Most people encounter planning once, at a meeting about something already decided, and conclude the process is theatre. Sometimes it is. Often the real opportunity was earlier and less advertised.

Engagement has formal and informal halves. The formal half is comment periods, advisory committees and public hearings, all minuted and answerable. The informal half is where positions are actually formed: technical workshops, committee agendas published in advance, and the questions a staff planner will answer by email if asked plainly.
Specific beats general, and local knowledge beats sentiment. "This corridor is unsafe" is noise. "The eastbound left turn backs into the through lane between 4 and 6 pm, and there have been three rear-end crashes at that approach in two years" is something a planner can act on, because it is checkable and points at a fix.
Advisory committee seats are frequently unfilled and recruit quietly. For anyone willing to read agendas, it is the cheapest available influence over how decisions are scored.