Regulation of Weights, Lengths, and Widths of Commercial Motor Vehicles Special Report 267 (2002) / Chapter Skim
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2. Past Evaluations of Changes in Truck Size and Weight Regulations
2. Past Evaluations of Changes in Truck Size and Weight Regulations
Pages 38-114
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From page 38... ...
Second, the analysis of changes in truck characteristics has not been integrated with the ongoing process of management and regulation of the highway system. As a consequence of these shortcomings, past studies, even when they have produced reasonable estimates of the consequences of changes in truck dimensions, often have not been successful in the design of improved policies or promotion of reform.
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From page 39... ...
Specifically, the DOT study assumes: · Constant highway user tax rates Changes in size and weight limits would not be accompanied by any change in the user tax structure. · Constant motor vehicle technology New trucks would be built with off-the-shelf components.
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From page 40... ...
carriers or s nippers · Delay at construction: change in highway user delay caused by change in the amount of highway construction · Delay from effect on traffic operations: change in delay caused by change in number and performance of trucks · Air pollution: cost of change in emissions caused by change in traffic volume, vehicle performance, and highway construction · Noise: cost of change in noise emissions · Energy consumption: external costs (if any) of change in petroleum consumption, other than pollution costs · Railroad profitability: change in welfare of railroad stockholders and employees (a distribution effect rather than a cost)
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From page 41... ...
The following two subsections examine these problems. Defining Objectives Truck size and weight regulations are a mechanism for balancing the potential public costs of truck travel against the benefits of lower shipper and carrier costs for freight transportation.
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From page 42... ...
Past studies' estimates of bridge costs illustrate the importance of aiming for objectives. The past DOT and TRB studies have identified regulatory options that appeared attractive considering freight costs, pavement wear, and truck traffic reduction, but were predicted, according to the conventional cost-estimating method, to generate high costs for replacement of deficient bridges to accommodate the new trucks.
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From page 43... ...
Because of their orientation toward evaluating the impacts of changing dimensions instead of seeking means of attaining objectives, most past studies have ignored some of the most promising policy alternatives, in particular, performance standards and pricing. Performance standards are regulations that require vehicles to pass specified performance tests demonstrating that they are safe and compatible with the design of the highway system.
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From page 44... ...
But to the extent that these objectives are applicable to size and weight regulations, they would be appropriate in the United States as well. As a second example, the following are the objectives set for regulatory changes recommended by the committee that authored the TRB Truck Weight Limits study (TRB 199Oa, 228~: · To select from the various changes in truck weight regulations proposed by industry groups and others the most practical means to realize the productivity benefits of increased truck weights while reducing or eliminating possible adverse effects; · To make changes in weight limits that would reduce truck accidents and encourage safety improvements in truck design and operation; · To provide mechanisms to match user fees with added costs for pavements and bridges; · To promote uniformity in the administration of truck weight regulations; · To balance the federal interest in protecting the national investment in the Interstate system and facilitating interstate commerce with the interests of the states in serving the needs of their citizens and industries; · To develop proposals that are realistic and feasible and would have a reasonable chance of being implemented.
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From page 45... ...
should only be undertaken after thorough safety analysis of all the benefits and costs of such an action to all highway users as well as the economy" (James 1994, 12~. The Comprehensive Truck Size and Weight Studly was begun at this time.
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From page 46... ...
The TRB study predicted that the share of twin-trailer combinations in nationwide combination truck travel would nearly triple by 1990 as a result of the 1983 law, whereas the actual increase was only about 60 percent (Bureau of the Census 1985, Table 13; Bureau of the Census 1995, Table 13~. As a second example, a study in Ontario examined how the trucking industry had utilized the features of new provincial weight limits introduced in the 1970s to develop a great variety of vehicle configurations for specialized uses, which could not have been predicted at the time the limits were enacted.
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From page 47... ...
benefits of changes in truck size anc! weight regulations in prominent past studies.
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From page 48... ...
The scenario assumes that existing state truck length and route restrictions are unchanged, as are federal axle weight limits. Truck weights are limited only by a new federal bridge formula.
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From page 49... ...
The truck configurations evaluated in the TRB and DOT studies include all the trucks that are commonly proposed for more widespread use in the United States: turnpike doubles, Rocky Mountain doubles, triples, short heavy double-trailer configurations (the Turner Proposal study vehicle and similar configurations) , and heavy six-axIe tractor49
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From page 50... ...
The policy options or scenarios in these studies all involve adding axles to trucks to better distribute loads or increase the total weight carried in the truck. None except the Turner Proposal study involves changing the federal weight limits for single and tandem axles.
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From page 51... ...
The DOT study and both TRB studies agree that even after allowing for increases in traffic that would be stimulated by cost reductions, truck traffic volume would be lower than if the regulations were not changed because the new trucks would be more productive than the ones they replaced. The three studies predict that highway agency pavement construction and maintenance costs would be unlikely to be greatly affected by a change in limits that allowed heavier trucks but did not increase axle weight limits and did not provide incentives for carriers to switch from tandem-axIe to single-axIe configurations.
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From page 52... ...
They conclude that increased use of larger trucks as a result of changes in size and weight limits would have little overall effect on highway safety because small possible increases in accident rates per truckVMT would be approximately offset by the reduction in truckVMT resulting from the new trucks' higher productivity. Accident rates per ton-mile of highway freight are predicted to decline.
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From page 53... ...
Truck Traffic Volume and Freight Costs Projections in the past studies of the effect of new regulations on truck traffic have proceeded according to the following steps: I Predict the new truck dimensions and configurations that would become attractive to carriers under the new regulations.
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From page 54... ...
Some studies use quantitative, calibrated models to perform at least some of these steps; for example, Road Work estimates an econometric mode! of carriers' choices of truck configuration that is analogous to the mode choice models used in transportation planning (Small et al.
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From page 55... ...
· The market models used to predict how changing truck costs would affect truck traffic, although based on cost and traffic data and plausible assumptions, probably are not highly reliable. A principal difficulty is assessing the consequences of vehicle characteristics that are not easily converted into cost differences, for example, the problem that carriers report in using double trailers to serve customers' docks directly (TRB 199Ob, 56-62~.
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From page 56... ...
In light of these characteristics of pavement response to loads, past studies have estimated that if gross weight and vehicle length limits are changed but axle weight limits remain unchanged, pavement costs will change only slightly. Thus, for example, the DOT "North American trade" scenario and the Truck Weight Study permit program are predicted to reduce annual highway agency pavement costs by $120 million and $10 million, respectively.
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From page 57... ...
In the DOT and TRB studies, the pavement cost of changing truck size and weight limits is estimated by assuming that the highway agency acts so as to maintain the same average pavement condition after the change by rescheduling the time of the next resurfacing of roads and by changing the design of future resurfacing treatments, following established pavement design methods, to accommodate the 57
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From page 58... ...
Road Test, a test of the effect of truck traffic on pavement wear conducted in 1958. This mode!
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From page 59... ...
This gap in the data is important because several prominent proposals for changing size and weight regulations involve extensive use of tridem axles. The Problem of Optimizing Vehicle and Highway Design To discover the best combination of policies, including size and weight regulations and highway design and management practices, it is necessary to examine whether changes in prevailing pavement design and management practices coupled with changes in limits could produce greater public benefits, considering road user costs that depend on pavement condition and highway agency pavement costs, as well as other shipper costs that depend on size and weight limits.
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From page 60... ...
The optimum axle weight limit will depend on bridge as well as pavement costs. Highway Bridge and Structure Costs In most past studies, the greatest predicted cost of allowing larger trucks is the cost of replacing bridges deficient for carrying the new heavier loads.
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From page 61... ...
The estimates in the TRB studies include costs of fatigue damage and of building future bridges to higher design standards, as well as the costs of replacing existing deficient bridges. Truck Weight Limits presents its estimate with an important qualification: "If all 35,000 additional foad-deficient bridges were replaced, total bridge costs would increase by $900 million per year under this scenario.
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From page 62... ...
As the table above shows, the DOT 2000 study estimates that the costs to highway users of delays due to bridge construction necessitated by increased limits would be four to six times the highway agencies' construction costs. Comparison of the studies' estimates of bridge costs with freight cost savings shows that these estimates appear to be decisive in judging whether costs exceed benefits for many proposed changes in size and weight regulations.
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From page 63... ...
Thus, for example, there is no evidence presented in the DOT 2000 study that the additional billions of dollars in user and highway agency costs required to replace bridges exposed to heavier trucks according to the criterion applied by that study, compared with a more lenient standard, would buy any significant public benefit. The second failing is that past studies generally have not systematically taken into account the possibility of intelligent management of bridge investment and maintenance decisions by highway agencies.
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From page 64... ...
Alternative treatments could produce the same degree of insurance against bridge failure as the extensive bridge replacements projected in past studies at much lower cost to the highway agency and to users. Finally, consideration of costs other than the highway agency's bridge replacement costs has been haphazard in past studies.
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From page 65... ...
used in past studies, the committee examined cost estimates for a small number of bridges selected from one state. Results obtained with the DOT study method for determining whether each bridge would require replacement if a particular new truck came into use were then compared with the results of analyses of options for each bridge as bridge engineers of that state would carry out such analyses.
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From page 66... ...
The state engineers' assessment suggests that fatigue costs are more important than past studies have indicated. Sensitivity of Cost Estimates to Assumptions To better understand the determinants of bridge cost estimates in the past studies, the committee next asked DOT to provide tabulations of California bridges that would require replacement according to the DOT 2000 study's method of analysis, and also according to an alternative, more lenient criterion dictating the threshold stress that would trigger replacement of a bridge if a specified larger truck were introduced.
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From page 67... ...
Nearly all Interstate structures were designed to the HS-20 design load or a more rigorous standard; the H-15 design load is typical of older bridges and less important roads. The bridge cost estimates of the TRB studies assume bridges would be replaced or posted if the operating rating were exceeded.
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From page 68... ...
GAO concluded that the latter criterion was reasonable to ensure safety, and that bridge costs could reasonably be reduced even further by judicious exclusions of Interstate segments with high bridge costs from the LCV network (GAO 1994, 24-25~. In another report, GAO comments on the fundamental shortcoming of the bridge cost estimation method used in past studies (GAO 2000, 6~.
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From page 69... ...
. A More Realistic Method of Estimating Bridge Costs As noted above, bridge cost estimates derived by the method of past studies assume replacement of bridges regardless of whether the cost of replacement is justified by the gain in safety and do not fully take into account the capabilities of highway agencies to maintain bridge safety by more cost-effective means than replacing all suspect bridges.
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From page 71... ...
3. Estimate the expected annual rate of bridge failures under the proposed new size and weight regulations.
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From page 72... ...
If the component is part of a highway bridge, this probability increases over time because the component is subject to deterioration and because trucks tend historically to become heavier (see Figure 2-4~. The original level of risk can be restored by either of two methods: the means of R and S can be moved apart (by posting the bridge to reduce truck weights, strengthening members, or replacing the bridge)
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From page 73... ...
If, hypothetically, a comparison were made between two highway systems one system with a rigorous, state-of-the-art bridge inspection and maintenance program within the framework of a comprehensive bridge management system, as well as effective weight enforcement, and with liberal policies regarding allowable vehicle loadings on bridges; and the other system with underfunded and unsystematic bridge inspection and maintenance and lax weight enforcement, and with restrictive bridge loading rules one could easily imagine that the first system might have the safer bridges and the lower fong-run user and highway agency costs. The method outlined here for estimating the costs of changing size and weight regulations assumes that highway agencies make optimal bridge management and construction decisions.
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From page 74... ...
· Regulation of Weights, Lengths, and Widths of Commercial Motor Vehicles | S .
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From page 75... ...
Although the method of past studies sometimes has been described as a behavioral approach, that is, an estimate of how highway agencies would respond if new trucks were introduced given the agencies' established practices, the projections of the traditional method are unlikely outcomes. It is highly implausible that states would undertake bridge investments of the magnitudes predicted by the method.
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From page 76... ...
1993~. The risk-based approach to estimating the bridge costs associated with changing size and weight regulations is more consistent than the method used in past studies with the way bridges are actually affected by increased truck weights and with how highway agencies manage bridges and respond to changes in traffic loadings in practice.
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From page 77... ...
life (Dicleli and Bruneau 1995~. Ontario's weight regulations have been among the most liberal in North America, allowing up to 140,000 Ib gross weight on eight-axIe combinations.
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From page 78... ...
Future extension of such an analysis, following the method outlined in the preceding subsection, should involve selecting an optimum risk level for regulating truck weights on existing bridges on the basis of economic criteria, taking into account the actions states can take to control the uncertainties in loadings and bridge conditions. State highway agencies are making progress toward having the kinds of data b as es and analytical cap abilities needed to contra f bridge failure risks through inspection and maintenance practices and to 78
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From page 79... ...
The Potential of Retrofitting Neither the TRB studies nor the DOT 2000 study quantitatively assesses the potential of bridge retrofitting as a means to accommodate greater loads. Retrofit strengthening of bridges to increase foad-bearing capacity and earthquake resistance is a technique employed increasingly in state highway programs, although no state is known to have undertaken a program of retrofitting specifically to accommodate larger trucks.
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From page 80... ...
Future truck size and weight studies should produce bridge cost estimates by predicting changes in expected frequencies of bridge failure caused by changes in size and weight regulations and in highway agency management practices; estimating the costs of increased risk; and comparing alternative methods of reducing risk to find the optimum combination of size and weight limits, bridge replacements and postings, intensity of bridge inspection and maintenance, and truck .
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From page 81... ...
· If new trucks are longer or less maneuverable or have less power in relation to their weight, each truck-VMT by the new trucks will cause greater perturbation of traffic than a VMT by the trucks replaced. · Changes in truck traffic volume and any resulting changes in congestion will alter the costs of highway travel for other highway users, who in response may change the time, route, or quantity of their highway travel.
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From page 82... ...
In all the projections, allowing more productive trucks reduced total truck-VMT since the only source of induced truck traffic considered was diversion from rail, which was projected to be too small to offset the effect of increased ton-miles per truck-VMT. Increasing federal limits was projected to increase truck traffic on some roads because trucks would be diverted from state-regulated secondary roads to primary roads subject to federal regulations in response to the liberalized limits.
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From page 83... ...
were as follows: Impact Peak volume/capacity ratio Off-peak volume/capacity ratio Annual oxides of nitrogen emissions Annual hydrocarbon emissions Population with noise exposure above a specified threshold Percent Change from Base Case -0.15 -0.25 -0.50 -0.30 +0.80 Thus the projected impacts are all very small and generally, with the exception of noise, are favorable. Because the traffic projections involve redistribution of truck traffic among routes in response to route-specific changes in regulations, some of the effects on the case study roads could be augmented or offset by effects on other nearby roads.
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From page 84... ...
TRB Studies Quantitative estimates of changes in congestion delay or emissions resulting from changes in size and weight regulations were not included in the TRB studies (TRB 1986; TRB 199Oa; TRB 199Ob)
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From page 85... ...
For the study's final estimates of congestion effects, it was assumed that newly introduced larger trucks would have the same power-to-weight ratio as existing trucks when fully loaded. This assumption is supported by examination of trends in power-to-weight ratios, which have been rising in recent years even as average truck weights have been increasing.
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From page 86... ...
~ g ~ AL ~ e o E it_ It .
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From page 87... ...
It would imply that a 20 percent drop in the price of truck transportation for the cargoes for which this configuration would be suitable would cause at least a doubling of the volume of those cargoes in trucks, which is much greater than any reported estimate of price sensitivity of truck traffic. The change in annual hours of delay was computed from the projections of changes in PCE volumes on the sample road segments for peak and off-peak periods.
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From page 88... ...
Present emission models provide no information on the effect of changing weight limits on heavy-duty vehicle emissions. There are almost no data on heavy-duty diesel vehicle emissions at alternative test weights.
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From page 89... ...
Summa* of Cost Estimates Under the assumptions stated in the preceding section, estimates of the changes in delay and air pollution costs for one of the DOT 2000 regulatory scenarios and one of the TRB studies are as follows: Change in Annual Highway User Delay Costs ($ millionsJ From Traffic Interaction Effects From Construction Delay DOT North American trade (51,000-lb tridem)
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From page 90... ...
Improved Methods The greatest shortcoming of the methods used in past studies to estimate congestion and pollution costs has been the oversimplified treatment of the complex interactions between trucks and other vehicles in the traffic stream. Changing the traffic volume, dimensions, and acceleration abilities of trucks will change how motorists drive around them, affecting other vehicles' patterns of acceleration and braking.
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From page 91... ...
Changes in business location decisions caused by changes in freight costs would eventually have some effect on residential location decisions and personal travel patterns. Choice of Firm's Location and Size ~ Choice of Supplier 1 1 r Choice of Inventory Strategy Choice of Mode and Shipment Size r Long Run Intermediate Run Short Run FIGURE 2-7 Freight decisions of a firm.
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From page 92... ...
If highway users paid the full costs of their travel, evaluation of truck size and weight standards would not depend on the magnitude of the change in truck traffic caused by a change in standards. However, the fees a highway user pays do not always match the cost to the highway agency of providing service, and highway travel generates external costs, for example, air pollution and congestion costs that shippers do not take into account when they make freight purchasing decisions.
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From page 93... ...
The recent TRB and DOT truck size and weight studies predict that liberalizing truck size and weight limits would lead to a decrease in annual truck-VMT. These studies most commonly assume that the total volume of freight traffic via all modes is unchanged and that the only source of new truck traffic in response to changing the limits would be freight diverted from rail.
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From page 94... ...
The practical way to reduce or avoid harmful land use consequences is to eliminate subsidies in the highway program and to control the environmental, safety, and congestion costs of truck traffic through regulation or imposition of fees. If the public were to decide that it would be desirable to limit the volume of truck freight transportation to promote environmental, safety, or other objectives, tightening truck size and weight limits might be a relatively expensive means of accomplishing this end.
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From page 95... ...
. serves, first, to indicate the need for extreme caution in the use of accident rates.
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From page 96... ...
It is a source of frustration that 60 years of research has not yielded definitive conclusions on these questions. Like the ICC study, later policy evaluations, including the DOT and TRB studies, have assembled their assessments of the likely impacts of allowing larger vehicles from multiple kinds of evidence: examinations of the relation of size and weight to vehicle handling and stability and of the relation of size and weight to the interaction of the vehicle with other vehicles in the traffic stream; reported experiences of carriers and drivers using larger vehicles; and statistical studies of accident involvement rates, accident severity, and types and characteristics of accidents.
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From page 97... ...
, but better understanding is needed of the relationship of accident risk to traffic volume and the mix of vehicles on a road (TRB 1996, 68-72~. The TRB Truck Weight Limits study attempts to simplify the safety risk comparisons for the trucks it considers by estimating two accident involvement rate ratios: the ratio of double-trailer to singletrailer rates and the ratio for a heavy tractor-semitrailer with respect to a conventional tractor-semitrailer.
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From page 98... ...
Regarding the systemwide safety impact of liberalization of size and weight regulations, in the TRB studies, for the recommended changes (or in the case of Twin Trailer Trucks, for the regulations already enacted) , accident involvement rates for combination vehicles per VMT are projected to increase, involvement rates per ton-mile of truck freight are projected to decrease, and total accidents are projected to decrease (TRB 199Oa, 12-17; TRB 199Ob, 5~.
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From page 99... ...
studies update the review of Twin Trailer Trucks by evaluating four more recent studies that used appropriate methodologies to isolate the effects of vehicle configuration on accident involvement rates. The Truck Weight Limits study committee decided to use a value of I.l for the ratio of double-trailer to single-trailer fatal and nonfatal accident involvement rates in its estimates of the impacts of changing size and weight regulations, citing the results of one study (Campbell et al.
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From page 100... ...
The 1988 study, with data from selected Interstate road segments in Washington, reports double-trailer involvement rates 2.5 to 3 times those of tractor-semitrailers. The committees that conducted TRB Turner Proposal and Truck Weight Limits studies examined traffic count data for the roads and time periods of the Washington study.
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From page 101... ...
The DOT study did not compare accident rates by weather conditions. Differences in weather conditions during operations may explain some part of the difference in LCV/non-LCV accident involvement rates.
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From page 102... ...
Truck Weight Limits presents graphs of Campbell et al.'s estimates of fatal accident involvement rates for single-unit and combination trucks by gross weight range and road class (TRB 199Oa, 127-131~. The conclusion offered is that "these data suggest a moderate increase in accident rates for higher gross weight, although the relatively small number of data points and the high degree of scatter make drawing conclusions from these data difficult" (p.
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From page 103... ...
Relation of Handling, Stability, and Traffic Interaction Effects to Accident Risk Most past studies, beginning with that of the {CC in 1941, have devoted considerable effort to examining how changing size and weight limits would affect certain dynamic properties and performance characteristics of trucks that are hypothesized to be related to safety. The goals of these examinations have been to understand the physical basis for any observed differences in accident rates among configurations, and to discover ways of redesigning trucks to counteract undesirable changes in dynamic properties brought about by changes in the limits.
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From page 105... ...
Wheel lock degrades vehicle controllability during braking and may lead to jackknifing. In addition to controlIability during braking, the other dimension of braking performance examined in past studies is stopping distance.
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From page 106... ...
For example, the Truck Weight Limits study concludes (TRB 199Oa,115-~16~: Existing five-axIe doubles have a unique handling and stability characteristic, namely, rearward amplification of the motion of the lead units, that is not shared by tractor-semitrailers.
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From page 107... ...
To verify judgments about the linkages among size and weight regulations, vehicle properties, and safety, two kinds of empirically derived relationships would be required: first, a mode! of how changes in regulations affect the handling, stability, and performance properties of trucks in use; and second, relationships, derived from observation, of accident involvement rates by level of severity as functions of these truck properties.
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From page 108... ...
Several of the vehicle performance characteristics hypothesized in past studies to be related to accident risk are correlated with gross vehicle weight. For example, rollover threshold, a vehicle property believed to increase the risk of certain accident types, tends to decrease with increasing vehicle weight for a given truck configuration.
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From page 109... ...
Among the unanswered questions are the relative safety of different sizes of doubletrailer combinations, the combined effects of weight and configuration, and the effectiveness of countermeasures. It is important to recognize that any measured differences in accident involvement rates between double trailers and tractor-semitrailers are likely to depend to some degree on specific vehicle characteristics, including the number and spacing of axles and the types of connections between trailers, and that changing these characteristics could change relative accident involvement rates.
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From page 110... ...
of how incremental changes in the volume and characteristics of truck traffic on a network of roads affect accident costs on the network, based on direct measurements of how changes in truck traffic affect the behavior of and risks to car drivers. Little is known about the effectiveness of the majority of the safety measures recommended by past studies as accompaniments to liberalization of size and weight regulations.
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From page 111... ...
Traffic Operations and Truck Size and Weight Regulations: Comprehensive Truck Size and Weight Study: Phase I Synthesis: Working Paper 6.
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From page 112... ...
2000. Effect of Changing Truck Weight Regulations on U.S.
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From page 113... ...
TRB.1990a. Special Report 225: Truck Weight Limits: Issues and Options.
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From page 114... ...
l990b. Special Report 227: New Trucks for Greater Productivity and Less Road Wear: An Evaluation of the Turner Proposal.
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