State patrol or state highway jurisdiction?

Hey , i live in washington so preferably a washington state user would help the best but federal law in general is pretty much the same. I was pulled over by a highway patrol cop for a seatbelt. My problem is that he pulled me over on a street.. however its not a small street, its a bigger 35 mph street. On the ticket it states i was driving on a public highway without a seatbelt. Also note the technical city limits starts just before where he pulled me over.. meaning i was already in city limits. He wrote the ticket as 15th st/ fleshman way.. 15th isnt a highway.. fleshman counts as one .. so i am wondering if he purposely wrote that to qualify it as a public highway. By the time he actually turned around and pulled me over i was on fleshman and with my seatbelt on so what would you say?