Model Capabilities and ApplicationsThe Highway Design and Maintenance Standards Model (HDM) was developed by the World Bank's Transportation Department to met the needs of highway authorities, particularly in developing countries, for evaluating policies, standards, and programs of road construction and maintenance. The latest version is the 1995 HDM System, issued in 1995.
The model simulates total life cycle conditions and costs for one road, a group of roads with similar characteristics, or an entire network of paved or unpaved roads, for a series of road agency construction or maintenance strategies, and provides the economic decision criteria for evaluating the strategies being analyzed. The primary cost set for the life cycle analysis includes the costs of road construction and maintenance and vehicle operating costs, to which travel time costs can be added. The costs of construction related traffic delays, accidents and environmental pollution can be entered in the model exogenously based on separate estimates. HDM can be coupled with the companion Expenditure Budgeting Model (EBM) to find the best way of using road agency funds under budgetary constraint.
General Use of the ModelCommon applications of the model examine questions such as:
W ithin the planning, budgeting and programming functions of a highway agency, the HDM model may therefore be used to establish:
- is a proposed construction or maintenance project economically justified?
- what is the construction or maintenance project alternative that yields the highest benefits to society?
- what is the economic benefit of spending another dollar on maintenance, compared with spending it on new roads or improvements to existing alignments?
- is it more economical to construct a strong, expensive pavement initially, thereby permitting the use of larger, more economical vehicles and reducing future road maintenance outlays, or to follow a stage construction strategy economizing on initial costs, restricting vehicle axle loads, and paying more for maintenance, with the intent of upgrading the road later on when traffic growth warrants it?
- how to define priorities among roads to be included in a proposed work program?
- how much should be spent to maintain paved roads, and how much to maintain and upgrade earth and gravel roads?
- does it matter much if certain road maintenance expenditures are deferred during years of financial stringency?
- what combination of maintenance strategies yield the lowest overall economic costs for a specified level of maintenance funding?
A n important feature of the model is the analytical support it can provide to make a convincing case to legislatures and top decision-makers for adequate maintenance funding to preserve the road infrastructure.
- desired budget levels that would minimize the total costs of road transportation;
- appropriate policies and standards for construction and maintenance programs that are consistent with minimizing total transport costs under existing resource constraints;
- long and medium-term investment and expenditure programs; and
- appropriate, economically derived intervention criteria to develop short-term programs and annual budgets, based on an appropriate pavement management system.
With appropriate choice of parameter values, it can also be used for applied research in engineering and economics. In engineering, for example, the model may be used to optimize road maintenance strategies, to determine economic thresholds for programming road maintenance strategies, to determine economic optimum for axle loads limits, or to simulate the type and extent of road deterioration as a function of cumulative traffic loads or time. In economics, the model has potential application in research related to user charges, pricing and taxation of automotive fuels, highway cost allocation, and road transport pricing.
Structure of the ModelThe broad concept of the HDM model is quite simple. For a given road and series of user-specified road agency strategies, three interacting sets of costs (related to the construction, maintenance and vehicle operation) are added together over time in discounted present values, where costs are determined by first predicting physical quantities of resource consumption and then multiplying these by unit costs or prices. Economic benefits are then determined by comparing the total cost streams for the various strategies with a base strategy (null alternative) usually representing minimal routine maintenance.
The model estimates for a given road and road agency strategy, year by year, the road condition and resources used for maintenance, as well as the speeds and physical resource consumption of the vehicles. After physical quantities involved in maintenance and vehicle operation are estimated, user-specified prices and unit costs are applied to determine maintenance and vehicle operating costs. The construction costs for each design option are directly specified by the user; relative benefits are then calculated for the different strategies.
HDM is designed to make comparative cost estimates and economic evaluations of different construction and maintenance options, including different time-staging strategies, either for a given road project on a alignment or for a group of roads on an entire network. It estimates the total costs for a series of road agency strategies (alternative project designs and maintenance alternatives) year by year for up to twenty five years, discounting the future costs at a postulated discount rate, so that the user can search for the strategy with the lowest discounted total cost. The user obtains net present values, rates of return and other economic indicators. Another capability, using HDM in conjunction with the Expenditure Budgeting Model, is finding the set of design and maintenance options that would minimize total discounted transport cots or maximize net present value of the entire highway system under year-to-year budget constraints. In addition to comparing strategies, the model can analyze the sensitivity of the results to changes in assumptions about key parameters such as agency costs and user benefits.
Adequate analysis of the many possible combinations of alternatives is too large a task for manual calculations. Even when the analyst have had access to computers of sufficient capacity, they have been hampered by the absence of two essentials:
U ntil recently, for example, data on the effects of road roughness on vehicle operating costs were limited and not fully reliable, as were data on the progression of road roughness under different maintenance practices and environmental regimes.
- an efficient simulation program embodying an appropriate model and procedures for using it; and
- an adequate body of empirically established relationships among the relevant variables.
The HDM model fills both of these needs. It is not only a readily usable program for handling the voluminous computations automatically; it is also a repository of the most extensive and consistent set of empirical information on the subject - including the qualitative structure and qualitative parameters of relationships among construction standards, maintenance, traffic characteristics, road deterioration, and vehicle operating inputs.
Foundations of the ModelHDM is a result of 17 years of research and analysis carried on collaboratively by the World Bank and major research institutions and highway administrations in Australia, Brazil, France, India, Kenya, Sweden, the United Kingdom, and the United States. The initial effort was directed at developing analytical methods and models, but it was soon found that quantitative data for implementing such models were deficient. Accordingly, the scope of the work was progressively expanded to establish the necessary quantitative foundations. These efforts focused largely on field collection of primary information to establish the underlying physical and economic relationships among vehicle speed, road user inputs, an road deterioration as related to road design and maintenance standards. Over the years between 1971 and 1982, four major empirical studies were carried out in Kenya, the Caribbean, India, and Brazil. The work is documented in a series of World Bank publications entitled.: The Highway Design and Maintenance Standards Study.
The physical and economic relationships underpinning the model derive from a particularly robust combination of analytical and empirical research and may also be applied independently in pavement management, cost allocation studies, and so forth. Validation of the relationships in and beyond the four major studies, has generally confirmed their broad applicability to different situations. There is also provision for model calibration to accommodate specific local conditions: for example, the effect of environment and materials on road deterioration or the effects of fuel efficiency on vehicle operation.
The present model has been extensively used by government planning agencies, consultants, and World Bank to prepare and evaluate road construction, maintenance, strengthening, and rehabilitation programs in more than 30 countries; they have also bee used for research and instruction in several universities as a standard methodology for highway investment planning.
Limitations of the ModelAlthough the model has a sound empirical base, model users should be aware of the following limitations:
T he user confronted with any of the above conditions should recognize that they are outside the empirical base of the present model and should make appropriate provisions or adaptations in the analysis. It is important to recognize, moreover, that the model is not intended to be used for final engineering design nor is it in itself a pavement management system. Rather, it is a tool for economic analysis of alternative strategies either at the project or the network level. The 1995 HDM-III system was replaced in March 2000 by the HDM-4 model which is the recommended software for evaluating highway investment options. The 1995 HDM-III system is available for downloading at the Road Software Tools page, together with the HDM Manager documentation and the HDM-III Volume 1 documentation.
- The road deterioration submodel does not explicitly include the effects specific to freezing climates, nor does encompass rigid pavements. Freezing climates can be simulated by adjusting the default road deterioration factors.
- The model does not explicitly account for the effects of varying basic routine maintenance (shoulder and drainage maintenance, grass cutting, and maintenance of side slopes) on pavement performance, but this can be handle indirectly by adjusting pavement strength parameters.
- The model doe not endogenously predict road accidents or their costs, nor environmental impacts such as air and noise pollution, nor traffic delay costs during road construction or maintenance. Facility is provided, however, to incorporate accident costs, delays, and environmental impacts where exogenous estimates are available.