It sounds like you might have a valid defense in that you were not within the specific speed zone of 55 MPH. I think this is the better defense than the fact that you were slightly over 55 mph, even though you were passing. This doesn't seem to work with most judges. But regarding the speed zone issue, the officer must indicate in his report that you were within a specific speed zone and many officers actually specify where the signs are posted. Overall, it appears you have some issues to challenge the ticket. As with any ticket, i would retain an experienced traffic attorney to handle it for you
I can't provide you specific legal advice, but I can observe that if a ticket is challenged police are required to provide certification that the radar unit is working properly. If the officer is moving at the time instead of stationary, then police also need to provide certification that the patrol car's speedometer interface with the radar is working properly. Additionally, there is a recognized margin of measurement error that should be accounted for, each unit is different but the officer should be required to provide testimony. When factored together, a ticket for 5 mph over could be subject to challenge.
I will note that I believe that the defense of slightly exceeding the speed limit in order to safely pass a vehicle does not apply when both lanes are going the same direction. You might want to talk to a lawyer to evaluate your options. Good luck.
I have been licensed to practice in the State of Oregon since 1990. I am not offering legal advice regarding your question, only general information regarding the law. You are not my client nor am I your attorney unless we sign a retainer agreement.
You are not allowed to go over a posted speed limit in Washington "with a reasonable margin of safety" on a roadway with two lanes in the same direction. The law in Washington is that you can only go over the speed limit to pass on a roadway with fewer than two lanes in each direction so that you may pass. Even then, if you speed it must be safe to do so.
In your case, you might have a defense to where your vehicle was on the roadway at the time you were going 5 over. If you were in a 60 zone, you might have a defense. There are probably others. Nevertheless, as a commercial driver you will want to try especially hard to keep this off your record, as a moving violation can create employment as well as insurance and driving privilege consequences.
This answer is given merely for informational purposes and does not create an attorney-client relationship. For specific advice, contact an attorney in your state to see if working together makes sense.
Since you were not yet in the 55 mph speed zone, I believe that this citation can be challenged on that basis. The argument that you were slightly exceeding the 55 mph limit that was about to commence due to passing is likely to be less convincing to a court. I would recommend retaining an attorney to handle the case. My advice to clients is to challenge all traffic tickets, since the first citations are more likely to be favorably resolved since the driver's record is clean. If you wait until you already have infractions on your record, prosecutors and judges are less inclined to give you the benefit of the doubt.