z₁ = (observed − expected) / σ

observed = this hour's mean congestion so far; expected = μ(calendar month, hour of day) + slope × months elapsed, with the trend term applied only where the Mann-Kendall p < 0.05; σ = the standard deviation of that same historical baseline cell.

Deviation from the node's expected congestion for this calendar month and hour of day, in standard deviations of the historical baseline. The sample count resets every hour, so a reading in the first few minutes after the hour turns over rests on very few intervals — shown alongside the value, not hidden.

A hollow dot on the map is a node with no seasonal value — not a zero.