Radar sensors have been used for active safety in cars for many years. An ongoing research project explores how radar sensors and technology common in automotive vehicles can be transferred for use on bicycles. Workshops have been used to generate ideas. A bicycle simulator is planned to be used for test and evaluation. Tests on a test track has been used to simulate high-risk scenarios. This paper describes the design process of this project, with focus on the user interface. High-risk scenarios and requirements are identified, followed by identified design challenges and design activities, including evaluation. Ideas for a dual HMI approach, directed towards the bicyclist and towards surrounding traffic are presented.