Demand or Congestion? Disentangling Event-Related Emissions Using a Directional Control on I-15

Before/after studies of planned special events routinely report elevated emissions and attribute the increase to event-induced congestion. We show that this attribution can be wrong, and we give a cheap test that separates the two causes. Using one-minute detector data from LATTICE, an operational freeway analytics platform deployed on the Las Vegas network, we analyze the 2026 Electric Daisy Carnival on Interstate 15 across both travel directions. Northbound, the arrival direction, loses 9.5 mph during a five-hour evening window. Southbound, over the same days and the same weather, loses nothing. Yet CO₂ rises by a nearly identical amount in both directions (+11.6% vs. +11.8%), which means the headline emissions increase is driven by weekend travel demand, not by the congestion. Treating the uncongested direction as a control, a difference-in-differences estimate isolates the congestion-specific component. Only PM_2.5 retains a substantial residual (+8.9 percentage points); CO₂ and NO_x retain essentially none. The residuals order monotonically by each pollutant’s measured low-speed emission sensitivity (Spearman ρ = 1.00, n = 4), a sensitivity we recover independently from the same detector stream. The practical consequence is that event-emissions claims based on a single direction should be treated as demand estimates unless a control corridor is included.