Keywords: sustainability, highways, indicators, multi-criteria decision making, analytic hierarchy process, quantification, economic valuation.

In the last two decades, transportation community worldwide shows an increasing interest in the concept of sustainability, as a result of the intensity of phenomena such as climate change, resource depletion, accidents, degradation of quality of life, and inequity of access. In order to incorporate sustainability into transportation decision making, the broad goals of sustainability should be translated into specific objectives and evaluation methods.

Previous research efforts have mainly focused on the definition of sustainable transportation, the identification of sustainability indicators, and the evaluation of alternative actions for the operational improvement of existing transportation systems. However, very little work has been done on sustainability evaluation of new highway projects taking into account all the phases of highway development, from design and construction to operation and maintenance.

This paper presents a framework for the evaluation of sustainability of highway projects. The proposed framework considers environmental, social and economic impacts and results in the development of a composite sustainability index by means of a multi-attribute decision making process. Such an index can rationalise the various impacts of highways on the environment, the economy, and the society, and can be a useful tool for sustainability assessment of new highway projects or for evaluating alternative design and construction strategies.

The development of the proposed sustainability index has resulted from a four-step process. Initially, a set of ten indicators are selected to capture the aspects of sustainability in the context of highway projects. The set includes the following indicators: ecological impacts, emissions, fuel consumption, use of recycled materials, noise, safety, accessibility, operator cost, user cost, and travel time. These indicators are selected from an exhaustive list of indicators that have been examined in previous studies considering their importance with respect to the research target and objectives, diversity, ease of understanding and measurement as selection criteria.

In the second phase, the selected indicators are quantified as a function of highway characteristics. Using models available in the literature, each indicator is related with the geometric, traffic and topography parameters of the highway.

The third phase deals with the weighting of indicators and their normalization in order to incorporate them into the composite index. Employing an analytic hierarchy process method, weighting factors are assigned to the indicators to capture their importance in achieving sustainability. Since indicators are measured in different terms, they are normalized using results of past valuation studies, transforming them into monetary terms.

Finally, the composite sustainability index is developed by means of multi-attribute decision-making for assessment of the sustainability of new or existing highway projects. Multi-attribute decision making can optimize multiple and conflicting objectives such as cost minimization, safety improvement, and environmental impact reduction.

The index developed would be a valuable tool for evaluating new highway projects, for assisting maintenance-reconstruction decisions of existing highways, or for prioritizing alternative highway projects on the basis of sustainability.

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