On a Saturday night in a strip mall parking lot in south Fort Collins, crowds of young motorcyclists, truck drivers and their friends gather to hang out and ogle each others' vehicles. A number of these men, including Jake Rogakis, roll coal.

"What it means to roll coal is to pretty much just floor it and blow as much black smoke as you possibly can,' said Rogakis "It's just fun. Go drive my truck. You'll have a blast."

Go to YouTube and type in "rolling coal," and videos of large diesel trucks spewing black smoke -- on Priuses, police cars, bikes, even women -- fill the screen. The trend itself isn't new. What is new is its appearance in more urban parts of Colorado, like Fort Collins and in the downtowns of small cities around Denver.

Read More: Your Questions About Rolling Coal, Answered

Rogakis, baby-faced and in his early twenties, said he tunes his truck to roll coal mostly because it boosts performance.

"I don't deliberately do it to someone unless they like, do something to make me mad, cut me off, ride my ass, anything like that."

But when asked if he had ever rolled coal on a person or bike, Rogakis shrugged and laughed, saying when he's in a car with friends, he sometimes goes a little farther.

"Can't say I haven't done it. Like going up to Horsetooth you see a bike. You just can't help but to do it. No matter how many times you do it you just get a smile on your face."

https://www.youtube.com/watch?v=iRu3xtTH9ng

Not everyone is smiling, though. Honore Depew rides his bike regularly around Fort Collins. He has been "rolled" twice, once while pulling his young son in bike trailer. He worries the phenomenon will frighten less experienced cyclists and keep them off the road. Having a truck roll coal on him made him feel vulnerable.

"I am reminded of how weak I am as a person traveling at a slow rate of speed without a protective encasement around me, compared to large powerful vehicle," said Depew.

Rolling coal on bikes and pedestrians is an increasing problem in Fort Collins, which has sent police officers out to crack down on the problem. It's not an easy issue, though, because there is no law specifically against the practice.

Instead, the city is using a law prohibiting "exhibition of speed" to enforce against rolling coal. In order to write the five-point ticket, an officer must observe a truck revving its engine or blowing smoke out as a way of demonstrating the truck's power.

At the parking lot in south Fort Collins, some truck owners were eager to differentiate themselves from rollers like Rogakis. Brock Anderson has also tuned his diesel truck, but the Greeley native, who works on a farm, keeps his smoke in the fields. There, the soot doesn't bother anyone.

"There's a time and a place, like out on the farm if I'm mudding, I'm going to be tearing up a field, nobody is going to be out there," said Anderson.

Anderson said he understands why some truck owners alter their vehicles this way, although he also said, "if you are just rolling coal you are not maximizing your horsepower. And for us, we run pretty clean tunes, we're not rolling coal."

His friend Cory Wiggins, originally from Louisiana, said he liked the look and feel of a truck that could roll coal, and so do women.

"It's just something cool, the ladies like it," said Wiggins. "It's just fun, it's exciting, it puts you back in your seat."

Both Wiggins and Anderson said the truck owners rolling coal in downtown Fort Collins gave them, "farm kids," and diesel enthusiasts, a bad name.

Even though Fort Collins police are trying to put a halt to those blowing smoke on pedestrians and cyclists downtown, those rolling are hard to find.

On a Friday night, officer Joel Tower heads toward Old Town in an unmarked vehicle. He cruises south down College Ave., but the street is quiet until he gets out of Old Town. Then, suddenly, the sound of diesel trucks fill the air and truck after truck pass, heading north.

"We'll see if we'll can follow them around we just gotta catch up with them," said Tower.

It's easier said than done. By the time the cop car makes a U-turn across busy College Ave., the trucks are long gone.

Tower drives up and down College for about an hour, even stopping into the strip mall parking lot to cruise through and look for any rollers. Tricked out trucks and twenty-somethings fill the parking lot, some boasting giant American and Confederate flags. A crew of youth practice roping a dummy steer. There's nothing illegal, though.

As Tower's shift comes to a close, he spies another big black truck with a giant exhaust pipe.

"If we followed him around probably for a while longer, we'd probably catch him rolling a little coal later on, looks like he's primed to do that."

Right now though, the truck only revs his engine a little. It's 10 p.m. and Tower's shift is over. He didn't catch any coal rollers tonight, but there's always next weekend.

Read More: Your Questions About Rolling Coal, Answered