A speedy trial waiver is usually required to get a continuance (postponement) of the Court hearing to another date. This is a very routine procedure.
Put simply, in Washington State you have to be brought to Trial within 90 days of arraignment or first appearance. This time-frame is shortened to 60 days if you are in Jail. In most DUI or Physical Control cases you will waive speedy trial several times. (you will execute a speedy trial waiver at Court) Typically, this means the 90 or 60 days begins to run anew . Usually, it is in your interest to have more time for your DUI attorney to interview witnesses, investigate and prepare your case as well as try to resolve it. (there are also exceptions to the speedy trial rule that can also extend the time for trial)