Electronic Supplementary Material
Long-Term Trends in Domestic US Passenger Travel:
The Past 110 Years and the Next 90
Andreas W. Schäfer
Precourt Energy Efficiency Center, Stanford University
Yang & Yamazaki Environment & Energy Building
473 Via Ortega, Room 387, Stanford, CA 94305-4205
aschafer@stanford.edu, 650 723-0884
and
UCL Energy Institute, University College London
a.schafer@ucl.ac.uk, 020 3108-5925
This study builds on long-term historical time series data describing the socio-economy
(population, GDP, amount of time worked), transportation-related figures in terms of
passenger-km traveled (PKT), and data describing vehicle costs and speed for the major
modes of passenger travel, i.e., automobiles, urban transit, commuter and intercity railways,
intercity buses, and commercial aircraft. Because of the very limited number of data points
describing school bus travel, it was excluded from the analysis.
Socio-Economic Data
The Bureau of Economic Analysis reports continuous GDP time series data from 1929 to
2010 (BEA 2011). Earlier data can be derived from the Historical Statistics of the US
(Department of Commerce, 1975). This source also provides population data until 1970;
more recent figures were derived from the Statistical Abstract of the US (Department of
Commerce 2012). The utility equations of the three choice models include the GDP-based
wage rate. To arrive at that figure, annual GDP levels were divided by the total work hours
per year. Time series data for work hours were derived from Maddison (1991), the Bureau of
Labor Statistics (2011), and the Historical Statistics of the US (Department of Commerce
1975).
Data Describing Travel Activity
This study accounts for the major motorized modes of transport, which are aggregated into
low-speed modes of public transport (urban mass transit, commuter and intercity rail and
intercity buses), LDVs (automobiles and those light trucks that are used for personal travel)
and aircraft.
Urban Mass Transit.The key data source describing travel by urban buses, light and heavy
rail, trolley buses, and paratransit systems is the historical section of the American Public
Transportation Association’s Fact Book (APTA 2012). For most modes, revenue passenger-
km traveled (RPK) are only reported from 1977 on. Hence, earlier figures were estimated by
multiplying the reported number of passengers with the constant average 1977 trip distance.
Over the 1977 to 2008 period, this distance has been very stable for especially urban buses,
light and heavy rail, and trolley buses.
Commuter and Intercity Rail.RPKfrom commuter and intercity rail were derived from
Historical Statistics of the US (Department of Commerce 1975) and ENO’s Transportation in
America data book (ENO 1983-2002). More recent commuter rail data originate from the
American Public Transportation Association’s Fact Book (APTA 2012), whereas those
describing intercity rail from the Transportation Energy Data Book (Davis et al. 2014).
Intercity Buses. RPKfrom intercity buses between 1925 and 1929 were derived from Barger
(1951); as intercity buses accounted for only around 1% of the PKT of automobiles in 1925,
their RPK before 1925 was assumed to negligible due to the general lack of bus-specific
data. RPK from 1929 to 1984 were derived from the American Bus Association (1966-1984).
Subsequent 1985 to 2001 RPK numbers were taken from ENO’s Transportation in America
data book (ENO 1983-2002). Underlying both sources are the monthly Transport Economics
reports by the Interstate Commerce Commission’s Bureau of Economics until 1979, the time
series of which starts in 1939 (ICC 1955-1979).RPK between 2002 and 2010 were estimated
by scaling the 2001 ENO-based number with the growth in vehicle-km traveled (VKT) of
buses operating on different categories of rural roads as reported in the Highway Statistics
(Department of Transportation 2002-2012).
Light-Duty Vehicles.PKT are the product of VKT and the average occupancy rate
(PKT/VKT). VKT was derived from the Highway Statistics (Department of Transportation
1995-2012), which report vehicle-miles by type of road and vehicle since 1936. Earlier
numbers were scaled with automobile gasoline consumption levels (1925-1936) using data
from the Department of the Interior (1937) and the automobile fleet (1900-1924) with data
from Department of Commerce (1975).
Only the passenger transport related LDV travel is considered here. While all
automobiles are assumed to be used for passenger transport, the share of light trucks
dedicated to passenger travel has increased from 35% of all light trucks in 1963 to 82% in
2002 (Census Bureau, 1965, 1970, 1974, 1980, 1985, 1990, 1995, 1999, 2004). Using these
observations, a logistic curve was fitted over time to estimate the share of light trucks used
for passenger travel over the 1950-2010 period. (Before 1950, the number of light trucks
used for passenger travel was negligible).In combination with the annual VKT, those
dedicated to personal travel were estimated. The associated vehicle occupancy rate was
estimated with a simple logistic equation over time, using the 1969, 1977, 1983, 1990, 1995,
2001, and 2009 observations from the US national travel surveys (Santos et al. 2011).
Travel Costs
LDV travel costs are derived from “The Nation’s Passenger Bill” table from ENO’s
Transportation in America databook (ENO 1983-2002). These figures are based on the
Bureau of Economic Analysis’ National Income and Product Account data (BEA 2011), but
are adjusted for business use of vehicles, interest on vehicle financing debt, vehicle
registration fees, etc. When deriving the costs per PKT, these costs need to be adjusted for
the use of light trucks for business purposes, which is excluded in this study. Because
(reliable) ENO figures only range from 1960 to 2001, previous (1929-1959) and more recent
(2002-2010) figures were derived from the BEA data directly, using the ENO adjustment
factors.
Cost figures for transit are derived from the historical tables of the American Public
Transportation Association’s Fact Book (APTA, 2012). Recent commuter rail cost figures
(from 1990 onwards) are also derived from the APTA source, while earlier numbers are
taken from the Historical Statistics of the United States (Department of Commerce, 1975)
and ENO’s Transportation in America data book (ENO 1983-2002). The two latter sources
also furnished the cost figures for intercity railways. Costs for intercity buses were taken from
the American Bus Association’s annual reports (1966-1984)and ENO’s Transportation in
America data book (ENO 1983-2002). More recent numbers are based on the assumption
that the 2001 level of revenue per PKT has remained unchanged. Finally, travel costs
associated with commercial air traffic were derived from the Air Transport Association of
America’s annual reports (ATA 1937-2010).
Travel Speeds
Travel speeds by mode were estimated on a door-to-door basis. Travel speeds strongly
depend on the travel distance—the longer the travel distance, the greater the probability of
choosing roads with faster moving traffic or public transport modes operating at higher mean
travel speeds. This direct relationship between travel speed and distance also applies to
aviation, where longer travel distances enable a longer (high-speed) cruise stage.
For surface transport modes, the speed-distance relationship was estimated using
the 2009 US National Household Travel Survey data (FHA 2011). The estimated power
function relationships between door-to-door speed and trip distance were then applied to
those average trip distances that were derived from the aggregate transport statistics
described above. The door-to-door speed of aircraft was estimated by dividing the average
stage length by the gate-to-gate travel time (augmented by delays) plus an extra 2.5 hours
for airport and airplane access, airplane and airport egress, and transfers (ATA 1937-2010).
As for all surface transport modes, the average trip distance for LDVs was derived by
dividing the reported VKT by the number of vehicle trips. Because the latter are not reported
by aggregate transport statistics, they had to be estimated as a function of daily VKT using
the reported values from the 1969-2009 US household travel surveys (Hu and Reuscher
2004; Department of Transportation 2013). The cross-country stability of these relationships
was validated using travel survey data from several European countries starting in the 1970s
(Schäfer 2000). Figure A-1 and A-2 report the speed distance relationships for LDVs and
public transportation modes.
[Figure A-1], [Figure A-2]
80
4.0
y = 0.1689x0.7631
R² = 0.9647
Average Vehicle Speed, km/h
70
3.0
2.5
2.0
1.5
1.0
60
y = 18.671x0.3535
R² = 0.9643
50
40
30
20
10
0.5
0
0.0
0
10
20
30
40
Daily Travel Distance, km
50
60
Figure A-1a
80
y = 3.8584x0.6885
R² = 0.8876
70
60
50
40
30
20
10
0
0
20
40
60
Trip Distance, km
Figure A-2
0
5
10
15
20
80
100
25
Average Trip Distance, km
Figure A-1b
90
Travel Speed, km/h
No Vehicle Trips per Day
3.5
30
35
40
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