In recent years, the importance of integrating sustainability into infrastructure and mobility projects has been increasingly acknowledged. While such ambitions are often clearly formulated during early planning phases, such as during policy alignment, scope definition, or ambition-setting, they frequently lose traction as projects move closer to execution. This thesis investigates this phenomenon, known as sustainability ambition erosion, by examining how and why initial sustainability goals weaken throughout the decision-making process and what success factors may help to counter this.

The study begins with a systematic literature review, which categorises the main barriers to sustained sustainability ambition into six thematic groups: conceptual and motivational, economic and financial, organisational and cultural, knowledge-related, governance and policy, and stakeholder and participation. For each category, corresponding success factors are identified to explore potential counterforces to ambition erosion. These theoretical insights are then empirically tested through a qualitative multiple-case study of three Dutch infrastructure projects, combining ten in-depth interviews with public clients and engineering consultancies. The projects span different scales, phases, and governance arrangements, and are analysed along three dimensions: the project life cycle, the stakeholder structure, and the decision-making levels: strategic, tactical, and operational.

The empirical findings confirm many of the literature-based barriers and success factors, but also reveal that several of the predefined categories were either too narrow or not sufficiently reflective of practical realities. In particular, certain codes, such as time pressure, or project-specific constraints, proved difficult to categorise within the existing thematic structure, while others overlapped across conceptual, organisational, and behavioural domains. As a result, the categorisation of both barriers and success factors was revised to better account for how these dynamics manifest in real-world projects. This led to a more practice-informed classification, which more accurately captures the interdependencies and contextual nuances of ambition erosion and reinforcement mechanisms.
Notably, intrinsic motivation, informal leadership, and team culture emerge as more influential than formal role or mandate. Furthermore, stakeholder influence is found to depend more on behavioural agency than on their specified roles. Intermediate users, such as project managers, technical managers, and sustainability advisors, played a decisive role in maintaining or abandoning sustainability goals, especially during transitions between project phases.

Ambitions are most vulnerable to erosion during the elaboration phase, where design choices and technical detailing occur, and during phase transitions, where lack of continuity, timing misalignments, and weak anchoring often lead to fragmentation. Financial constraints, conceptual vagueness, and temporal pressure and misalignment were particularly salient across cases. At the same time, success factors such as intrinsic motivation and leadership, engagement and steering power, and contractual and procedural governance mechanisms were found to support the retention of sustainability ambitions. Additionally, several context-specific barriers and success factors emerged that could not be generalised, underlining the importance of project-specific reflection and flexibility.

The study concludes that maintaining sustainability ambition throughout the project lifecycle requires both systemic and human-centred interventions. Rather than relying solely on frameworks or instruments, projects need an integrated strategy that combines structural anchoring (e.g. KPIs, contracts), behavioural ownership (e.g. motivation, leadership), and procedural attentiveness (e.g. continuity, timing, phase-specific tools). These three conditions, structure, behaviour, and procedure, must be addressed in parallel if sustainability ambitions are to withstand the complex realities of infrastructure development and result in lasting impact.