This is an important question to ask yourself, particularly if you are in the car with your spouse. Ask yourself, "Is the awful weight of the decisions that must be made in the next few seconds and minutes, that will affect whether I and my loved ones live or die, mine to bear? Is it my skill and alertness, and those things alone, that stand between a crippling, fiery impact with multiple tons of hurtling steel? Indeed, am I the one holding the wheel?"
If the answer to these questions is "no," then you are the driven. Try to restrict your commentary on the driver's style and habits: it seems that for some reason such criticism can be particularly hurtful (and distracting) when it comes from a loved one. Of course, if you think something bad is about to happen, it's better to be safe than sorry, and you should speak up. On the other hand, if it's just a matter of taste, perceived efficiency, or something else: the car's not the place to bring it up. Remember that your spouse has just answered "yes" to the questions above, and no matter how comfortable they are in the driver's seat, they probably have at least an inkling of the gravity of what they're doing. If you tell them they're doing something wrong, they will get defensive, and maybe a little scared, which you don't want for a number of reasons.
Of course, if you are in fact the driver, you had best stop pondering this and get your eyes back on the road!