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Measuring Exposure to Vehicle Accident Risk Using the American Time Use Survey. Lee Giesbrecht and Jonaki Bose Bureau of Transportation Statistics RITA, U.S. DOT. What Do We Mean by Exposure. Exposure measures form the denominator of accident rates

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Measuring exposure to vehicle accident risk using the american time use survey l.jpg

Measuring Exposure to Vehicle Accident Risk Using the American Time Use Survey

Lee Giesbrecht and Jonaki Bose

Bureau of Transportation Statistics

RITA, U.S. DOT

TRB 2007 Annual Meeting – Travel Survey Methods Committee Meeting


What do we mean by exposure l.jpg
What Do We Mean by American Time Use SurveyExposure

  • Exposure measures form the denominator of accident rates

  • Defining the numerator may be simpler – number of accidents, fatalities, injuries

  • However, defining exposure or the opportunity to be involved in any of the numerator incidents is not that clear cut – using total population counts is obviously a poor choice

TRB 2007 Annual Meeting – Travel Survey Methods Committee Meeting


Defining exposure l.jpg
Defining Exposure American Time Use Survey

  • Vehicle miles traveled

  • Numbers of registered drivers

  • Numbers of registered vehicles

  • Time spent on roadways/outdoors

TRB 2007 Annual Meeting – Travel Survey Methods Committee Meeting


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Why Bother with the Denominator? American Time Use Survey

  • Better understanding of the nature of accidents

    • Are there variations in accident rates of different subgroups?

    • Are there interactions between subgroup status and factors such as time of day, type of vehicle, urbanicity, etc.

  • For example even though there are similar numbers of men and women in the population, men tend to drive longer and further, thus increasing their potential exposure

TRB 2007 Annual Meeting – Travel Survey Methods Committee Meeting


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Literature American Time Use Survey

  • Travel survey data has been explored as a denominator measure

    • Data on miles traveled used in New Zealand and Australia to compute crash rates

    • Data on miles traveled and travel times used in Canada to compute average speeds for subgroups

  • Miles traveled from the NHTS and its predecessors has also been explored

TRB 2007 Annual Meeting – Travel Survey Methods Committee Meeting


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Combination of Two Previous Studies American Time Use Survey

  • Bose, J. and J. Sharp. “Measurement of Travel Behavior in a Trip-Based Survey Versus a Time Use Survey: A Comparative Analysis of Travel Estimates Using the 2001 National Household Travel Survey and the 2003 American Time Use Survey”. Presented at the American Time Use Survey Early Results Conference, December 8-9, 2005.

  • Giesbrecht, L., J. Bose, and M. P. Cohen. “Developing Exposure Measures to Evaluate Vehicle Accident Risk”. Proceedings from the Survey Research Methods Section, American Statistical Association, 2005.

TRB 2007 Annual Meeting – Travel Survey Methods Committee Meeting


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Our Objectives American Time Use Survey

  • Continue to highlight the NHTS as a source of data, especially since it includes –

    • children under five

    • vehicle occupancy

    • time of travel, distance and mode

  • Compare the ATUS with the NHTS as a source of exposure data

TRB 2007 Annual Meeting – Travel Survey Methods Committee Meeting


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Background on the 2001 NHTS American Time Use Survey

  • Nationally representative RDD survey

  • 26,000 households, 60,000 persons

  • Daily and long distance trips, vehicles

  • All household members

  • Randomly assigned travel day

  • Approximately 250,000 daily trips, 45,000 long distance trips (in the national sample)

  • Data collection within 6 days

  • Proxies accepted

TRB 2007 Annual Meeting – Travel Survey Methods Committee Meeting


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Background on the 2003-4 ATUS American Time Use Survey

  • Continuous survey

  • Nationally representative telephone survey based on area probability sample (retired CPS panels)

  • 21K people 15 years old and above

  • Data for one household member

  • Data on household and personal characteristics, as well as on all activities

  • For each activity data collected on start and end times, detailed nature of activity, who else was part of the activity

  • Travel was considered a primary activity

TRB 2007 Annual Meeting – Travel Survey Methods Committee Meeting


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Similarities American Time Use Survey

  • Nationally-representative surveys

  • Data collected over the phone

  • Data collected for a 24-hour period (4 am start)

  • Data on travel activities/trips collected at a person level

  • NHTS defines trips as movement from one address to the other. In the ATUS movement from one address to the other is coded as travel, regardless of any other activity

TRB 2007 Annual Meeting – Travel Survey Methods Committee Meeting


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Differences American Time Use Survey

  • Trip-based (NHTS) vs. activity-based (ATUS)

  • NHTS data collected for 2001, 2003-4 for ATUS

  • NHTS allowed proxies; ATUS did not

  • NHTS used a travel diary; ATUS used recall

  • Assigned reporting day constant in NHTS; could rotate to other weeks in ATUS

  • 6-day collection period for the NHTS; 1 day for ATUS

  • NHTS collected data for ALL household members including children; One person 15+ for ATUS

  • More transportation-related details collected for NHTS; more contextual activities in ATUS

  • RDD sample used for the NHTS, area probability for ATUS

TRB 2007 Annual Meeting – Travel Survey Methods Committee Meeting


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ATUS is More Limited than NHTS American Time Use Survey

  • No information on household vehicles (vehicle type, age, mileage)

  • Larger sampling errors due to smaller sample size

  • Lower estimates of time spent in travel, but this may not necessarily be a limitation

    • Bounding activites may lead to better estimates

    • Sample day substitution may lead to bias toward days with less travel

TRB 2007 Annual Meeting – Travel Survey Methods Committee Meeting


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A Few Details on the Analysis American Time Use Survey

  • Used data from FARS for the numerator for illustration purposes

  • Appropriate weights were used in estimates (used national NHTS sample and weights)

  • Complex survey design was taken into consideration when estimating sampling error

TRB 2007 Annual Meeting – Travel Survey Methods Committee Meeting


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Minutes of Vehicle Travel per Day American Time Use Survey

TRB 2007 Annual Meeting – Travel Survey Methods Committee Meeting


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Minutes of Vehicle Travel per Day by Age American Time Use Survey

TRB 2007 Annual Meeting – Travel Survey Methods Committee Meeting


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Minutes of Walking per Day by Age American Time Use Survey

TRB 2007 Annual Meeting – Travel Survey Methods Committee Meeting


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Minutes Bicycling per Day American Time Use Survey

TRB 2007 Annual Meeting – Travel Survey Methods Committee Meeting


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Fatalities per Million Minutes of Vehicle Travel American Time Use Survey

TRB 2007 Annual Meeting – Travel Survey Methods Committee Meeting


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Fatalities per Million Minutes of Vehicle Travel by Sex and Age

TRB 2007 Annual Meeting – Travel Survey Methods Committee Meeting


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Pedalcyclist Fatalities per Million Minutes Bicycling per Day

TRB 2007 Annual Meeting – Travel Survey Methods Committee Meeting


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Possible Reasons for Differences Day

  • Use of travel diary in NHTS; specific probe for walk and bike trips

  • Reconciliation of trips among household members

  • Exclusion of exercise and dog walking trips from ATUS

  • Substitution bias in ATUS– people might be more likely to respond when they are less busy

  • Lack of other activities to bound trip times

TRB 2007 Annual Meeting – Travel Survey Methods Committee Meeting


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Summary Day

  • These were just a few results to illustrate the utility of ATUS in this area

  • Also points out the higher utility of the NHTS for this purpose because of richer data and a larger sample – the ATUS is not a very good replacement

TRB 2007 Annual Meeting – Travel Survey Methods Committee Meeting


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