Some of the drivers are nominated by their verbally abusive spouses. The hosts will point out to them how much of a detriment they are to their driving spouses, and they actually turn around their own behaviors. Sometimes, the epiphany is a Tear Jerker: one verbally abusive spouse (considered the worst they'd had on the show) went on a drive with the host with his own verbal abuse being played back at him during the drive. The man was absolutely crushed to realize how bad he was and made a conscious effort to change. His wife's driving performance (to say nothing of their relationship) improved drastically.
Aaron's graduation in Season 7. He consistently refused to graduate because he wanted to get everything he could out of rehab and he wanted to be a warning to the rest of the contestants. He was therefore the last graduate of the season. You can't help but smile when he does his final challenge (a drive on public roads) and performs almost perfectly (after accidentally running a stop sign); he is grinning from ear to ear and Andrew is quite relaxed.
Really, any time a driver graduates looking like he or she has become a good driver — not just "less bad than when they arrived" or "the least bad of those remaining".
The show really caring for Angelina's mental health, as evidenced by Dr. Kiru's request to turn off the cameras, and the offer for Angelina to obtain the necessary treatment away from the show. The show even took time to advertise a Mental Health institution saying that staying in silence is no way to help those who need it. Andrew's compassion and concern towards Angelina and Christine in their final episode also qualifies, especially given that he is the one to drive them back to the drivers rehab centre, as opposed to Christine doing so.
Brad, not only improving his driving and becoming the final graduate, but also his wife Donna learning that she was part of the problem and changing her ways, resulting in not only him being a better driver but them being stronger as a pair. Donna apologized to Brad, and when he was announced to be the final graduate, she cheered and said "He worked hard for this!"