For those not in the know, rolling coal is the name for blasting black smoke out the exhaust of a diesel-engined vehicle. It is done by modifying the air and fuel mixture in forced induction diesel engines to make the engines run rich, the result of which is a cloud of soot blowing out the tailpipes. This is done for the childish sake of annoying people, typically anyone that a coal-roller doesn't like, which includes but is not limited to hybrid drivers, convertible drivers, bicyclists, pedestrians, and anyone who so much as looks like they don't attend the same evangelical megachurch. Why do people do it? Nobody one can say for sure, though it has been suggested that it has its roots in anti-environmentalism.
I will be the first to say it: you look like a dimwit when you roll coal. If you are doing it to antagonize people, then it isn't a protest; it's the automotive equivalent of it's just a prank, bro! Even if done with the intent of being a legitimate form of protest, it falls short due to its futile nature and likelihood of galvanizing the opposition.
Because it cannot be demonstrated to be anything other than stupid, rolling coal has been banned in Colorado, following a vote by the Colorado General Assembly, which passed the bill by a wide margin of 40 in favor, 25 opposed. It is now bound for Governor John Hickenlooper's desk. This is the third attempt to regulate rolling coal, with previous iterations of the bill failing to pass a Republican-controlled Senate due to concerns regarding overregulation that would disparately impact blue-collar industries such as trucking and agriculture. Fine-tuning of the bill on its third introduction, to educate traffic officers on the difference between a smoky old work truck and coal rollers in their mall crawlers, should help prevent unfair enforcement against drivers of well-kept vehicles. Citation for rolling coal will carry a $100 fine, and will serve as a data point on heat maps around Colorado which will highlight areas with a high density of offenders.
It will likely turn into another rarely-enforced law, a symbolic victory, because nobody is dumb enough to roll coal around a cop ... right?