EPA's Proposal for MOBILE6      
                     Fleet Characterization

                          March 14, 1997     

Abstract

     The MOBILE model incorporates data characterizing the in-use
vehicle fleet in the United States to model emissions from motor
vehicles. Vehicle population is characterized by the total number
of vehicles in operation within certain vehicle categories, the
age distribution and fuel type (gasoline or diesel) within each
vehicle category, and the annual mileage accumulation rates by
age, fuel type vehicle category.  MOBILE5a contains fleets data
based on 1984 Polk, 1982 Truck Inventory Use Survey, and 1984
National Purchase Diary (NPD) data.  Acurex Environmental, under
contract to EPA, will analyze more recent sources of  fleet
characterization data which may be used in MOBILE6.
 
Summary of the Proposal

    Acurex Environmental is analyzing updated fleet
     characterization data for use in MOBILE6.  They were asked
     to determine:

               Updated national registration distributions by age
                         for each vehicle type and total national vehicle
                         registrations by vehicle type
          
               Total national registrations for HDGVs and HDDVs
                         by gross vehicle weight class
          
               Gasoline/diesel sales fractions for LDVs and LDTs
                         by MY
          
               Average annual mileage accumulation rates by age
                         of reach vehicle type
          
    The above data shall be provided for the following vehicle
     types:

       Vehicle Type                       Gross Vehicle Weight (lbs)
          LDGVs                              All
          LDGTs                              All
          LDGT1s                             0-6,000
          LDGT2s                             6,001-8,500
          HDGVs                              >8,500
          LDDVs                              All
          LDDTs                              0-8,500
          Class 2B HDDVs                     8,501-10,000
          Light (classes 3-5) HDDVS          10,001-19,500
          Medium (classes 6-8A)HDDVs         19,501-60,000
          Heavy (class 8B) HDDVs             >60,000
          Buses                              All  

Supporting Data

    Acurex Environmental has identified the following sources of
     data as most useful for providing the necessary data:

       1993 Truck Inventory and Use Survey (CD-ROM), U.S.
                 Department of Commerce, Economics and Statistics
                 Administration, Bureau of the Census (TIUS)
  
       Current data files procured from R.L. Polk Co.
                 containing vehicle registration information (Polk)
  
       "Nationwide Personal Transportation Survey,"
                 Federal Highway Administration, U.S. Department of
                 Transportation (1990) (NPTS)
  
      These data sources are ~ 5-10 years more current that those
                 sources included in MOBILE5a

Preliminary Observations/ Issues

     Based on NPTS data, mileage trends for LDVs generally follow the
          MOBILE5a curve, although magnitude is ~2,000 miles per
          year greater (approximately 10% increase over MOBILE5a)

     Annual mileage for HD vehicles and trucks shows very high
          variability by age

     Mileage accumulations for HDDVs are higher at younger vehicle
          ages, slightly lower at older ages

     Will have to determine how to handle projections for total number
          of vehicles by vehicle category, and gasoline/diesel
          sales fractions for LDVs and LDTs