Most projects require a permit. Most permits require insurance.
Budget for it, plan on it, and get good coverage. The End.
You will need commercial general liability insurance, usually for $1,000,000 in coverage. It'll cost somewhere between $400 and $600 for basic events, but the price will vary with complexity and size. Call up your insurance agent or get Google on it.
Truth is, anything can go wrong at any time, and public space projects, by their nature, invite interaction and it's cousin, mayhem. Unless you're planning on patrolling your installation 24/7 or be at all places at once during your event, insurance will protect you from any unforeseeable uh-oh moments without needing to test Murphy's Law.