The safest drivers exhibit a healthy lifestyle that includes freedom from legal and illegal substances, sufficient rest, and control over personal stress. That's true.
The safest drivers have many good qualities. They live a healthy lifestyle. Driving while have any illness or medical problem could led to disastrous results.
They want a whole freedom: legally and from illegal substance like drugs or alcohol during their driving time. They are also free from stress. Their driving judgement isn't controlled by their emotions (mental condition).
They also take sufficient rest. They rarely drive more that 8 hours a day and with taking few breaks between their work.