According to the AAA Foundation for Traffic Safety, driver inattention is a major contributor to highway accidents. Driver distraction is one form of inattention and a leading factor in most vehicle crashes and near crashes. Distraction occurs when a driver is delayed in the recognition of information needed to safely accomplish the driving task because some event, activity, object, or person within or outside the vehicle compels or induces the driver attention away from the driving task. Although some indexes of driving performance have measured distraction, they are the results of the distraction and not the distraction itself. We directly and quantitatively employ biological signals to measure the distraction by finding useful biological indexes from candidates of various biological signals. Our experimental results using a driving simulator showed useful indexes derived from EEG and ECG.