Vessel queuing procedures at maritime ports may induce strategic changes in the polluting activities of transport operators. This paper documents a reduction in vessel emissions following the introduction of San Pedro Bay's new ETA-based queuing system. Using geospatial vessel position data in monitored US coastal areas, alongside records of vessel departures at foreign ports of origin, I show that the global emissions of containership voyages servicing Los Angeles and Long Beach declined, on average, by 10%. The added certainty of vessel admittance times incentivized ships to slow their voyage speeds and strategically limit local wait times. However, guaranteed queue positions also attract greater commercial activity, which led to an overall deterioration in local air quality, despite per-vessel emissions falling.