According to the Noble Eightfold Path, is it poor practice to speak poorly in solitude? I had this question yesterday, when I was using my navigation system to navigate somewhere completely new in my car. I, by accident, took a wrong turn that resulted in multiple miles of detour. When I realized my mistake, I verbally spoke an explicit word out loud. I was completely alone in the car at the time.

I immediately realized my second mistake in speaking as such and I verbally corrected myself to a non-explicit word.

The Noble Eightfold Path puts emphasis on right speech, and not saying anything to harm others and self. I was not meaning to harm anyone in my outburst, but to express the anguish of a delayed trip.

Buddhism writings (specifically the Dhammapada) also put emphasis on sage qualities, more specifically in the quote "a sage tames himself". Certainly what I did in that situation was not a kusula action, and thus can help breed other akusula actions.

My question is: Is it bad practice to display this kind of poor self-control when in solitude?
Solitude is the best time to practice skillful behavior!  When you don't have anyone else watching you, it's that much easier to lag in your discipline.  The best noble qualities are ones that are intrinsic to your character.  They show that your integrity is deep and that your practice is strong.