## 5 Concluding Discussion
### Contributions, Limitations, and Future Research  
This study contributes to the literature regarding policy implementation, construction management, and sustainable public procurement through using a cross-country, cross-level comparison, highlighting different ways to implement and disseminate SPP in infrastructure construction. Our study has shown that:  
* International climate policy was important to legitimize and enable individuals to act as champions in the cases we identified as drivers. In other national contexts, it is the basis for organizations to become frontrunners, pushing for carbon emission reductions in the sector as a whole. In the cases we identified as translators, the national level guides how projects work in improving project-level performance. This means that to assess pathways for carbon emission reduction, the country context is essential. * Factors influencing implementation that are deemed critical are consequently dependent on the national context, views on perceived costs and benefits, market maturity, and the level of carbon knowledge in procuring organizations and projects. * As stated in the SPP literature, visions, leadership, and motivation facilitate multilevel learning processes and successful SPP. * Learning is, however, not only built at the project level, but depends on the whole SPP governance structure and is central to the development of collaborations at a market level or in relation to institutionalizing collaborations between clients and procurers. * Market interaction and a high client competence and engagement, sustained over several projects and longer time horizons, are needed; otherwise, suppliers will not invest in new technology or new knowledge. * Ongoing dialogue and learning between government clients and market actors are central for driving long-term change and the development of sustainability in the infrastructure sector, since the two main obstacles for successful implementation are: the lack of (i) knowledge in public organizations; and lack of (ii) sustainable products on the market. It is, however, central to acknowledge that this learning can, and will, take different forms in different national contexts. The SPP field has matured in terms of both managerial and scientific aspects. Thereby, a focus on factors influencing carbon emission ambitions at the project level has become partly obsolete, and a more holistic view encompassing multiple administrative levels involved in governing and implementing SPP needs to become more central to the research field. Even if factors influencing the SPP are similar in the different cases, the methodsof navigating between them are different in different national contexts. This means that suggestions on how to further improve SPP need to be contextualized to avoid requirements that increase bureaucracy and/or transaction costs without delivering results in terms of real carbon emission reductions. The study indicates the importance of engaging in industry collaboration and joint development of products and standards, a perspective that is shared with the sustainable supply chain management field [34; 36; 60]. This is especially visible in frontrunner projects; it is, however, clear that it is not solely a market-based approach, as goals and visions are in line with national ambitions. Some of the frontrunner projects rely heavily on a \"cut carbon, cut cost\" discourse, motivating the industry to innovate jointly. It is, however, important to note that such policies will eventually reach a dead-end where more proactive strategies are needed to meet long-term goals. [PERSON] and [PERSON] [37] argue that the resource efficiency agenda is at a cross-roads where policymakers advocate for proactive approaches, while industry leans on a business-as-usual scenario and resource scarcies are addressed within the current paradigm.