We have developed a simulator to help with the design and evaluation of assistive interfaces. The simulator can predict possible interaction patterns when undertaking a task using a variety of input devices, and estimate the time to complete the task in the presence of different disabilities. We have evaluated the simulator by considering a representative application being used by able-bodied, visually impaired and mobility impaired people. The simulator predicted task completion times for all three groups with statistically significant accuracy. The simulator also predicted the effects of different interface designs on task completion time accurately. Categories and Subject Descriptors D.2.2 [Software Engineering]: Design Tools and Techniques – user interfaces; K.4.2 [Computers and Society]: Social Issues – assistive technologies for persons with disabilities General Terms Algorithms, Experimentation, Human Factors, Measurement Keywords Human Computer Interaction, Assistive Techn...