Improved decision support is deemed essential for the planning and implementation of sustainable transport solutions, but limited evidence exists that decision-relevant information is effectively used for these purposes. This paper applies a framework inspired by research in “knowledge utilization” to examine to what extent various kinds of decision support are used and have become influential in three different planning situations—a local cycle plan in Copenhagen, the Stockholm congestion charging trial and the UK national transport strategy. The results reveal the extensive use of decision support but also the difficulty of unpicking its exact role in each case. Stockholm presented the most successful case, with a mix of academic and experience-based knowledge inputs facilitating understanding and acceptance. The cycle plan example revealed very limited influence of cycling design guidance. The UK national transport strategy fell somewhere in between with evidence of assessment and monitoring of the plans being well bedded in the culture of the organizations involved, but less supportive of sustainability objectives. While decision support and monitoring are clearly relevant, they provide no guarantee for the implementation of sustainable transport solutions.