Road bike, average 12 mph, longer distances, non-competative. Shoulder to shoulder is 42 cm, but my drops are positioned at the upper bend, 39-40 cm end to end.

So, according to posts, handlebar should be 42 cm to match shoulders, but with my drops at 39-40 cm, should I opt for a 44 cm width such that, when drops are used, I am actually at the "correct" 42 cm width?

nrsmd, Handle bar width is measured at the drops, center to center;you can also measure left side to left side. There are many variations on the drop bar and the shoulder width/bar width fitment guide is a good starting point.

Brad

That shoulder-width recommendation is just a starting point and a guideline. Do you feel too cramped with your current bars? If so, go wider, if not, don't bother. As bradtx noted, the width is specified at the ends of the drops and can be specified as center-to-center (c-c) or outside-to-outside (o-o) depending on the maker.

Most catalogs that offer various makes of handlebars will say if the listed width is as c-c or o-o. As a general rule the o-o measurement will be 2 cm larger than the c-c measurement on any given bar so one maker's 42 cm c-c bar will be the same width as another makers 44 cm bar specified o-o.

The main reasons not to go too narrow are to avoid restricting breathing, and to keep it easy to control. The main reason not to go to wide is for clearance, particularly in traffic.

2cm isn't going to make a whole lot of difference in that picture; once you're close, it's a matter of preference.