A policy design is a dynamic mix of goals and instruments that develop over time through processes of layering, drift, conversion, replacement and exhaustion. In the face of these dynamics, it is a key concern for policy designers to maintain fit between policy design elements by sustaining goal coherence, instrument consistency and the congruence of goals and instruments. Even though the temporal aspect is fundamental to new policy design thinking, few studies have dealt with the interrelation between policy dynamics and fit. With a longitudinal case study of Dutch transport planning, this research aims to provide insight into this interrelation and to highlight practical implications. This study reveals an intricate and ongoing fitting process between goals and instruments, in which any moment of coherence, consistency and congruence is temporary. During this fitting process, goals and instruments developed in different and largely separate trajectories. In this case, layering successfully improved congruence, but at the same time created inconsistencies between old and new instruments. To resolve some of these inconsistencies, conversion was used. These findings show that policy design is an ongoing process. The main practical implications of this study are that integrating the design of goals and instruments is an essential first practical step, that the ongoing monitoring and evaluation of policy design performance should be a central component in the ongoing process of policy design, and that a combination of layering and conversion can be a successful design approach to adjust instrument mixes to changing goals