Economic Viability of Upgrading Low-Volume Roads

Transcription

1 34 TRANSPORTATION RESEARCH RECORD 1291 Economic Viability of Upgrading Low-Volume Roads H. R. KERALI, M. S. SNAITH, AND R. C. KooLE In many countries, particularly in the developing world, a large percentage of the road network is made up of gravel roads. Although such roads are suitable for low traffic volumes, many countries have had rapid increases in traffic levels. This process has resulted in large expenditure on maintenance and rehabilitation of gravel roads as well as high costs of vehicle operation. At high traffic levels, it is therefore desirable to upgrade such roads with bitumen or asphalt pavements to reduce total costs. A computer package incorporating the World Bank Highway Design and Maintenance Standards Model (HDM-III) has been developed to determine the traffic levels at which it becomes economic to upgrade road pavements. Depending on the level of accuracy required, the computer package may be used to give approximate or detailed results. At the simplest operation level, only a few data variables are required to accomplish a quick analysis giving approximate results. A graphical plot of the total costs is displayed on the computer screen showing the crossover point at which upgrading becomes viable. The purpose of constructing a road is to provide safe and reliable vehicle movement. In many countries, roads have to be constructed along route corridors that pass through areas with soils that are both weak and susceptible to the ingress of water. The construction of an appropriate standard of pavement is intended to provide a smooth riding surface in addition to protecting the underlying soil subgrade from the adverse effects of traffic loading and excessive moisture. The function of a highway engineer is therefore to design cost-effective roads with appropriate geometric and pavement standards that meet the objectives. In order to perform this function, there are a number of different road types lying between engineered gravel roads and heavy-duty bituminous or asphalt pavements that the highway engineer can adopt in the pavement design stage, depending on the economic viability of the proposed road. The progression from one of the above pavement standards to the next represents a significant improvement in the capacity of a road to withstand traffic loading and the adverse effects of the environment. An increase in construction expenditure would be required, but this is countered by lower rates of pavement deterioration caused by traffic loading and the environment. Each successive improvement in pavement standards may therefore be justified by consequent savings in maintenance expenditure, in the cost of vehicle operation, and in other road user costs. Hence, it is clear that with high trnffic, or severe environment, there is a greater likelihood that construction of a stronger pavement would be justified. H. R. Kerali and M. S. Snaith, School of Civil Engineering, University of Birmingham, Birmingham, B15 2TT, United Kingdom. R. C. Koole, Shell International Petroleum Company, London, United Kingdom. In many developing countries, a large percentage of the road network is unsealed (earth or gravel roads), and despite the generally low traffic volumes on such roads, there is a need to protect them from being washed away, particularly in hilly or mountainous regions with high rainfall. The ingress of water can be prevented by sealing a road surface using a relatively cheap bituminous layer such as surface dressing or other surface treatment. With increased traffic loading, a road requires additional strength to withstand the development of permanent deformation in the subgrade and in other underlying granular layers. In order to reduce the stresses, an extra thickness of pavement is required. This is usually provided by gravel in sealed or surface-dressed roads, but even gravel is becoming increasingly scarce and requires continual replenishment. At a certain combination of gravel cost and traffic, it becomes cheaper to add the required strength to the road in the form of a load-spreading structural bituminous layer. A simplified method of economic appraisal has been developed for calculating the costs and benefits of the transition from unsealed roads to sealed roads and then further to roads with a structural bituminous load-bearing pavement layer. The transition point is defined in terms of the number of vehicles using a road per day required to justify a higher pavement standard. This is referred to as the break-even traffic level for upgrading from a weak to a stronger type of pavement. For a given set of circumstances, the transition point between pavement standards depends on the costs of road construction, road maintenance traffic composition, and the resulting vehicle operating costs (VOCs). These costs will largely depend on the location of the project together with the effects of topography and climate. Several economic analyses were conducted to determine typical break-even traffic levels in a number of environments. The results are intended for use in chart form to identify situations in which upgrading a road pavement structure may be beneficial. SHELL INPUT DATA PROGRAM A user-friendly microcomputer program has been developed to be used as a front-end data preparation tool. The primary objective of the Shell Input Data (SID) program is to prepare data for the World Bank HDM III model in the form required for the break-even analysis. HDM-III is then used to calculate the transport costs incurred on any two pavement types at different traffic flow levels. The package therefore provides a user-friendly front end that can be used to run HDM-III with the specific purpose of determining the optimum traffic levels for upgrading road pavements under different environments.

2 Kerali et al. 35 TABLE 1 TYPICAL DATA BANDS USED IN SID, LEVEL 2 Parameter Level 2 Default Values Low Average High Environment: Rainfall Altitude (mm/month) (mm) Geometric Characteristics: Horizontal Curvature (deg/km) Vertical Alignment (m/km) Carriageway Width (m) Pavement Standard: Surface Layer Thickness (mm) Roadbase Thickness (mm) Subbase Thickness (mm) Subgrade CBR (%) The SID program provides users with three modes of operation (Levels 2, 1, and ), corresponding to the simplicity required in data processing. In Level 2 operation, only a few parameters required to run HDM-III are requested; other input parameters are assigned default values. Data entry using Level 2 is intended to give approximate results from which a decision may be made for further investigation. In this mode, only a few data cards can be edited to specify parameters such as the unit costs of construction, maintenance, vehicle operation, and traffic characteristics. Other physical characteristics of the road to be analyzed can be selected from low, average, or high values. Table 1 presents examples of default values suggested by the SID program in Level 2 operation. The default values are intended to give the orders of magnitude believed to be typical for the area of analysis and can be changed by the user. However, Level 1 operation requests all parameters necessary to run HDM-III and is designed to provide more accurate results. It is intended to be used by experienced HDM-III users, who are familiar with all input requirements. Levels 1 and 2 are tailored to provide data to HDM-III for the comparison of any two types of pavement structure to obtain the break-even traffic level. Level of the data input program can be used to specify input parameters to HDM-III for any type of economic analysis (e.g., the appraisal of maintenance alternatives). ECONOMIC ANALYSIS OF BREAK-EVEN TRAFFIC LEVELS The economic analysis involved in deriving the break-even traffic level is a straightforward comparison of transport costs calculated for any two road types. For example, to determine whether a gravel road or a sealed road is more suitable under a given set of circumstances, HDM-III is used to calculate the annual transport cost matrices for the two alternatives. For example, if the present value of costs for a gravel road alternative exceeds that of a sealed road, it can be concluded that, under the given set of circumstances, a sealed road has lower total transport costs and therefore its construction would be economically viable. In this example, the set of circumstances refers to a road with known climate, topography, and traffic flow characteristics. The method of appraisal could therefore be used to answer the question, Is it cheaper to operate a gravel or a sealed road under the present traffic loading pattern? In order to answer the next question-at what traffic flow level does it become cheaper to operate a sealed instead of a gravel road?-it is necessary to calculate the total transport costs for both road types for a range of traffic flow levels and to determine the average daily traffic (ADT) at which the transport costs for the two pavement types are equal. Weak pavements deteriorate faster than strong pavements under traffic loading and other environmental effects. At low traffic levels, the rate of pavement deterioration is low, and therefore transport costs are likely to be low. Consequently, by comparing the transport costs calculated for a range of traffic loading on two pavement types, it is possible to interpolate and obtain the ADT at which transport costs are equal for the two pavement types. The economic analysis required to determine the breakeven traffic is conducted by using HDM-III to calculate transport costs for any two pavement types under a range of traffic flow levels. Table 2 presents the default traffic flow levels used in HD M-III to compare transport costs calculated for gravel, sealed, and premix roads. These values represent initial ADT with a specified traffic composition, that is, observed traffic flow in the first year of analysis (the base year). The traffic levels were chosen to include the upper and lower limits for the break-even ADT likely to be found in most countries. When these are used in a particular country, the unit costs of construction, maintenance, and vehicle operation are required together with the maintenance standards commonly applied. These unit costs, maintenance standards, and traffic mix affect the break-even ADT level. For countries in which costs of vehicle operation are high, a low break-even ADT can be

3 36 TRANSPORTATION RESEARCH RECORD 1291 TABLE 2 TRAFFIC FLOW LEVELS USED IN HOM-III COMPARISONS Alternative Gravel to Sealed Sealed to Premix Gravel to Premix ALTl 2 ALT2 5 ALT3 75 ALT4 1 ALT5 2 ALT6 4 ALT7 7 ALTS expected. Similarly, if the maintenance costs for the weaker pavement are high (e.g., cost of regraveling), a low breakeven traffic can also be expected. Typical Results Using Data From Cyprus Trial runs of the HDM-III program were conducted using data from Cyprus with the objective of determining typical break-even traffic flow levels for upgrading from gravel to sealed and from sealed to premix pavements. For these analyses, the gravel road was assumed to have 15 mm of gravel surfacing; the sealed road, 25 mm of surface dressing on 15 mm granular road base; and the premix road, 4 mm of premix surfacing layer on top of 3 mm of selected granular road base material. The results from the trial runs produced for a comparison of a sealed road against a gravel road by HDM-III are summarized in Table 3. The break-even traffic level is given by the ADT level for which the net present value (NPV) is zero (i.e., when the transport costs of the two pavements being compared arc equal). It may be observed from the table that the optimum or break-even traffic level required to justify the construction of a surface-dressed road varies with the discount rate used in the analyses. If the economic costs are not discounted, the break-even traffic flow is just over 5 vehicles per day (vpd). Presentation of Results There are a number of ways in which the results given in Table 2 can be presented to illustrate the break-even traffic level. Three of these are of particular interest when presented in graphical form: Total transport costs versus initial ADT flows, NPV versus initial ADT flows, and Internal rate of return (IRR) versus initial ADT flows. Figure 1 shows the first of these with the total transport costs for the gravel and sealed roads on the vertical axis. This graph is similar to the graph produced on the computer screen. The point at which the two curves intersect marks the breakeven ADT value. Figure 1 indicates that the break-even traffic level is just under 2 vpd for a 2 percent annual traffic growth rate. Figure 1 can also be used to estimate the size of benefits to be derived by upgrading a road. If the daily traffic flow is known, the benefits derived from upgrading are given by the difference in total transport costs. Figure 2 shows the sewml method of presentation with the NPVs at four discount rates plotted on the vertical axis. The break-even traffic level for each of the discount rates is given by the points at which the curves cross the horizontal x-axis when the NPV i Lt;JlJ. Tlte 1 µe1ct::11l uiswu11l line C!Usses the x-axis at a traffic flow of just under 1 vpd. TABLE 3 COMPARISON OF A SEALED ROAD WITH A GRAVEL ROAD FOR 2 PERCENT TRAFFIC GROWTH RATE Initial Net Present Value at Discount Rate of I.RR Traffic % 5% 1% 15% (%)

4 Kerali et al. 37 Cost (C m) Traffic Growth - 2% Discount Rate - 1% _,.,,-' Sealed ,,., lnial Average Daily Traffic (ADl) FIGURE 1 Relationship between total transport cost and traffic level. Table 3 presents IRR values calculated for each of the initial ADT flows. The IRR values represent the discount rates at which the corresponding ADT would justify upgrading from a gravel to a sealed road. Figure 3 shows the relationship between IRR and traffic flow for Cyprus conditions. The figure represents all ADT levels required to justify upgrading a gravel road to a surface-dressed road in Cyprus for any given traffic growth and discount rate. For example, at a discount rate of 1 percent, the break-even traffic flow for surface dressing is just under 2 vpd for a 2 percent growth in traffic. Factors Affecting the Break-Even Traffic Level The break-even traffic level discussed in the previous section depends on a number of factors, notably the unit costs of construction and maintenance, traffic growth rates, discount rates, and the traffic composition that has a direct effect on the VOC component. The unit costs of construction and main- tenance in most cases are determined before the analysis is conducted and are then assumed to remain constant. The break-even traffic level therefore depends largely on traffic growth and the discount rates used in the economic analyses. If the voe component is included in the analyses, the size of the total saving in voe will depend on the traffic growth rate. If high traffic growth rates are used, the traffic loading during the analysis period will be high, resulting in a faster deterioration of the weaker pavement and giving higher savings in VOC; consequently, upgrading to a stronger pavement will be justified at lower traffic flow levels. Figure 3 shows the effects of these factors on the break-even traffic levels. EFFECT OF ENVIRONMENT ON BREAK-EVEN TRAFFIC LEVELS Economic analyses were conducted using HDM-III to determine the break-even traffic for a range of environments using NPV (C m) ,.,-; Discount Rate % _,- 5% 1% 15% Initial Average Daily Traffic (ADT) FIGURE 2 Relationship between NPV and traffic level.

5 38 TRANSPORTATION RESEARCH RECORD 1291 IRA% _ Traffic Growth 1%...- 5% % Initial Average Daily Traffic (ADT) FIGURE 3 Relationship between IRR and traffic level. data from Cyprus. The objective for this procedure was to quantify the effects of various environmental factors on the break-even traffic levels. The factors included in the analysis were rainfall, topography, and subgrade soil strength in terms of the California bearing ratio (CBR). In the course of the analysis, it was found that some of the environmental factors had little or no effect on the break-even traffic results (e.g., rainfall on sealed or premix roads). The Shell pavement design manual recommends pavement layer thicknesses based on four factors: Traffic loading in terms of equivalent standard axles, Subg1aJe sl1eugll1 mujulu, Prevailing weather conditions in terms of the weighted mean temperature, and Bitumen and mix properties. The effect of traffic loading and subgrade strength on the design thickness for pavements used in the break-even analysis is discussed in the next section. The effect of temperature on the performance of bituminous pavements is not explicitly modeled within HDM-111. However, temperature variations in different climatic regions can be taken into account by calibrating the performance equations built into the model, such as rut depth progression and other defect progressions. The effects of the environment are also taken into account in terms of the rainfall and altitude. In HDM-111, rainfall is assumed to affect the performance of unpaved roads, particularly the rate of gravel loss. Pavement Design Standards Traffic loading is used in the break-even analysis as the independent variable that governs the size of benefits derived from upgrading a road from one pavement type to another. The normal practice is to design pavement thicknesses according to predicted cumulative traffic loading and prevailing subgrade strength conditions over the design period. The break-even analysis is essentially conducted by calculating the total transport costs incurred on a road using a range of traffic flow levels. Ideally, each traffic range used in the analysis should be assigned a unique pavement design (and hence cost of construction). However, to maintain the simplicity of the calculation of the break-even traffic level, the same pavement structure is used in all analyses. This pavement structure represents the minimum possible design thickness. For example, the minimum pavement structure for premix roads is assumed to include a 5-mm asphalt concrete (or equivalent) surfacing and 3 nun of granular road_ base. The minimum sealed road structure is a surface dressing of 25 mm with a granular base of 3 mm. For unpaved roads, the minimum gravel thickness commonly used is 15 mm. This assumption is compensated for in the analyses by using a condition-responsive maintenance policy within HDM-111. The effect is to apply frequent maintenance and rehabilitation on undcrdcsigncd pavements when the pavement attains critical condition. For example, a pavement composed of 5 mm of asphalt concrete and 3 mm of granular road base is used for ADT levels from 1 to 5, but is expected to receive more overlays during the analysis period under the heavier traffic load. Effect of Rainfall and Topography Only rainfall in combination with high vertical alignment had noticeable effects on the break-even traffic level. The analyses were therefore performed for a combination of low rainfall in a flat area and high rainfall in a mountainous area. The results obtained for intermediate combinations of rainfall with topography indicated deviations in the break-even traffic level of less than 2 vpd. The break-even traffic levels for upgrading roads from gravel to sealed pavements range from 5 to 15 vpd depending on

6 6 5 MODERATE RAINFALL LOW LYING AREA Annual Traffic Growth,.-., i!r._, s llj Ill QI Cl Ill "' % 2% r-r (Thousands) Average Daily Traffic (.ADT).9,-.. i!r._, ::l "' Ill II) QI QI "' Annual Traffic Growth -.- 1oi HIGH RAINFALL MOUNTAINOUS AREA 5% 2% Average Daily Traffic (.ADT) FIGURE 4 Effect of rainfall and topography on break-even traffic level: (top) upgrading gravel to premix road and (bottom) upgrading gravel to sealed road.

7 4 the traffic growth rate and the discount rate used in the analysis. The combined effect of rainfall and severe topography is to lower the break-even traffic level by between 2 to 5 vpd, again depending on the traffic growth rate and the discount rate. Figure 4 shows the results of break-even analyses conducted for a low-lying area with moderate rainfall and for a mountainous area with high rainfall. The comparison of sealed-road performance against that of premix roads gave some unexpected results, because the breakeven traffic level from sealed to premix was between 5 to 75 vpd. This comparison refers to premix roads with the minimum pavement structure (i.e., 5 mm asphalt concrete on 3 mm granular road base). This result is in contrast with ADTs of over 1,5 vpd, commonly quoted as the minimum for premix standard pavements. The break-even traffic level for upgrading from a gravel to a premix rn;ici is surprisingly low, ranging from 1 to 25 vpd. This result indicates a rapid increase in transport costs once the break-even traffic level for paving a road is exceeded. It therefore suggests that sealing a road at an early stage will yield high benefits. The preceding results point to a high degree of dependence on the unit costs of construction used for gravel roads, sealed roads, and premix roads together with the unit costs for maintenance activities. Gravel roads, though least expensive to construct, have a high voe component because of high surface roughness values. The difference in roughness progression on sealed and premix roads, however, is similarly high, resulting in a lower break-even traffic level than is commonly expected. CONCLUSIONS A computer package incorporating the World Bank's HDM-III model can be used to determine the traffic levels TRANSPORTATION RESEARCH RECORD 1291 at which the transition from one pavement standard to the next becomes economic. A number of analyses were conducted using the package to study the effects of the climate, topography, and sub grade soil strength on the break-even traffic levels. The results of the analyses indicate that the break-even traffic level for upgrading roads from gravel to sealed is surprisingly low, implying a rapid increase in transport costs once the break-even traffic level for paving a road is exceeded. This confirms the view that sealing a road at an early stage is advisable. The combined effects of heavy rainfall and severe topography result in a lower break-even traffic level than might be expected. The comparisons of sealed-road performance and that of premix roads also gave some unexpected results, because the break-even traffic level from sealed to premix occurs at much lower ADT flow levels than is commonly expected. ACKNOWLEDGMENTS The work described in this paper was sponsored by Shell International Petroleum Company (SIPC). The Highways and Geotechnics Research Group of the University of Birmingham is greatly indebted to SIPC for providing the resources required in the research study. REFERENCES 1. World Bank. The Highway Design and Maintenance Standards Model. Johns Hopkins University Press, Baltimore, Md., The SHELL Pavement Design Manual for Asphalt Pavements and Overlays for Road Traffic. Shell International Petroleum Company, London, A Guide to Road Project Appraisal. Overseas Road Note 5. Transport and Road Research Laboratory, Crowthorne, Berkshire, England, 1988.