Rolling coal is the practice of modifying a diesel engine to increase the amount of fuel entering the engine in order to emit an under-aspirated sooty exhaust that visibly pollutes the air. It also may include the intentional removal of the particulate filter. Practitioners often additionally modify their vehicles by installing smoke switches and smoke stacks.
Rolling coal is a form of conspicuous air pollution, for entertainment or for protest. Some drivers intentionally trigger coal rolling in the presence of hybrid vehicles (a practice called ‘Prius repellent’) to taunt their drivers, who are perceived as being environmentally motivated in their vehicle choice. Coal rolling may also be triggered at foreign cars, bicyclists, and pedestrians. Users cite ‘American freedom’ and ‘a stand against rampant environmentalism’ as reasons for coal rolling.
In addition to environmentalism concerns, it’s been noted that this phenomenon also has health risks associated with it, particularly respiratory issues, and the American Cancer Society has linked exposure to diesel exhaust to lung cancer. A more actionable concern is road traffic safety violation, as the black smoke can intentionally impair visibility, risking motor vehicle crashes.
In 2014, the EPA stated that the practice was illegal, as it violated the Clean Air Act which prohibits the manufacturing, sale, and installation ‘of a part for a motor vehicle that bypasses, defeats, or renders inoperative any emission control device [and] prohibits anyone from tampering with an emission control device on a motor vehicle by removing it or making it inoperable prior to or after the sale or delivery to the buyer.’
In 2015, New Jersey Governor Chris Christie signed a bill into law prohibiting the retrofitting of diesel-powered vehicles to increase particulate emissions for the purpose of coal rolling. The bill was introduced by state Assemblyman Tim Eustace after a pickup truck blasted smoke at his Nissan Leaf while driving on the New Jersey Turnpike. Proposed bans have failed have in Colorado and Maryland.