Civil Engineering Reference
In-Depth Information
The transportation model should indeed help citizens and public offi-
cials decide what to do, but its purpose is not to automate the decision—to
tell public officials what is the scientifically correct thing to do. Transporta-
tion models are not reliable or complete enough to provide such a solution.
Rather, the model lets decision makers compare alternatives (say different
alignments for roads) and rank alternative projects (which most reduces
congestion, which costs least, which generates the least pollution). In a
democratic society, the ultimate decision is political, and ideally should
represent the judgment of properly elected representatives operating under
the rule of law. Models are meant to inform their decisions and those of
the persons they appoint.
Among the methods available for modeling traffic in an urban area, the
most commonly used is the
four-step model.
It actually has two parts. The first
part requires a description of the physical capacities of the area's road and
transit network; estimates the numbers and locations of people who will use
the system; and provides scenarios for the future of. The second part then
estimates the patterns and volumes of traffic through which people travel from
origins to destinations within the area. This second part of the modeling is
done in four steps—for which this entire two-part model is named. We will
now consider what it takes to construct these two parts of the model.
PART 1: CAPACITY AND USERS
Modeling the Present
The first part of our model should describe the present capacity the physi-
cal system (roads, intersections, subways, buses) has to carry vehicles and
their riders in region; it should also tell us the numbers and locations of
the system's users.
We should begin to describe the system's capacity with an inventory of
the present road network. This may be as simple as a map that depicts major
and minor roads and bridges and their interconnections. At a minimum, it
must also include numbers of lanes in each direction and speed limits, but if
we can afford further research, it should also have turn curvatures (that may
slow traffic below its limit), areas with common slowdowns from accidents,
efficiency of traffic flow at intersections, and other data. The combined
information tells us the roads' capacity to carry traffic. Similarly, we should
inventory the numbers of buses and subway cars, and their routes, average
speeds and headways (time lapses between runs), and locations of stations,
so we can estimate their capacity to carry passengers.
Now we want to know how many drivers and passengers use the
system. Obviously, we need to find the size of the population and how it
