Despite centuries of studying human body remains a great mystery but human brain is a much bigger one. I don't believe in hypnosis when someone can make another person do stuff he would never do willingly and don't even remember that. It must be just some kind of a show trick. But still, I do believe that we have at least two consciousnesses - conscious and subconscious.
When I studied psychology I had to memorize all the definitions because there were too many of them and in my point of view they were too complicated even for simple notions. When people try to define things that cannot be touched, measured or can be perceived differently by different people there is no right way of doing it and inevitably you will turn it into a philosophy session. It is like going to a party, all dressed up and beautiful, to seduce a guy you like only to end up in a kitchen, discussing if cubical planets were ever possible or was Doom better than Duke Nukem and going all the way to the meaning of life dilemma. Surprisingly, you may reach at some wise conclusions but that is not why you came. The same is true for most theoretical sciences like physics, for example. You have to think about how something works and why, without knowing for sure that it even exists so all you can do is make predictions based on indirect evidence and calculate probabilities, hoping that someday more advanced experiments will either prove or disprove your concept. The problem is, you may be long dead before it can actually be achieved, if ever.
That is why I am not sure if any definition is necessary as intuitively we all know what conscious and subconscious means but for the sake of providing unnecessary facts I chose the simplest definitions from the merriam-webster.com dictionary, the ones that work best for my understanding of the self-awareness and the topic of this article:
Conscious - the state or fact of being conscious of an external object, state, or fact
Subconscious - existing in the mind but not immediately available to consciousness
While learning to drive a car with a manual gearbox, I had to force myself to pay attention to so many things - steering was the easiest one, but a change of gears, signs, traffic lights, pedestrians, other cars and most importantly, my instructor's cup of tea on the dashboard... Well, a hell of a lot was going on around me and I was not able to track it all, my head was about to blow up after every lesson I had. With practice and experience some automaticity appeared and one by one I stopped noticing how I do things. First, I stopped looking at the speedometer and tachometer and changed gear when I felt that it was the right moment, then I stopped thinking about using mirrors, I just looked when necessary, the last thing I got used to checking was the traffic lights and signs. I still have problems with noticing them in new places but it is mostly due to their bad positioning (or that is what I prefer to believe). So at some point, I learned to drive automatically almost without using my brain. Sounds a little disturbing, yes, but in a way, it is a more superior way of doing things then overloading the brain with unnecessary activity. Ability to do things without thinking about further actions in detail using your subconscious leaves you the time for more interesting things you can engage your conscious with. Automaticity or a muscle memory is the key to highway hypnosis, only when you get to this point of doing something you can experience it. So what is it anyway?
The concept was first described in the 1920s as a "road hypnotism" and in 1960s it transformed into a "Highway hypnosis". Recently, new terms "Driving Without Attention Mode" (DWAM) or "Driving without awareness" (DWA) were introduced and are being used in researches and studies of this phenomenon. Though naming changes, the meaning remains the same. To put it simply, it is a trance-like state when a person can perform some familiar actions without being aware of it or being able to recall any details of this activity later. The "Highway hypnosis" or the "White line fever" term is used specifically in relation to long-distance driving but it can be applied to any activity like walking, running, doing housework, anything that can be done on autopilot without conscious effort.
Actually, there may be an answer to what our subconscious is and why we are able to perform without thinking. The "Default Mode Network" (DMN) is a network of interacting brain regions that is mostly active while we are not focused on any external task, consumed by our inner world, our thoughts, dreams, memories, future expectations. This default mode deactivates when we have a task to perform and it activates in a fraction of a second after the task is completed and no specific cognitive demand exists.
Driving, as an example demonstrates that the phenomenon can be induced during monotonous, uneventful and predictable trips on straight and smooth roads without traffic lights, crossings or even bumps (though bumpy road, in general, can add to the effect if it is happening repeatedly or at a certain rate) often accompanied by a gaze at a fixed point like a lane divider. When experiencing this state, level of attention decreases, person may keep staring into space without a specific focus, blinking time increases, micro-sleep may occur and that may end very badly as reaction time to unexpected situations increases hugely. At this point, it is most important to try to avoid this state fully and there are rather obvious ways of doing it:
Make stops, take rests
Listen to music, irritating one is better
Make yourself uncomfortable, turn AC on at max, sit straight
Talk to your passenger if you are not alone
Do something mind engaging, requiring cognitive activity, look around you noticing details
So if you ever found yourself in a situation when you arrive at your destination without remembering any details like route, any kind of events, or just any details of the surroundings and it feels that the trip that usually takes hours lasted just minutes, this is it, you may have experienced the highway hypnosis.
As a time saving feature this may seem tempting, still, I would recommend keeping attention level high as it is not only your health and life at stake. Drive carefully, be vigilant and don't risk it if you are tired as fatigued driving is considered to be much more dangerous as falling asleep on wheel leaves you little chance to wake up.