Using the <highlight> tag inside <layer> or <condition>, you can draw a line over or parallel to a road or any other line object. For example, a green line next to a road is commonly used in road atlases to indicate a scenic route. Another application, particularly suited to dynamic maps, is to show traffic density. Here's an example:

The highlight is added using this code:

<highlight distance='120%' width='40%' fill='green'/>

You can have more than one highlight, and highlights can be placed on either side of the line. Negative distances indicate that the highlight goes on the left of the line, and positive distances place it to the right. Left and right are defined by the direction of the line data supplied by the map database. Highlights can have borders and repeated symbols, just like ordinary linear features.