In the last years, a great deal of research has demonstrated that driving simulator studies can be a suitable alternative to field studies. However, a driving simulator must be correctly validated for each specific aspect of driver behavior such as speeds, steering control, driver response, etc. This paper aims to investigate a behavioral validation of the interactive fixed-base driving simulator in order to verify the DGSIT driving simulator's usefulness as a tool for distraction research on a two-lane highway. Fifteen drivers were asked to complete three levels of a cognitive distraction task (n-back task) concurrently while driving either an instrumented vehicle or the simulator. The three pairs of distacted driving behaviors were analyzed in terms of absolute validity and relative validity. Results suggested that the participants' simulated driving behavior were not significantly different from the on-road driving behavior, establishing the relative validities. Especially, heart rate established absolute validity as well.