| IMPACT ANALYSIS AREAS |
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| FREIGHT
DIVERSION AND MODE SHARE |
| Summary: Changes in
the Nation's TS&W limits, which determine the maximum payload that
vehicles may carry, will influence motor carrier productivity. For
high-density--weigh-out--freight such as farm products and natural
resources, a vehicle's maximum payload is controlled by truck weight
limits. For low-density--cube-out--freight, such as computer equipment
and snack foods, vehicle size limits constrain payload as opposed to weight limits.
In general, increases in TS&W limits would increase the tonnage
and/or volume of freight that may be carried per vehicle per trip.
Consequently, fewer trips would be required to carry the same amount
of freight, decreasing vehicle-miles-of-travel (VMT) and reducing
trucking costs. Alternatively, more restrictive TS&W limits would
increase trips, VMT, and trucking costs.
Changes in truck costs and rates may cause, for some shipments,
a change in the selection of transport mode. For example, changes
in truck rates could induce some shippers to switch from rail to
truck services. Further, changes in other shipper logistics costs
impacted by TS&W variables (such as the size and frequency of shipments)
may also influence intermodal (truck/rail) diversion. Examples of
these costs include warehousing, order processing, and loss and damage.
A collection of state-of-the-art diversion models was developed
for the CTS&W Study to predict the impact of TS&W changes on mode
choice and truck configuration selection. For long-haul shipments,
the Intermodal Transportation and Inventory Costs (ITIC) Model estimates
each shipment's transportation and logistics costs for rail, rail-intermodal,
and eight truck configurations for a large database of truck and
rail shipments. The lowest cost mode is selected.
Short-haul truck shipments, defined as those under 200 miles,
are analyzed separately from the ITIC model. Rail-to-truck diversion
is not an issue for this market segment because trucks operating
under 200 miles typically do not compete directly with rail traffic.
Short haul diversion is estimated separately for heavy single unit
and combination trucks.
For straight trucks, a procedure was developed to estimate: (1)
the change in truck weight distributions within a truck configuration
class, and (2) the change from one vehicle configuration to another
that would likely result from changes in TS&W limits. It assumes
that carriers will consider purchase price, operating costs and
likely productivity improvements and will adopt the most economical
equipment options into their fleets. For combination trucks, diversion
estimates are extrapolated from the long-haul diversion model.
The determination of inter- and intra-modal diversion is extremely
important to the overall CTS&W Study as most of the impact analysis
methodologies require input regarding VMT by mode and truck configuration type. |
SHIPPER COSTS AND RAIL
INDUSTRY COMPETITIVENESS |
| Summary: Beyond the
issue of motor carrier productivity is that of shipper costs. The
motor carrier industry is considered sufficiently competitive that
cost savings are assumed to be passed on to shippers as lower rates.
This is generally true of the rail industry as well. A shipper that
can shift to more productive truck configurations would realize lower
total transportation and logistic costs. However, rail shippers that
could not economically switch to trucks might face increased costs
as railroads spread fixed costs over a smaller shipper base. Inter-
and intra-modal diversion, therefore, has the potential to change
costs borne by the Nation's shippers.
The ITIC model captures the impact of reduced truck costs for
shippers using motor carrier services and for those rail customers
which experience lower rates resulting from rail industry attempts
to maintain traffic in the face of lower truck rates. However, the
impact of freight diversion from rail to truck on the rates for
the remaining rail customers and the viability of the rail industry
is addressed using an independent analysis.
Specifically, the rail analysis estimates the necessary increase
in rates for traffic remaining on the rail system after diversion.
These increases would result from the fact that less traffic would
be available to cover fixed costs. The contribution to capital lost
from diverted traffic would be re-couped by increasing rates for
the remaining traffic, potentially impacting future demand for rail
service and therefore the financial status of the rail industry. |
| SAFETY
AND TRAFFIC OPERATIONS |
| Summary: Considerable
debate has focused on the safety of larger and heavier trucks, and
whether allowing TS&W limits to increase would affect safety. Safety
is a primary issue in the CTS&W Study because of the great public
concern about the implications of mixing large trucks with passenger
cars on our highways and because of the Department's enhanced priority
on safety as the pre-eminent Departmental goal.
As discussed earlier in the section on freight diversion, providing
the opportunity to deploy vehicles of differing sizes, weights,
and/or configurations will alter travel patterns, changing the distribution
of VMT by configuration and roadway type. As a result, the overall
motor carrier accident experience may change, both in terms of accident
rates and aggregate numbers. Quantifying the new safety profile
on an aggregated basis, however, is extraordinarily difficult because
historical accident rates can not be reliably applied to the new
travel patterns as they would reflect what would have occurred under
existing operating conditions and not what could occur under new conditions.
New truck travel patterns would likely result if the scenario
vehicles were allowed to operate in significantly different environments
than in the past. More specifically, certain configurations could
operate in different regions of the country, on different functional
classes, under different weather conditions and at different times
of the day relative to the current fleet of vehicles.
Another factor complicating the estimation of accident rates given
changes to TS&W policies is the fact that the population of commercial
trucks represents a small subgroup of all vehicles, and consequently,
there is a shortage of data directly correlating TS&W factors to
type, frequency, and cause of roadway crashes.
Further, TS&W effects must be isolated from other safety variables
before precise numbers of accidents may be determined. The physical
characteristics of vehicles play a role in motor carrier safety
experiences along with the important and interrelated factors of
driver performance, roadway design and the traffic environment.
Although accident rates may not be reliably predicted for each
scenario, valuable information about relative vehicle stability
and control properties is available. Work commissioned for the CTS&W
Study focused on comparing the dynamic properties of scenario vehicles
with the vehicles they would be replacing.
Vehicle performance tests and engineering analyses indicate significant
differences in the stability and control properties of different
sizes, weights, and configurations of trucks. Some larger and heavier
trucks are more prone to experiencing a rollover event than are
other trucks; some are less capable of successfully avoiding an
unforeseen obstacle when traveling at highway speeds; some negotiate
tight turns and exit ramps better than others; some can be more
reliably stopped in shorter distances than can others; and some
climb hills and maneuver in traffic better than others.
Differing vehicle stability and control properties combined with
new truck travel patterns will affect accident rates and numbers.
For example, all vehicles (including trucks) traveling on lower
standard roads experience significantly increased crash risk compared
to those traveling on Interstate and other higher quality roadways.
The majority of fatal crashes involving trucks occur on highways
with lower standards. Also, higher traffic densities in populous
areas exacerbate this problem.
In addition to safety impacts, the introduction of new truck configurations
could have significant effects on the operational characteristics
and quality of service on the highway network. The Study examines
passenger car equivalents (PCEs) for a variety of truck configurations
and provides estimates of the differences in overall delay that
may occur with operation of the new truck configurations. The associated
cost of this delay is also included in the Study. |
| PAVEMENT
PRESERVATION |
| Summary: Pavement wear
is of interest because rough pavement affects the cost of travel.
These costs include vehicle operating costs, delay, and crash or accident
costs. The life of a pavement is determined by a number of factors:
vehicle loading (axle loads, tire pressure and GVW), traffic volume
and mix, environment, subgrade condition, initial pavement design,
initial construction practices, maintenance and pavement age.
According to engineering principles, pavement deterioration increases
with axle weight and with the number of axle loadings which a pavement
experiences. The Study relies on FHWA's NAPCOM model to simulate
pavement deterioration patterns given alternative vehicle weights,
and axle configurations (with axle-group weight limits generally
not changed) and to predict the requirement for road maintenance
and construction expenditures. The NAPCOM model was developed to
estimate cost responsibilities for the Department's 1997 Highway
Cost Allocation (HCA) Study. |
| BRIDGE
PRESERVATION |
| Summary: While the
relationship between pavement deterioration and axle or axle group
weight is well documented, the role of trucks with respect to bridge
wear is not as well understood. Bridge engineers base new bridge designs
on expected truck loadings with a safety margin to ensure against
failure. These margins are significant and reflect uncertainty about
bridge materials, construction practices and actual loads.
There is much controversy regarding the margin of safety appropriate
for loads crossing a bridge. Most bridges in the United States were
designed based on one of two standard loadings. "HS-20" designs
are typical for Interstates and other highways where heavy truck
traffic is expected. "H-15" loadings were used to design older bridges
which are still in use and generally appear on the lower order functional
class facilities. It should be noted that when bridges on lower
order systems are replaced, they typically are replaced by HS-20
designs. Changes in TS&W limits may impact these safety margins,
possibly increasing the number of bridges that must be replaced or posted.
The Federal bridge formula generally establishes truck axle spacings
and loadings to limit bridge stresses to no more than 30 percent
above the design loading of an H-15 bridge and no more than 5 percent
above the design loading of an HS-20 bridge. As noted, design loadings
incorporate significant margins of safety such that even the 30
percent overstress allowed on H-15 bridges does not put that bridge
in danger of sudden failure.
The bridge formula reflects the fact that loads concentrated over
a short distance are generally more damaging to bridges than loads
spread over a longer distance. It provides for additional gross
weight as the wheel base lengthens and the number of axles increases.
It should be noted that the number of axles influences pavement
wear more than bridge stress.
State transportation agencies rate bridges using an "inventory
rating" or an "operating rating" approach to determine when a bridge
should be posted to prevent its use by certain vehicles. The inventory
rating is more conservative than the operating rating, requiring
a greater margin of safety (55 percent of yield stress as opposed
to 75 percent of yield stress for the operating rating). Past TS&W
studies used either the inventory, operating or some compromise
assumption between the two, to indicate the requirement for bridge
replacement or posting. Not surprisingly, studies using the inventory
rating found more bridges needing replacement and therefore, higher
bridge replacement costs than studies using the other assumptions.
The current Study uses the bridge stress criteria as established
by Bridge Formula B to indicate bridge replacement requirements;
Bridge Formula B is not used. This means that for H-15 bridges,
71.5 percent of yield indicates the requirement for bridge replacement;
for HS-20 bridges, 57.8 percent of yield indicates the need for replacement.
The Study relies on a state-of-the-art bridge stress engineering
model to predict the impact of changes in truck traffic on bridge
conditions. The model generates bridge reconstruction and maintenance
capital costs as well as user delay costs resulting from bridge
construction activities. |
| ROADWAY
GEOMETRY |
| Summary:In some cases,
the scenario vehicles will perform differently than vehicles in the
current fleet. For example, long double-trailer combinations may have
difficulty negotiating interchange ramps. In addition, some require
staging areas where they can be assembled or broken down, allowing
pickup and delivery with shorter combinations. Such performance characteristics
may necessitate modifications to existing roadway geometric design features.
Work commissioned for this Study examined the relationship between
the operating characteristics of the replacement configurations
and the geometric elements of the current system. Geometric improvements
required to accommodate the new vehicle operational characteristics
were determined. The cost of upgrading roadway geometry as well
as the cost of providing staging areas are estimated.
Data from nine States were examined and facilities with geometric
designs that were problematic from the standpoint of current or
scenario vehicles were identified. For analysis purposes, this sample
was then expanded to cover all the States. The underlying assumption
was that all interchanges would be reconstructed to meet the requirements
of the "worst" vehicles. Of particular interest is the issue of
low-speed offtracking. This phenomenon refers to slow turns where
the rear wheels of a turning vehicle do not follow the same path
as its front wheels. |
ENVIRONMENT QUALITY AND
ENERGY CONSUMPTION |
| Summary: Environmental
impacts being evaluated for the CTS&W Study include air and noise
pollution. Procedures developed for the HCA Study are being applied
for the CTS&W Study. In general, environmental quality and energy
consumption impact assessments are a function of VMT.
Motor vehicles produce emissions that damage the quality of the
environment and adversely affect the health of human and animal
populations. The cost of changes in air pollution levels resulting
from alternative TS&W policy scenarios are not currently available.
The Department is working with Environmental Protection Agency to
develop estimates that adequately reflect the latest understanding
of the costs of motor vehicle emissions. If this work is complete
by the July Conference, the approach will be presented.
Noise emissions from motor vehicle traffic are a major source
of annoyance, particularly in residential areas. Vehicle weight
is a key variable affecting noise emissions. Noise costs were estimated
using information on the reduction in residential property values
caused by noise emissions. Estimates of noise emissions and noise
levels at specified distances from the roadway were developed using FHWA noise models.
Also of interest are greenhouse gas emissions. Most scientists
believe that increasing concentrations of greenhouse gases in the
atmosphere will cause climate changes. Because of the tremendous
uncertainty in climate change costs, no estimates of cost related
to highway transportation are developed for this Study. However,
changes in greenhouse gas emissions associated with the TS&W scenarios
would be directly related to changes in VMT.
The change in fuel consumption given alternative vehicle configurations
is also of interest. This was estimated, for each scenario, based
on fuel economy by vehicle weight using engine performance models.
Fuel consumption is also a function of VMT. |
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