Soundproofing your windows with a double glazing.

Secondary glazing is when an extra glazing panel is placed on the inside of an already existing window. This second panel will help soundproof you window from outside noise sources.
One of the issues with this approach is if your existing windows are drafty, this will allow moisture from the outside to come in, and in the right conditions, the moisture creates condensation between the glass and settles at the bottom of the window.