After emissions standards were made stricter in California, thousands of pollution-emitting trucks became unsuitable in the state. Truck owners sold the rigs to neighboring Oregon where the environmental protection standards are more lenient.
From the story: “The shift undercuts Oregon regulators' limited work to reduce the number of older, higher-polluting diesel engines, which pollute the air that Oregonians breathe daily. Oregon, like other states, hasn't done anything to prevent California's unwanted big rigs from coming in, despite plenty of warning. California adopted its rules in 2008.”
Read the full story by The Oregonian/OregonLive.