A traffic ticket is a notice of violation. The statute of limitations is to prevent one from being accused of a crime when the witnesses may no longer be available and defense difficult. In this case, you have already been notified of the violation and have not defended against it in the time allotted. As such, a traffic ticket does not expire and is not subject to a statute of limitations.
The above is true but I just called Wisconsin DMV and they have no record of my 1996 no seat-belt ticket, so the is no SOL but they do go off your record after some time :). The best thing to do is to call DMV and find out.