There are a few different ways that people measure road handlebars, but the most common way is from the center of the bar to the outside edge. This gives you a good idea of how wide the bars are, and is generally the measurement that is used when talking about handlebar width. Another way to measure road handlebars is from the stem to the end of the bars. This method is less common, but can be useful if you want to know how much space you have for your hands on the bars.