“Human Factors” in highway transportation refers to the capabilities and limitations of the driver. The interface between the human being and the machines he/she uses must not overtax human capability. Drivers cannot be presented with situations that exceed their perception-reaction time, that require supernormal visual acuity, that present them with an excessive workload, or that distract them during the driving task. Any of these situations - and others - can create unsafe situations and the potential for accidents.Throughout the field of Intelligent Vehicle/Highway Systems (IVHS) there is a major need to consider human factors. The particular human factors requirements vary from application to application and cover a wide range of human characteristics and capability limits. This paper is intended to focus on one example of the major importance of human factors considerations in the development of IVHS hardware. The example is the use of variable message signs in Advanced Traffic Management Systems. This example serves to demonstrate that human factors must be very carefully considered throughout the development of IVHS.