Research projects in the field of Cooperative Intelligent Transport Systems (C-ITS) are characterized by high complexity. Coordinating various mutually dependent subsystems throughout the development process, while simultaneously ensuring responsiveness and quality assurance, represents a key challenge for future large-scale projects. Since existing process models often fail to manage such complexity in practice, the need for new development approaches in C-ITS has become evident. Building on four established process models, we combined their respective advantages and designed a new, unified, and user-centered process model for multi-layered C-ITS projects. Results from a subsequent evaluation workshop suggest that this unified and user-centered model can enhance cooperation and reduce complexity for practitioners. By proposing an alternative approach to structuring C-ITS projects, this work contributes to advancing sustainable, highly connected transportation and mobility systems and addresses current challenges in the sector more effectively.