Civil Engineering Reference
In-Depth Information
increases in highway capacity are directly responsible for increases in traf
c. In fact,
the relationship between increases in highway capacity and traf
c is very complex,
involving various travel behavior responses, residential and business location
decisions, and changes in regional population and economic growth.
”
Travel demand on new or expanded roadways consists of several major com-
ponents. These include (1) the traf
c diverted from other roadways, other times, or
other modes, and (2) the new, or
c demand resulting from the
improved service level made possible by the added capacity.
The components of induced traf
“
induced
”
traf
c include longer and/or more frequent trips, as
well as the additional traf
c generated from new land developments
“
induced
”
by
the improved access provided by the new or expanded road.
17.4.2 What Is the Source of Induced Traffic?
The conceptual analysis below illustrates the effect of capacity expansion on trip
costs and traf
c volumes for both existing and future travel demand. The results are
shown graphically in Fig.
17.15
through Fig.
17.17
.
Two time frames are discussed below to clearly differentiate (1) the proportion of
traffic growth in metropolitan areas that is induced by highway capacity expansion
and (2) the proportion that is the product of population and employment growth:
one for existing demand, the other for future years demand.
17.4.2.1 Existing Demand
Increasing roadway capacity reduces the generalized cost of travel (including both
travel time and out-of-pocket costs). Lowering the cost of travel increases the
volume of traf
c on the roadway (Fig.
17.15
)
—
an outcome consistent with mi-
croeconomic theory of consumer demand.
Travelers who are attracted to the improved route include those who, to avoid the
congested route, had (1) diverted to other less congested routes; (2) switched to
different modes or times; (3) traveled to other destinations; or (4) decided not to
take a particular trip. In addition, because of higher speed, trip distance tends to be
longer on the improved facility.
The volume of traf
c diverted and induced by the improved route [(V
1,0
)
-
(V
0,0
)]
is shown in (Fig.
17.15
). This additional traf
c experiences a lower congestion on
the improved route (P
1,0
) and it creates lower congestion on the routes which lost
trips diverted to the improved route.
