I don't know which roads ought to have been closed, I didn't do the safety case for the air show. My point is that some of the people who did, failed to have it carried out.
They didn't know, but they suspected, and agreed that it was important to mitigate the potential, then failed in their duty to do so. This is not hindsight, they saw it coming and formulated a plan to minimise the danger, then failed to execute the plan.