INDUCED OR GENERATED TRAFFIC
Since the advent of modern highway travel, engineers, planners and
policy makers have been attempting to alleviate traffic congestion. Most
attempts to eliminate or reduce traffic congestion on a particular
road segment were by means of the construction of new or larger roads.
Although adding road capacity is intended to provide some congestion
relief, an unintended consequence is "induced" traffic.
The definition of induced traffic, or "generated" traffic,
is "the additional travel resulting from a transportation improvement,
which would not have otherwise occurred."2
Causes of Induced Traffic
Due to improved highway conditions, people change the way in which
they make travel decisions. They:3
- Change their route;
- Change their mode of travel (e.g., shift from mass transit to private
vehicles);
- Change their place of residence or workplace; and
- Change the number of trips they tak.
Studies done for the Transportation Research Board have concluded
that increases in capacity induce demand which will then manifest
itself over a long period of time as travelers alter their travel
patterns due to the improved capacity.
Although many of the studies look only at the addition of capacity
as a cause of induced travel, the study entitled If We Build It, Will
They Really Keep Coming? A Critical Analysis of the Induced Demand
Hypothesis, indicates that the unprecedented increase in urban travel
is not just a response to an improved LOS but is also the result of
redistribution of some new traffic which would have been created anyway
independent of the LOS change.
This new traffic has occurred as a result of changes in demographic
and social trends over the last twenty years: growth in population
and labor force, effect of women entering the labor force, decreasing
household size, saturation of vehicle availability, and dispersion
of residences and workplaces. All five of these trends are seen to
be stabilizing and since they were considered one-time effects, they
have effectively played themselves out. As a result, the authors state
that these unprecedented levels of growth in urban travel should not
be expected to continue, regardless of the amount of additional capacity
that is added.
Population and the Labor Force
- Impact on VMT
- Dramatic growth in both areas between 1960 and 1980; and
- Tendency to drive alone also magnified the increase.
- Trend
- Labor force growth has slowed from its high in 1970 and is expected
to fall substantially during the next decade; and
- Effect on VMT - should diminish due to decrease in the number of workers.
Female Labor Force Participation
- Impact on VMT
- Contributed by adding more commuting trips; and
- Helped to sustain large gains in real income which allowed for more
travel.
- Trends
- Female participation rate is now rapidly approaching that of men -
which has not increased for almost 50 years; and
- Although it represented the #1 structural change in the economy, the
increase in female participation will no longer be a major drive of
VMT growth in the future.
Household Size
- Impact on VMT
- As VMT increased, commuting by driving alone also increased; and
- Much of the increase in SOV came at the expense of carpooling -
people preferred to carpool only with other household members.
- Trends
- Average household size appears to have stabilized at 2.6 persons;
and
- Total workers per household increased only slightly in 1990 after
remaining constant since 1960.
Vehicle Availability
- Impact on VMT
- Vehicles per capita and vehicles per household both increased over
the last twenty years;
- Allowed trips that would have been chained to be made separately;
and
- Individuals with cars no longer had the need to carpool or use transit.
- Trends
- Market indications show saturation;
- Number of vehicles grew more slowly than the number of workers between
1960-1990;
- Average number of vehicles per worker declined slightly;
- Majority of one-adult households have one or more vehicles available;
- Majority of two-adult households have two or three vehicles available;
and
- The total number of vehicles in the U.S. exceeds the number of licensed
drivers - another indication of market saturation.
Residential and Workplace Dispersion
- Impact on VMT
- Population has increasingly tended to settle in metropolitan areas,
but the growth has been primarily in the suburbs of the metropolitan
area rather than in the central cities;
- Significant migration during the 1980's within the metropolitan
areas has caused the central cities to lose over 2 million residents
per year to the suburbs;
- Job growth has been primarily in the suburbs, with more jobs located
there than anywhere else;
- Suburb-to-suburb commute has been responsible for more than 58% of
the growth in commuter travel;
- Increases in VMT shows that as a function of the increase in population
and employment, the dispersion of residences and workplaces has increased
income and differences in development patterns; and
- Reorientations of communities to a less concentrated pattern has had
a strong positive effect on private vehicle travel.
- Trends
- Unclear as to its future effect; and
- Primary motifications for moving are complex and reflect lifestyle
choices and qualitative differences between suburban communities and
central cities.
Costs of Induced Travel
The research report entitled Accounting for Induced Travel in Evaluation
of Metropolitan Highway Expansion stated that the critical issue is:
"Not whether highway capacity results in induced travel, but
whether the benefits to induced travelers and pre-existing travelers
and other economic benefits exceed the total social costs occasioned
by induced highway travel and public, social and environmental costs
incurred in providing the added capacity."
The results of the study indicated that induced travel, as a function
of latent demand, will increase when capacity is added to an uncongested
freeway. In addition, the magnitude of the induced travel, as well
as the social costs associated with it, increase as levels of initial
congestion and user benefits increase.
The impact of additional capacity, evaluated in a time range, are:
- Short-term - Route changes and time of travel are the primary
impacts;
- Mid-term - Mode shifts and destination shifts become more important;
and
- Long-term - The origin of trips may also shift or new trip origins
may be generated due to new development. In addition, new destinations
further away may generate new trips as homes and businesses locate
in the improved corridor in response to improved accessibility.
The long-term diversion of homes and businesses from other parts of
the region will increase the rate of new trips to a corridor. The
increase in new trips will increase the rate of development in the
corridor. However, the rate of increase is dependent upon other location
characteristics of the improved corridor, such as the availability
of vacant land.
The final result may be that as travel increases due to induced demand,
the initial price reduction that was given to the travelers in the
corridor will slowly erode as traffic increases.
Conditions Needed for Induced Traffic
In 1994 the United Kingdom Department of Transportation Standing Advisory
Committee on Trunk Road Assessment (SACTRA) study identified the types
and categories of highway improvements for which induced traffic is
likely to be significant. The SACTRA report states induced traffic
will occur when the following conditions are present:4
- The network is operating or is expected to operate close to capacity;
- Traveller responsiveness to changes in travel time costs is high,
as it would occur where trips are suppressed by congestion, and then
released when the network is improved.
- The implementation of the improvement causes large changes in travel
costs.
I-71
Applying the SACTRA conditions to the I-71 network suggests the following:
- The I-71 network at the points where the highway widening is proposed
in Medina County is not operating at capacity. However, I-71 in Cuyahoga
County from US 42 south to the Medina County line is operating over
capacity, indicating a congested network segment as determined by
LOS D/E.
- Upon completion of the proposed road widening in the congested segment
in Cuyahoga County, the congestion would be reduced but still present.
The reduction in congestion will reduce travel time and costs somewhat.
The concept of induced traffic in the I-71 corridor is demonstrated
by actual experience. Before the construction of I-71, the study area
was served by SR 42 as the major transportation segment between southeast
Cuyahoga County and Columbus, Ohio. The travel time and costs associated
with the use of SR 42 were very high. When I-71 was completed, the
reduced travel time to Medina County and the induced traffic which
resulted, are the reason why the population of the county has changed
by almost 100%. Any further increases to I-71 capacity will only perpetuate
this induced traffic.
2Littman, Todd. Determining Generated Traffic External Costs, Victoria Transportation
Policy Institute, (Victoria, BC), 1995, p.3.
3Expanding Metropolitan Highways, Special Report 245, National Academy Press
(Washington, D.C.), 1995, p. 144-145.
4SACTRA, Trunk Roads and the Generation of Traffic, UKDOT, HMSO (Londson), 1994,
p. 205.
