TRAFFIC ACROSS THE BRIDGE - Bridges - page 110

year, when an investor offers to open up headquarters on the waterfront
for a giant software firm, the legislators can quickly drop the old plan and
substitute a new one. So it is that transportation modelers should make
no claim of predictive precision—the transportation forecast should not be
confused with the engineering predictions that tell us how a satellite will
enter orbit. Rather, the forecasts help us more clearly assess the potential
values of a project, such as a bridge, under different future scenarios. To
assess the value more completely, we have to go on to the second part of
the modeling process.
PART 2: TRAVEL PATTERNS—THE FOUR-STEP PROCESS
Having assembled a description of road and transit capacities, and of popu-
lation and land use, and also scenarios for the future, we can go on to the
rest of our effort: to figure out the patterns of traffic by which people get
from their origins to destinations in our region. We will use the “four-step”
process for which this entire modeling method is named.
To set up a four-step model, we must divide our metropolitan area into
transportation analysis zones, known as TAZs. Here we just call them “zones,”
but specifically mean TAZs, not any other kind of zone. Depending on the
complexity of the model, a zone may be as small as a few city blocks or
as large as a rural township. Every part of the metro's land area should be
placed in a zone, except maybe a wilderness area.
We set up these zones because we are not going to try to estimate
individuals' travel patterns from their specific home addresses to their various
specific destinations, whether school or theater or workplace. As of the time
when we are writing (when methods are changing because of new geospatial
technologies), we do not have such information and do not wish to snoop
into peoples' lives. Instead, for the particular metro area we are working
on, we estimate trips not for actual individuals but for average behavior by
occupants of zones.
Before we estimate travel between our zones, we should remember that
many trips come into or head out of the region: they have origins outside
our metro area, or have destinations outside it, or are just passing through
but using local infrastructure. To account for these, modelers also establish
external zones . Typically, over 90 percent of all trips taken in a metro have
origins and destinations within the metro—trips from and to internal zones .
In recent years, the National Capital Region Transportation Planning
Board, which serves Washington, DC, has used a model that divides its
metro area into 2200 zones and recognizes 28,000 road segments, plus many
transit lines. The model we soon present is rather smaller than Washington's.
It is the model for an invented place called Square City, which has only