Driver fatigue and distraction are major road safety issues globally; developing driver state detection and warning technology to help reduce impairment-related incidents is a promising approach. The aim of this case study was to design an effective Human Machine Interface (HMI) for a near-market driver warning system primarily aimed at commercial truck driving. A human-centered design (HCD) process was employed for the development and evaluation. Application of HCD here was a multi-stage iterative process: a comprehensive literature review; developing a context of use description; undertaking truck driver interviews; identifying user needs and associated design requirements; conducting two design workshops; operationalizing the design; running HMI evaluation studies, and finalizing the HMI concepts. As a result of the iterative HCD process, the HMI comprises a multi-modal warning system (visual, auditory and tactile) with two levels for driver fatigue and an escalating system for driver distraction. Following this successful HCD process, further on-road evaluation work is proposed before the final version of the HMI is manufactured.