Research Question: How can the driver’s workload be estimated in order to adapt information and entertainment systems? Approach: Smartphone sensor data, situational factors and basic user characteristics are collected. This data is tested whether it significantly influences workload and can be used to estimate it. Method: Workload is measured with a smartphone-based representation of the NASA-TLX and the RSME during a user study with 20 participants on different road types. Results: Driving situation, gender and driving frequency significantly influence workload. Using only this information and smartphone sensor data the driver’s current workload can be estimated with 86% accuracy using a decision tree.