Comprehensive Truck Size and Weight Limits Study: Pavement Comparative Analysis Technical Report
Chapter 4: Scenario Impacts
4.1 Impacts of Scenarios on Pavement Costs
The next step in the study was to explore the impacts of the six scenarios for the pavement life cycle costs. Life cycle costs considered include only those related to highway agency costs for rehabilitation following the rehabilitation strategy and schedule presented in Table 10. The five-year nominal rate to be used for discounting the nominal flows that are often encountered in lease-purchase analysis is 1.9 percent. This figure is based on the Office of Management and Budget 2015 budget figures (OMB, 2013). In terms of unit costs for rehabilitation treatments, the American Concrete Pavement Association was consulted leading to a recommendation of an average cost of $3.43 per SY over the last 5 years (2009 to 2013) for concrete diamond grinding. Recent asphalt overlay construction data was gathered from the National Center on Asphalt Technology and State asphalt pavement association representatives. The total cost of milling and overlay (including milling, tack, overlay placement, and traffic control) for a 1.5-inch thick overlay is approximately $8.00 per square yard and between $11 and $12 per square yard for a 2-inch or 2.5-inch thick overlay.
It should be recognized that there are many variables involved with pavement rehabilitation that will greatly affect the cost; for example, the quantity of existing asphalt that is milled, new asphalt that is overlaid, section length, pavement thickness and geotechnical and hydraulic conditions. The asphalt concrete overlay and diamond grinding estimates are based on large-scale projects; however, smaller quantity projects could significantly increase the estimated values used in this analysis.
The rehabilitation strategies described in Table 10 were applied beginning with the time of initial failure for the base case and each scenario using unit costs of $68,400 per lane mile for a 2-inch asphalt overlay, $74,800 for a 3-inch asphalt overlay, $87,400 for a 4.5-inch asphalt overlay, $3.43 per square yard for diamond grinding, $1.65 per square yard for milling before asphalt overlays, and a discount rate of 1.9 percent.
An example of the calculations applied for the base case and two scenarios for the high-volume flexible pavement section in the Geographic Location #1 is presented in Table 11 to illustrate the cost comparison approach.
The example in Table 11 shows that the initial flexible pavement rehabilitation (triggered by rutting) occurred at 32.0 years for the base case, at 32.1 years for Scenario 3, and at 31.2 years for Scenario 4. The pavements in each scenario were given a 3-inch asphalt overlay (Rehab Activity 1) at each of those times at a total cost of $86,400 per lane mile (including milling), with present value costs as shown on the third line. Additional 3-inch asphalt overlays were placed after a 12-year interval, and the present value costs of each were accumulated. Since the second overlay was near the end of the 50-year life of each scenario, the cost was prorated based on its 12-year anticipated life. The base case pavement, for example, was charged for only 6.0 years’ worth of the overlay.
It should be recognized that in practice, the rehabilitation strategies may not be applied at the exact time that a failure is predicted for reasons such as funding availability, rehabilitation programming methods, weather conditions conducive to proper construction, and other priorities. For a fair, direct comparison between the different traffic scenarios, however, the service interval predictions presented in this Study represent the best estimate based on model prediction (for initial pavement service interval) and survival analysis of when the subsequent rehabilitation timing will occur.
Note that each of the scenarios included in the example above showed initial pavement service intervals at ages between 32 and 33 years, and a total of two rehabilitation activities were required during the 50-year life cycle. Appendix L compiles the changes in service intervals for each pavement section and Appendix N contains the tabulations and results of the life-cycle cost analysis on each pavement section. The shoulder pavement costs and all other costs were not included in the cost analysis.
Table 12 shows the weighted average percentage changes in initial service intervals and life cycle costs for each Scenario. Life cycle costs are shown as ranges as a result of using two discount rates applied to modeled treatment costs over the fifty year life of the pavement sections. To derive the weighted averages, sample pavement sections were weighted based on the number of lane-miles of pavement of each type, thickness range, and highway type. Scenario 4 increased LCC the most, by approximately 1.8 percent more than the base case. Scenario 3 decreased LCC the most by 2.6 percent below the base case.
Conclusions on Percent Life Cycle Cost Changes for Scenario 1
Scenario 1 (allows 3-S2 trucks to increase from an 80,000 lb. to an 88,000 lb. GVW and allows tandem axle weights to increase from 34,000 lb. to 38,000 lb.) was found to shift tandem axle weights modestly upwards but slightly decrease overall truck travel. (See the Volume II: Modal Shift Comparative Analysis for an in-depth discussion of the anticipated shifts between truck and rail modes and between vehicles and operating weights within the truck mode as a result of each Scenario.) Cumulative life cycle cost estimates were found to vary somewhat among the sample pavement sections for Scenario 1, with a few sections showing slight decreases and most showing slight increases in expected life cycle pavement costs.
Overall, Scenario 1 showed a small overall increase in LCC over the base traffic scenario.
Conclusions on Percent Life Cycle Cost Changes for Scenario 2
Scenario 2 (allows 3-S3 trucks to operate at weights up to 91,000 lb. and allows tridem axle weights up to 45,000 lb.) was found to result in fairly significant reductions in average loads per axle. The long-term pavement costs tended to decrease under this scenario.
Overall, Scenario 2 showed no change or decreased long-term rehabilitation LCC over the base traffic scenario for all pavement sections, with an overall weighted average LCC reduction of 2.4 percent.
Conclusions on Percent Life Cycle Cost Changes for Scenario 3
Scenario 3 (allows 3-S3 combination trucks to operate at weights up to 97,000 lb. and allows tridem axle weights up to 51,000 lb.) was also found to result in fairly significant reductions in average loads per axle. The pavement costs were observed to decrease accordingly, but slightly less under this scenario than under Scenario 2.
Overall, Scenario 3 showed decreased LCC over the base traffic scenario for nearly all sample pavement sections, with an overall weighted average 2.6 percent reduction in LCC.
Conclusions on Percent Life Cycle Cost Changes for Scenario 4
Scenario 4 (allows trailer lengths on 2-S1-2 trucks to increase from 28 feet to 33 feet) was found to result in significantly higher average loads per axle as well as significant shifts to single axles from tandem axle groups. As a result, the estimated pavement costs increased more under this scenario than any other.
Scenario 4 showed no change or increased LCC over the base traffic scenario for all sample pavement sections, with an overall weighted average 1.8 percent increase in LCC.
Conclusions on Percent Life Cycle Cost Changes for Scenario 5
Scenario 5 (allows seven-axle triple trailers up to 105,500 lb. on a designated highway network) was found to decrease truck travel and shift weight to slightly lower average axle loads on the designated network but slightly higher average axle loads on off-network highways.
Scenario 5 showed changes in LCC that varied from positive to negative when compared with the base traffic scenario for various pavement sections, with a slight (0.1 percent) overall weighted average increase in LCC.
Conclusions on Percent Life Cycle Cost Changes for Scenario 6
Scenario 6 (similar to Scenario 5, allows nine-axle triple trailer configurations up to 129,000 lb. on a designated network) was found to move significant numbers of tandem axles to lighter tandems and slightly heavier single axles.
While all the other scenarios used actual WIM observations to develop the distribution of axle weights and types for the scenario vehicles, Scenario 6 was unique in that it had no WIM observations for the target vehicle. There were many WIM observations for seven-axle and eight-axle triple trailer configurations in the full range of operating weights up to 130,000 lb., so axle weights observed on seven- and eight-axle triple trailer configurations were used to estimate a distribution of axle weights for the nine-axle triple trailer configurations at each operating gross weight. Steering axles were assumed to keep the same weight distributions, as were tandem axles on the eight-axle triple trailer configurations. Single load axles were assumed to be equally likely to have been converted to tandem load axles across the full range of observed axle weights.
Similar patterns of life cycle cost estimates and variations were observed for Scenario 6 as were observed for Scenario 5, showing changes in LCC that varied from positive to negative when compared with the base traffic scenario for various sample pavement sections, with a slight (0.1 percent) overall increase in weighted average LCC.
4.2 Impacts on Local Roads
The study focused on Interstate and NHS highways—key corridors in which trucks are delivering the highest vehicle miles traveled (VMT). More than 80% of total annual truck miles travelled occurs on the NHS. There is a more than four million center line miles of public roadway mileage in the United States with most of those miles located off of the NHS. Except in cases of very rare exception, there is little quantitative information available regarding travel, by facility, occurring on this non-NHS roadway network and on how these pavements are designed, built, and maintained. There is minimal to no history of HPMS data from local roads, and hard data on both how many and how often trucks use these facilities is in many cases not readily available. These data limitations have made it prohibitive to perform an accurate and representative study on the impacts of loading scenarios on local roads at this time. The lack of pavement structure characteristics, pavement surface type and typical travel levels for local system roadways yields it impossible to develop sampling based approaches that would produce results supported with adequate statistical confidence.
In order to better investigate the qualitative impacts of the six loading scenarios on local road pavements, a framework would need to start by gathering information on existing pavement sections, pavement design standards used, construction specification details, maintenance frequency and application types, materials properties of the pavements and underlying soils, and the traffic amount and distribution. It would be particularly beneficial to quantify the truck traffic volumes, truck type distributions (classes), and truck traffic variations (seasonally, hourly, monthly, etc.) as a basis for comparing the effects of introducing any of the six loading scenarios in this study.
4.3 References
California Department of Transportation, “Chapter 5 Diamond Grinding and Grooving,” MTAG Volume II - Rigid Pavement Preservation 2nd Edition, California Division of Maintenance, 2007, http://www.dot.ca.gov/hq/maint/RPMTAGChapter5-DiamondGrindingandGrooving.pdf.
Darter, M., and K. Hall, “Performance of Diamond Grinding,” Transportation Research Record No. 1268, Transportation Research Board, National Research Council, Washington, D.C., 1990.
Hoerner, T., Darter, M., Gharaibeh, N., and T. Crow, “Comparative Performance and Costs of In-service Highway Pavements, I-15 Utah”, Technical Report, ERES Consultants, Inc., 1999.
Office of Management and Budget, “Discount Rates for Cost-Effectiveness, Lease Purchase, and Related Analyses,” Circular A-94 Appendix C, OMB Circular No. A-94, Washington, D.C., 2013, http://www.whitehouse.gov/omb/circulars_a094/a94_appx-c.
Oman Systems, Inc., BidTabs, 2013, http://omanco.com/index.php/product/bidtabs/
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