Rijkswaterstaat has introduced the two-stage model in the Dutch construction industry in order to reduce risks in large and complex projects (Rijkswaterstaat, 2020a). However, it is also possible that the two-stage model can influence the sustainability of infrastructure projects since the risks are reduced and a different way of collaboration between market parties and public parties is introduced. Nevertheless, it is unknown if the two-stage model makes an impact on sustainability of infrastructure projects. Boskalis Nederland, where this research is performed, is interested in if the two-stage model can make an impact on sustainability of infrastructure projects because of the high relevance of these two topics in the Dutch infrastructure sector. Therefore, the main objective of this study is to explore the impact of the two-stage model on sustainability of infrastructure projects. Taking into account the knowledge gap, the problem statement and the research objective, the main research question is defined as follows: "What is the impact of the two-stage model on sustainability of Dutch infrastructure projects?".

The expectation regarding the impact of the two-stage model on sustainability of infrastructure projects is that the model will offer market parties and public parties more room to discuss, incentivise and implement sustainable solutions in infrastructure projects because of the integral collaboration between the parties in the project team. The impact of the two-stage model on sustainability could be seen when comparing the model to a one-stage collaboration model. For example, a technical specification (which is a one-stage collaboration model), is characterized by having result-oriented specifications prescribed by the public client which results in leaving little freedom for the contractor for sustainable input in infrastructure projects. In contrast, the two-stage model is characterized by collaboration between the involved parties which creates more freedom for input on sustainability from the involved parties. This may lead to a bigger chance to implement sustainable solutions in infrastructure projects. The essential difference between the two collaboration models is the collaboration aspect between the involved parties, which eventually may make a difference in chances to implement sustainable solutions in infrastructure projects.