It's not always a fully contious decision. I have seen people get DUI's while having a plan not to drive or not planning on drinking/just having one drink, but they end up getting drunk and plans fall through and then they make the determination while they are drunk that they are "sober enough" to drive home. The failure was in not having a plan and back up plan to get home safely before he went out, but I would suspect he didn't go out that night with a plan to get drunk and drive, or maybe he did, but none of us knows that.