommuter’sexperiencethanasimpleaveragetraveltime.Forexample,atypicalbefore-and-afterstudyattemptstoshowthebenefitsofanincidentmanagementprogram(seeFigure2).Lookingataveragetraveltime,theimprovementmayseemmodest.However,traveltimereliabilityprovidesadifferentperspectiveoftheimprovement:theworstfewdayshavebeendramaticallyimproved.Travelersmakeittotheirdestinationsontimemoreoftenorwithfewersignificantdelays.
How do agencies measure travel time reliability?
Traveltimereliabilitymeasuresarerelativelynew,butafewhaveproveneffective.Mostmeasurescomparehigh-delaydaystothosewithanaveragedelay.Themosteffectivemethodsofmeasuringtraveltimereliabilityare90thor95thpercentiletraveltimes,bufferindex,andplanningtimeindex,explainedinthefollowingsections.
Severalstatisticalmeasures,suchasstandarddeviationandcoefficientofvariation,havebeenusedtoquantifytraveltimereliability.However,theyarenoteasyforanontechnicalaudiencetounderstandandwouldbeless-effectivecommunicationtools.Theyalsotreatearlyandlatearrivalswithequalweight.Butthepubliccaresmuchmoreaboutlatearrivals.
Jan. July Dec.
Trav
el T
ime
Annual average
How traffic conditionshave been communicated
Jan. July Dec.Tr
avel
Tim
e
Travel times vary greatly day-to-day
. . .and what they remember
What travelers experience. . .
Figure 1. Averages don’t tell the full story
Before After
2003 2004 2005
Trav
el T
ime
Average Day
Small improvement inaverage travel times
Before After
2003 2004 2005
Trav
el T
ime
Worst Dayof the Month
Large improvement intravel time reliability
Figure 2. Reliability measures capture the benefits of traffic
management
2
90th or 95th percentile travel times
Thismethod,the90th or 95th percentile travel times,isperhapsthesimplestmethodtomeasuretraveltimereliability.Itestimateshowbaddelaywillbeonspecificroutesduringtheheaviesttrafficdays.Theoneortwobaddayseachmonthmarkthe95thor90thpercentile,respectively.Usersfamiliarwiththeroute(suchascommuters)canseehowbadtrafficisduringthosefewbaddaysandplantheirtripsaccordingly.Thismeasureisreportedinminutes(asshowninFigure6).
Buffer index
Thebuffer indexrepresentstheextratime(ortimecushion)thattravelersmustaddtotheiraveragetraveltimewhenplanningtripstoensureon-timearrival.
Forexample,abufferindexof40percentmeansthatforatripthatusuallytakes20minutesatravelershouldbudgetanadditional8minutestoensureon-timearrivalmostofthetime.
Averagetraveltime=20minutesBufferindex=40percentBuffertime=20minutes×0.40=8minutes
The8extraminutesiscalledthebuffertime.Therefore,thetravelershouldallow28minutesforthetripinordertoensureon-timearrival95percentofthetime.
1.00
1.20
1.40
1.60
1.80
2.00
2.20
12 AM 2 AM 4 AM 6 AM 8 AM 10 AM 12 PM 2 PM 4 PM 6 PM 8 PM 10 PM 12 AM
Time of Day (weekdays, non-holidays only)
Inde
x Va
lue
Travel Time Planning Time
Travel time index
(average)
Buffer time between
expected (avg.)
and 95th percentile
travel times
Planning time index
Los Angeles, 2003 citywide data shown
Figure 3. Reliability measures compared to average congestion measures (Source: http://mobility.tamu.edu/mmp/)
Planning time index
Theplanning time indexrepresentshowmuchtotaltimeatravelershouldallowtoensureon-timearrival.Whilethebufferindexshowstheadditionaltraveltimethatisnecessary,theplanningtimeindexshowsthetotaltraveltimethatisnecessary(seeFigure3).
Forexample,aplanningtimeindexof1.60meansthatforatripthattakes15minutesinlighttrafficatravelershouldbudgetatotalof24minutestoensureon-timearrival95percentofthetime.
Free-flowtraveltime=15minutesPlanningtimeindex=1.60Planningtime=15minutes×1.60=24minutes
Theplanningtimeindexisespeciallyusefulbecauseitcanbedirectlycomparedtothetraveltimeindex(ameasureofaveragecongestion)onsimilarnumericscales.Thetraveltimeindexisameasureofaverageconditionsthattellsonehowmuchlonger,onaverage,traveltimesareduringcongestioncomparedtoduringlighttraffic.
Figure3illustratestherelationshipbetweenthebufferindexandtheplanningtimeindex.Thebufferindexrepresentstheadditionaltimethatisnecessary,whereastheplanningtimeindexrepresentsthetotaltraveltimethatisnecessary.
3
Figure 4. A methodical approach can be used to develop
reliability measures
STEP 1. Determine how measures will be used
• Definethestructureandcontentofprogram
• Quantifybenefitsforelectedofficialsandkeydecision-makers
• Monitorconditionsforfine-tuningoperationalprocedures
• Comparealternativemulti-modalinvestmentscenarios
STEP 2. Develop a plan based on uses and users
• Definetravelmodes,routes,trips,days,timesofinterest
• Definedatasourceandcalculationprocedures
• Developcommunicationtoolsforresults
STEP 3. Collect and process required data
• ContinuousdatacollectionfromIntelligentTransportationSystems(ITS)mostdesirable
• Othermethodstocollectorestimatedataarepossible
• Usequalityassurancemethods
• Calculaterouteortriptraveltimes(basicdataelement)
STEP 4. Calculate reliability measures
• 95thorotherpercentiletraveltime
• Bufferindex
• Planningtimeindex
STEP 5. Communicate measures in meaningful way
• Annotatedgraphicsthatavoid“tech-speak”
• Relatetotraveler’sexperience
How does an agency begin using travel time reliability measures?
Puttingthesemethodstoworkrequiresanoverallevaluationandimplementationprocess.Figure4brieflyshowsthestepsinvolvedinmeasuringtraveltimereliabilityandhowtoputthatinformationtoworkfortravelersandtrafficmanagers.
wHo is currently using travel time reliability measures?
Eventhoughtraveltimereliabilitymeasuresarerelativelynew,severalagencieshavealreadybegunusingthem.AgenciessuchastheFederalHighwayAdministration(FHWA),MinnesotaDepartmentofTransportation(Mn/DOT),andtheWashingtonStateDepartmentofTransportation(WSDOT)haveprimarilyusedtraveltimereliabilityasaperformancemeasuretosupplementmeasuresofaveragecongestion.
FHWA
FHWAsupportsanationaltrafficmonitoringprogramthattracksreliabilitymeasuresinmorethan30cities.FHWAcommunicatesthisinformationtokeydecision-makersthroughamonthlydashboardreport(seeFigure5).Thereportincludestrendinformationontheduration(hoursofcongestedtravelperday),magnitude(traveltimeindex),andreliability(planningtimeindex).
Figure 5. A reliability measure is included in FHWA’s Monthly Congestion Dashboard Report
status: Progress:
NATIONAL CONGESTION INDICATORS
Hours of congested travel Per day
travel time index Planning time index
Current Quarter 4.823 1.284 1.690
Same Quarter, Previous Year 5.181 1.294 1.707
Change vs. Previous Year 6.91% 0.77% 1.00%
National Congestion Pattern # of Cities
DOWN >5%
# of Cities NO CHANGE
# of Cities UP
>5%
# of Cities
DOWN >5%
# of Cities NO CHANGE
# of Cities UP
>5%
# of Cities
DOWN >5%
# of Cities NO CHANGE
# of Cities UP
>5%
Total Cities: 19 9 4 6 2 17 0 4 13 2
green
➡ ➡
green
➡
4
Figure 6. WSDOT provides reliability measures for traveler information
(Source: http://www.wsdot.wa.gov/traffic/seattle/traveltimes/reliability/)
FHWA-HOP-06-070 TTI0601.0106.2M
wHere can i find more information?
Aguidetotraveltimereliabilityisavailableathttp://www.ops.fhwa.dot.gov/.Itcontainssupportinginformationandtechnicaldetailsondevelopingtraveltimereliabilitymeasures.
contact information
Formoreinformation,contact:RichTaylor,FHWA(202)[email protected]
Mn/DOT
In2000,Mn/DOTusedtraveltimereliabilitymeasurestostudytheeffectsofarampmetershutdownonMinneapolis-St.Paulfreeways.Inthisstudy,Mn/DOTreportedthatturningofftherampmeteringsystemcausedtraveltimereliabilitytoworsenby91percent.Incomparison,theaveragetraveltimesworsenedbyonly22percent.ThesefindingssupporttheconceptspresentedinFigure2—operationalimprovementshaveagreatereffectonday-to-daytraveltimereliabilitythanonaveragetraveltimes.Asaresultofthislegislativelymandatedstudy,Mn/DOTwasabletocontinueoperatingitsrampmeteringprogramin2001.
WSDOT
WSDOTtrackstraveltimereliabilityinitsperformance-monitoringeffortsandprovidesreliabilityestimatestocommuters.ApageontheWSDOTwebsite(seeFigure6)allowscommuterstoselectatripandgeneratea95thpercentiletraveltimebasedonhistoricaldata.Commuterscanthenusethetraveltimeestimatetoensuretheyarriveontimeforthatparticulartrip.
WSDOTalsousesreliabilitymeasuresinreportingtheperformanceoffreewaysandhigh-occupancyvehicle(HOV)lanes
Produced by Texas Transportation Institute with Cambridge Systematics, Inc.
(http://depts.washington.edu/hov).Inparticular,WSDOTusesthe90thpercentiletraveltimeandthefrequencyofcongestionperformancemeasurestodetermineoperatingstrategiesandprioritizeimprovements.