Cognitive resources are not required of me to initiate and guide the complex sequence of behaviors required to bring the vehicle safely and smoothly to a stop. Rapid, accurate, effortless performance that makes no demands on valuable conscious resources has obvious advantages. The problem is changing a behavior - after it has become autonomous. For example, an experienced driver would take longer to learn to reliably stop at a green light, than he originally took to learn to stop at a red light. Until the new habit is acquired, the driver must pay attention in order to over-ride the well practiced behavior of driving through a green light.
Autonomous behavior can be over-ridden, but it requires conscious attention to do so.
The karma of repeatedly following the path to immediate gratification is that this path becomes autonomous. As a result, whenever conscious resources are occupied by a demanding social situation, powerful emotional state, or diminished due to fatigue or intoxication, one tends to follow this default path. An absent-minded relapse occurs when mindful processing, which is necessary to interrupt the autonomous sequence, is not invoked when needed. This may occur when a person was simply not conscious of the original commitment until the relapse sequence was already well under way. Less dramatic but quite common, the person was more or less aware of the unfolding of the sequence of events leading to the lapse, and was also fully aware of the previous intention to remain abstinent, yet simply failed to dedicate the conscious effort required to interrupt the autonomous behavior chain.