First, there is a specific statutory provision that allows a majority (or more) of the city council to travel together so long as there is no discussion of city business amongst them. RCW 42.30.070 states in part:
(It shall not be a violation of the requirements of this chapter for a majority of the members of a governing body to travel together or gather for purposes other than a regular meeting or a special meeting as these terms are used in this chapter: PROVIDED, That they take no action as defined in this chapter.
Note that "action" is defined in RCW 42.30.020(3) to include discussion of city business:
(3) "Action" means the transaction of the official business of a public agency by a governing body including but not limited to receipt of public testimony, deliberations, discussions, considerations, reviews, evaluations, and final actions.
The above restrictions apply when a quorum of the council travels together, such as, for example, to Washington D.C. to meet with Congressional representatives, or to a meeting held by another agency. For a seven-member city council, four councilmembers constitutes a quorum, so two or three councilmembers can travel together and discuss city affairs without any OPMA implications.
Councilmembers should avoid during their trip any conversation regarding city business that is done when a quorum is present (even if only two members are speaking), and the council should avoid any serial conversations on a city issue that would result in the participation in that conversation of a quorum of the council.
At the last meeting prior to the council trip, it might be wise to have the city attorney (if present at the meeting) or the mayor remind the councilmembers of the OPMA restrictions on discussions of city business by a quorum outside of an open public meeting. That lets the public know that the council is aware of the OPMA restrictions. Keep in mind, however, that some members of the public may nevertheless be distrustful of council compliance with the OPMA when a majority of more of the council travel together.
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