The incident took place Thursday, when Johnson said he pulled over to rest after feeling faint.
During a Thursday night news conference, Johnson said that, after he had been out with friends for dinner, he realized that he did not take his blood pressure medication.
On the ride home, Johnson said he felt faint and pulled over the vehicle.
He continued, "Responding officers did come.
Johnson said that it was important to "avoid the appearance of impropriety," and called for the internal investigation.