Freight moving between ports, rail yards and warehouses travels along a limited set of corridors. The emissions from that movement are concentrated where the corridors run.
Why freight routes are narrow
Trucks serving ports and distribution centers use designated truck routes, chosen for road weight ratings, turning radii and interchange access rather than for population distribution.
Warehousing clusters near those interchanges because proximity reduces drayage cost, and each facility generates a predictable volume of truck trips per day.
The result is heavy repeated traffic on specific streets, with adjacent blocks carrying almost none, producing sharp differences in exposure within a short distance.
What concentration does to local air
Diesel exhaust contains fine particulate matter and nitrogen oxides, and concentrations of traffic-related pollution fall off with distance from the roadway.
That decline means the effect is intensely local. Homes, schools and playgrounds within a short distance of a heavy corridor experience conditions unlike the metropolitan average.
Regional monitoring networks are designed to characterize an area rather than a block, so a corridor's local conditions may not appear in the regional figure at all.
Idling and yards add to through traffic
Emissions come not only from vehicles in motion but from queueing at gates, idling during loading, and yard equipment operating in a fixed location.
Refrigerated units run while stationary, and rail yards and ports host locomotives and cargo handling equipment with their own emissions profiles.
Because these sources are stationary or slow-moving, their output accumulates in the immediate area rather than dispersing along a route.
Why land use decisions set the pattern
Industrial and heavy commercial zoning tends to sit alongside transportation infrastructure, and housing near such zoning has historically been less expensive.
Highway routing decisions made in earlier eras determined where corridors run, and those routes are effectively permanent because relocating them is rarely feasible.
Current warehouse siting decisions follow the existing corridors, which reinforces the pattern rather than redistributing it, and this is where much local dispute now occurs.
What is being tried and what limits it
Responses include anti-idling rules, cleaner fleet requirements, electrification of yard equipment, buffer distances between facilities and homes, and truck routing restrictions.
Authority is split: vehicle emission standards are largely federal and state, while siting, buffers and local routing are decided by cities and counties.
That division means a community can win a local siting condition and still face traffic governed elsewhere, which is why disputes often run in several forums at once. Rules vary by jurisdiction and change over time.