Current research and practices on guide signs studies mainly focus on how to set up a particular guide sign. Less attention is on the tolerable or optimal number of advance guide signs on a sign structure. This paper will assess drivers' workload on various freeway guide sign structures and sign information units, and propose the desirable number of guide sign panels and symbol signs on one freeway guide sign structure. A posterior questionnaire survey was conducted after drivers finished the driving simulator test. Drivers' perception and cognition on different sign structures are carefully analyzed using the NASA-Task Load Index. With the statistical analyses of questionnaires and the recorded data from driving simulator tests, the optimal advance placements of guide signs considering drivers' workload are proposed. Recommendations are provided on how to better deliver necessary information to motorists under various geometric and traffic conditions.