This paper describes an effort to develop a valid and reliable process for comprehension testing of candidate automotive symbols and to conduct comprehension testing on a set of new symbols being considered for in-vehicle active safety systems. The comprehension testing process was developed though a multi-year effort, supported by Society of Automotive Engineering (SAE) and other organizations, aimed at generating a test methodology that would: yield high-quality comprehension data for new automotive symbols, provide clear and specific guidance back to symbol developers based on the test results, and could be adopted and performed internationally to support international standards efforts. Seventeen (17) candidate symbols were evaluated for three classes of in-vehicle active safety systems: forward collision warning (4 symbols), side collision warning (6 symbols), and lane departure warning (7 symbols). So far, testing has been completed in Germany, Sweden, Japan, and the United States. In the US testing, the study yielded comprehension data, appropriateness rankings, and diagnostic design feedback for all 17 icons tested. Based on these data, US recommendations have been made to the International Standards Organization (ISO) for all three classes of in-vehicle safety systems. This paper describes the process associated with developing the procedure, including the international outreach required to obtain support from major ISO countries, as well as the methods and results from the US testing.