I always thought of texting as a way of saying some non-urgent thing without the expectation of an immediate reply. If it's an emergency obviously it's a phone call, or if it's something like "hey, can you pick up some peanut butter on your way home?" it is also a phone call because with a lot of people, they may not see the text before they leave and some of them are even smart enough to not read their phone while driving. They can hear the phone ring, and they have the choice to either answer if it's a convenient time or at least listen to a message when they can--which is more likely than a text because a phone call just has a more time-sensitive aspect to it than a text.

Personally, half the time I don't even respond to texts because there is nothing to say in response. I don't want to be that person that just constantly says "ok" after every single text. I usually reserve that for when I'm having an argument and come to a point I see we just won't see eye to eye on it.