In the past decades, sustainability has become increasingly important for the construction industry, in particular highway infrastructure, as it can generate both positive and negative impacts on the social and physical environment. As a result of the impact on the environment and the growing awareness of environmental protection, the effects on society and economic development, there is a pressing need and sense of urgency for the sector to become more sustainable. For a highway to be truly sustainable it needs to take the social, environmental and economic dimensions of sustainability (People, Planet, Prosperity) into account. Over the past decades, some sustainability assessment (SA) approaches have been developed in an attempt to integrate sustainability in the assessment of infrastructure projects. Despite these attempts, these SA approaches do not address all the dimensions of sustainability thoroughly and are biased to either an environmental or an economic assessment, while the social dimension is taken less into consideration. However, how designers and decision-makers can integrate these dimensions into the assessments of highway design options is less known. This lack of integration of the three dimensions of sustainability makes it impossible to evaluate and assess the sustainability consequences of highway design choices and options and thus represents a significant limitation.

To address the observed problem, the main objective of this research is to not only provide designers and decision-makers with a SA tool that integrates all three dimensions of sustainability, but also with an understanding of how this can be used.

The triple bottom line (TBL) theory coined by John Elkington, a well-known concept in sustainable development (SD) was selected as a sound theory regarding the three interdependent dimensions of sustainability. A total of 64 SA criteria from the construction and infrastructure sector were identified through surveying recent literature, which formed the preliminary list of SA criteria. This was followed by a filtering process on the SA criteria and subsequently a questionnaire survey to establish which are relevant to the Dutch highway context. This resulted in the conceptual SA framework which consists of 36 SA criteria categorised in their corresponding dimensions of sustainability and related 9 themes. Next, the assessment procedure was provided for the conceptual SA framework, to form the proposed SA tool. In this tool, the best-worst method (BWM) is used to determine the weighting factors of the themes depending on the project. The tool was tested by applying it to a reference case (a highway project in the Netherlands). Based on the results from the reference case, the applicability of the tool was evaluated by an expert involved in the project. In the developed SA tool, the three dimensions of sustainability are integrated into one comprehensive framework specific for the Dutch highway context.

The conceptual SA framework provides a systematic overview of all possible aspects of sustainability that can be taken into account during the planning- and design phase of a highway project. In addition, it can help designers and decision-makers understand how the dimensions of sustainability can be used to decrease the negative impacts of highway projects and embrace the principles of sustainability with respect to environmental protection, economic profitability and human well-being, to make their highway designs more sustainable and to contribute towards SD.

The proposed SA tool can assess, compare, evaluate and rank design options based on the SA criteria. The SA criteria assess to what extent a design choice and option contributes to the creation of sustainable added value. In this tool, design options can be explicitly weighed upon all dimensions of sustainability, related themes and corresponding SA criteria which can support the decision-making.