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Chapter Listing
Conditions and
Performance Home Page
Introduction
Summary
Economics-Based Approach to Transportation
Investments
Highway
Investment Requirements
Bridge
Investment Requirements
Combined
Highway and Bridge Investment Requirements
Transit
Investment Requirements
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HERS initiates the investment requirement analysis by evaluating the current
state of the highway system using information on pavements, geometry, traffic
volumes, vehicle mix, and other characteristics from the HPMS sample dataset.
Using section-specific traffic growth projections, HERS forecasts future
conditions and performance for four 5year periods. At the end of each period,
the model checks for deficiencies in eight highway section characteristics:
pavement condition, surface type, volume/capacity (V/C) ratio, lane width,
right shoulder width, shoulder type, horizontal alignment (curves), and
vertical alignment (grades).
When HERS determines a section's pavement or capacity is deficient, it will
identify potential improvements to correct some or all of the section's
deficient characteristics. HERS evaluates seven kinds of improvements:
reconstruction with more lanes, reconstruction to wider lanes, pavement
reconstruction, major widening, minor widening, resurfacing with shoulder
improvements, and resurfacing. For each of these seven kinds of improvements,
HERS evaluates four alignment alternatives: improve curves and grades, improve
curves only, improve grades only, or no change. Thus, HERS has 28 distinct
types of improvements to choose from. When analyzing a particular section HERS
actively considers no more than six alternative improvement types at a time;
one or two aggressive improvements that would address all of the section's
deficiencies, and three or four less aggressive improvements that would address
only some of the section's deficiencies.
When evaluating which potential improvement, if any, should be implemented
on a particular highway section, HERS employs incremental benefit/cost
analysis. HERS defines benefits as reductions in direct highway user costs,
agency costs, and societal costs. Highway user benefits are defined as
reductions in travel time costs, crashes, and vehicle operating costs. Agency
benefits include reduced maintenance costs and the residual (salvage) value of
the projects. Societal benefits include reduced vehicle emissions. These
benefits are divided by the costs of implementing the improvement to arrive at
a benefit/cost ratio (BCR) that is used to rank potential projects on different
sections. The HERS model implements improvements with the highest BCR first.
Thus, as each additional project is implemented, the marginal BCR and the
average BCR of all projects implemented declines. However, up until the point
where the marginal BCR falls below 1.0 (i.e., costs exceed benefits), total
benefits will continue to increase as additional projects are implemented.
Investment beyond this point would not be economically justified, since it
would result in a decline in total benefits.
| Q How closely does the HERS
model simulate the actual project selection processes of State and local
highway agencies? |
| A The HERS model is intended to
approximate, rather than replicate, the decision processes used by State and
local governments. HERS does not have access to the full array of information
that local governments would use in making investment decisions. This means
that the models may recommend making some highway and bridge improvements that
simply are not practical due to factors the model doesn't consider. Excluding
such projects would result in reducing the "true" level of investment
that is economically justifiable. Conversely, the highway model assumes that
State and local project selection will be economically "optimal" and
doesn't consider external factors such as whether this will result in an
"equitable" distribution of projects among the States or within each
State. In actual practice, there are other important factors included in the
project selection process aside from economic considerations, so that the
"true" level of investment required to achieve the outcome desired
under the scenarios could be higher than that shown in this report. |
| Q Does HERS identify a
single "correct" level of highway investment? |
| A No. HERS is a tool for
estimating what the consequences may be of various levels of spending on
highway condition and performance. If funding were unlimited, it might make
sense to implement all projects identified by HERS as cost-beneficial. In
reality however, funding is constrained, and highways must compete for funding
with other public sector priorities. The investment requirement scenarios in
this chapter estimate the resources that would be required to attain certain
levels of performance, but are not intended to endorse any specific level of
funding as "correct". |
Travel Demand
Elasticity
The States furnish projected travel for each sample highway section in the
HPMS dataset. The HERS model uses these projections as an initial baseline, but
alters them in response to changes in highway user costs on each section over
time. Travel demand elasticity procedures have been added to HERS to recognize
that as a highway becomes more congested, travel volume on the facility is
constrained, and that when lanes are added to a facility, the volume of travel
may increase.
| Q What assumptions does the
HERS model make about the travel forecasts in the HPMS dataset? |
| A HERS assumes that the
forecasts for each sample highway segment represent the travel that will occur
if the level of service remains constant on that section. This implies that
travel will only occur at this level if pavement and capacity improvements made
on the segment during the next 20 years are sufficient to maintain highway-user
costs at current levels. Note that at current funding levels, HERS assumes that
VMT will grow more slowly than the HPMS baseline forecasts, particularly in
large urbanized areas. |
The basic principal behind demand elasticity is that as the price of a
product increases, consumers will be inclined to consume less of it, and either
consume more of a substitute product or simply do without. Conversely, if the
price of a product decreases, consumers will be inclined to consume more of it,
either in place of some other product or in addition to their current overall
consumption.
The travel demand elasticity procedures in HERS treat the cost of traveling
a facility as its price. As a highway becomes more congested, the cost of
traveling the facility (i.e., travel time costs) increases, which tends to
constrain the volume of traffic growth. Conversely, when lanes are added and
the highway user costs decreases, the volume of travel will tend to increase.
The travel demand elasticity values used in this report are higher than the
values used in the 1997 C&P report. This increase further con-strains
travel growth in congested urbanized areas. This change was made partly to
capture some of the effects of Travel Demand Management (TDM) programs that
were previously simulated by reducing the HPMS baseline forecasts. The
rationale for this change is explained in Appendix G.
| Q What are some examples of
the types of behavior that the travel demand elasticity features in HERS
represent? |
| A If highway congestion worsens
in an area, this increases travel time costs. This might cause highway users to
shift to mass transit, or it might cause some people living in that area to
forgo some personal trips they might ordinarily make. For example, they might
be more likely to combine multiple errands into a single trip, because the time
spent in traffic on every trip discourages them from making trips unless it is
absolutely necessary. In the longer term, people might make additional
adjustments to their life-styles in response to changes in user costs that
would impact their travel demand. For example, if travel time in an area is
reduced substantially for an extended period of time, some people may make
different choices about where to purchase a home. If congestion is reduced,
purchasing a home far out in the suburbs might become more attractive, since
commuters would be able to travel further in a shorter period of time.
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The particular values of elasticity used in this report are within the
ranges of the available literature on this subject, and are intended to reflect
that the majority of the impact on travel demand will occur in the short term,
within 5 years.
For short term elasticity, HERS now uses a value of 1.0. An additional
-0.6 (total, 1.6) is used for long-term elasticity. The short-term
elasticity is used within the 5-year period being analyzed and long-term
elasticity is used in the remainder of the overall analysis period.
For example, if highway-user costs on a given highway facility increased by
10 percent, the model predicts that travel on the facility would decline
by 10 percent below the baseline forecast within 5 years, and by an
additional 6 percent within 20 years. Conversely, a reduction of user
costs would cause a corresponding increase in highway travel on the facility.
As a result of travel demand elasticity, the overall level of highway
investment has an impact on the projected travel growth. For any highway
investment requirement scenario that results in a decline in average highway
user costs, the effective VMT growth rate will tend to be higher than the
baseline rate. For scenarios in which high-way user costs increase, the
effective VMT growth rate will tend to be lower than the baseline rate. This
effect is discussed in more detail in Chapter 9.
| Q How do the travel demand
elasticity features in HERS reflect the effects of Transportation Demand
Management (TDM) programs? |
| A To some extent, the HERS
elasticity features mimic the effect that transportation demand management
programs would be expected to have on the level and location of future travel
growth. The elasticity features suppress highway travel growth in areas where
widening is not feasible, or congestion is increasing. The model assumes that
individual highway users will change their driving patterns and lifestyle
choices in response to these factors, which will slow the rate of highway
travel growth in large urbanized areas. However, these shifts will not occur at
the assumed rate unless these drivers have viable alternatives. Federal,
State and local TDM programs serve to provide these alternatives. The 1990
Clean Air Act Amendments require States and localities to reduce vehicular
emissions by implementing transportation control measures to manage travel
demand and improve traffic flow. These measures include TDM programs that
provide alternatives to single-occupant-vehicle travel such as options for
carpooling, transit, and bicycling. These include:
- Bicycle/pedestrian facilities - provision of paths, special lanes, lockers,
showers, or other facilities.
- Area-wide ridesharing - a program that provides carpool matching and
information services.
- HOV lanes - highway lanes reserved for high-occupancy vehicles, i.e.,
buses, vanpools, and carpools.
- Park & ride facilities - parking lots or facilities located to provide
access to a transit station, HOV lane, bus service, or to encourage carpooling.
- Transit improvements - transit service expansion or improvements.
In addition, the following TDM measures are available for implementation by
employers:
- Compressed workweeks - extension of the typical workday in order to reduce
the number of days worked, thereby reducing the number of work trips.
- Telecommuting - arrangements allowing employees to work at home or at
satellite offices close to home.
- Employer trip reduction - a State or local government regulated program
requiring employers, usually above a certain size, to implement plans that
encourage employees to reduce vehicle travel to work.
The HERS elasticity values are set at a relatively high level. If the TDM
programs listed above are less than fully successful in providing viable
transportation alternatives, VMT growth will probably exceed the levels
predicted by HERS. If TDM programs are more successful than the elasticity
values in HERS imply, then VMT growth could be lower than the level projected
by HERS. Chapter 10 explores the effects that different travel growth
assumptions would have on the investment requirement projections.
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| Q Are the travel demand
elasticity values used in HERS appropriate for use in other types of
applications? |
| A Since HERS analyzes
individual highway segments in isolation, rather than corridors, or the highway
network as a whole, the elasticity values need to account for trips that might
shift to or from a parallel highway route, as well as trips that might shift to
or from other modes of transportation, or that might be induced or suppressed
entirely. For network analysis, it would be more appropriate to use lower
elasticity values. |
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