You will not know what possible defenses are available unless you request a contested hearing and file for discovery. As part of discovery you will receive the officer’s sworn statement in which he/she will testify as to why you were cited. It’s possible you could find logical flaws in the statement to argue in court. Read the sticky thread Procedural Guide to Traffic Tickets in Washington State for more information.
I doubt there is any harm in contesting but really it boils down to how motivated you are and what your time is worth. It might well be easier just to pay the fine. A few years ago I was also stopped at a speed trap in Bellevue, cited for 5 over and advised by the officer to just ask for a deferral. I didn’t believe I did anything wrong and hadn’t been pulled over for speeding in over 30 years, so instead I fought and won a dismissal.

