Your subconscious mind is the feeling mind. It doesn't respond to words, it just associates feelings.
So you say "I do not want to have a car accident" the subconscious mind picks up the words "car accidents" and that causes a feeling.
Alright, take the following 2 sentences:
"I do not want a car accident"
"I would like to arrive perfectly safe"
Which one feels better? Now that "I do not want a car accident" turns into nervousness instantly, and boom, you just greatly increased your odds of a car accident.