To develop easy-to-use control panels, it is essential to measure driver performance, compare it with behavioral specifications, modify the design based on driver feedback, and then retest. Rapid prototypers help engineers do this quickly. This paper identifies the I/O capabilities, ease of use, ability to record user behavior, and real-time performance for several prototypers.Two example HyperCard prototypes are described here. The first, a car clock, shows how HyperCard can vary button size and location, labeling, auditory feedback, and the mapping of switches to system functions. The second, a car radio, shows that continuous controls and digitized sound can be handled.