11/1/1999
Why are the Roads so Congested? Methodology
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Methodology
The data for this analysis comes from
the Texas Transportation Institute’s annual report, Urban Roadway Congestion.
To read that report, visit TTI’s website at http://mobility.tamu.edu. We are very
grateful to TTI, particularly Tim Lomax and David Schrank, for giving us access to their
data and permitting us to perform our own, independent analysis. Our analysis covers the
entire 16 years of data collected by TTI, and used TTI’s Travel Rate Index for
ranking comparisons. See TTI’s study for an explanation of their data source and
rankings.
Perceived Population Growth
The perceived population growth was calculated
by multiplying each metro area’s population in 1982 by the percentage increase in
vehicle miles traveled in each of those metro areas. For example, Los Angeles, California
had a population of 9.9 million people in 1982. Multiplying this by the growth in vehicle
miles traveled (56%) gives us the perceived growth in population of 5.5 million people.
Comparison of Congestion Indices
In order to compare the congestion indices of
metro areas which built many roads between 1982 and 1997 and those that didn’t, we
divided TTI’s 68 metro areas into three groups. The group which built many roads
during the period increased their road capacity by an average of 28 percent per person.
The group which build the fewest roads during the period actually experienced a decline in
road capacity per person of 11 percent. We then averaged the Travel Rate Index for each of
the groups, for all years from 1982 to 1997, and plotted the metro areas with high
road-building rates against metro areas with low road-building rates.
Metro Areas which Built Many Roads
(averaged 28% increase in lane miles per capita) |
Metro Areas which Built Few Roads
(averaged 11% decrease in lane miles per capita) |
Albuquerque NM
Austin TX
Charlotte NC
Chicago IL-Northwestern IN
Dallas TX
Detroit NM
Fort Worth TX
Houston TX
Jacksonville FL
Laredo TX
Louisville KY-IN
Memphis TN-AR-MS
Milwaukee WI
Nashville TN
New Orleans LA
New York NY-Northeastern NJ
Pittsburgh PA
Portland-Vancouver OR-WA
Providence-Pawtucket RI-MA
Rochester NY
St. Louis MO-IL
Tampa FL
Tucson AZ |
Bakersfield CA
Baltimore MD
Beaumont TX
Boston MA
Boulder CO
Brownsville TX
Colorado Springs CO
Columbus OH
Denver CO
Eugene-Springfield OR
Fresno CA
Ft. Lauderdale-Hollywood-Pompano Beach FL
Hartford-Middletown CT
Las Vegas NV
Norfolk VA
Oklahoma City OK
Phoenix AZ
Sacramento CA
San Antonio TX
San Bernardino-Riverside CA
San Diego CA
San Jose CA
Tacoma WA |
Increase in Gas Tax
Finally, to calculate the additional average
gas tax required to keep congestion rates steady, we used TTI’s estimates of highway
capacity deficiencies, and multiplied those numbers by $1.45 million per lane mile (a
conservative estimate of the cost of road construction; from USDOT). Dividing this figure
by the number of gallons of gasoline consumed per year (in the affected metro areas) gives
the average gas tax increase required, over and above what would be needed to continue
normal building practices.
It should also be noted that wherever we use
the terms ‘miles of highway’ or ‘miles of roadway,’ this has a
specific definition and refers to lane miles of Interstates, freeways, expressways and
principal arterials.
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