From my experience: non-emergency situations, I try to consider who is the *best*, first person to deal with this. For example, if there's an issue in the classroom, I would email the teacher and cc the principal. Emergency situation: walk into the office and talk to the secretary. She would discern if the situation was something to schedule time with the principal for (and I've asked to schedule time for non-emergency issues which arose where I felt she needed to be informed) OR the secretary will try to call the principal to meet with you ASAP. When my son was in first grade and being physically hurt regularly by a child who was bullying him, I decided to go back to the school and pull my son out until they could come up with a safety plan for him. (That was a last-resort move, but it got the staff's attention, finally) At that point, the principal and assistant principal did drop what they were doing to address the situation and it was resolved for the rest of that school year.
So, it helps to consider what level of import your communication has. If it's about the previously-mentioned traffic circle, they may have already had an earful and are just in 'dealing with this' mode. If there is a safety issue, don't wait for a call back, go to the office.
And remember-- principals are people too. They just have about a gajillion parents to answer to plus fires large and small to put out. Don't stand on propriety on this one and don't take it personally if you don't get a call back. I only once expected a call from our principal, and that was due to some scary, threatening behavior on the part of a classmate who had already physically hurt my kid. (same one as first grade). And even then, we ended up communicating via email, but only after I'd addressed it with the teacher and received an apathetic response.