Counties imposing a tax under this chapter shall enter into an agreement through the interlocal cooperation act with the department of transportation. The agreement shall provide an opportunity for the department of transportation, cities and transit agencies having within their boundaries a portion of the existing or planned high occupancy vehicle system as contained in the regional transportation plan, to coordinate programming and operational decisions affecting the high occupancy vehicle system. If two or more adjoining counties impose a tax under RCW 81.100.030 or 81.100.060, the counties shall jointly enter one interlocal agreement with the department of transportation.