Fewer motorists along metro Phoenix freeways are littering, that’s according to a new survey by “Don’t Trash Arizona.” 

The study found number of drivers admitting to throwing litter out their window has dropped 15 percent over the last year. Kelly Taft with the organization said smokers throwing cigarette butts out the window remains problematic. 

“They’re so small. It’s not something that is easily picked up by ADOT clean-up crews. And so it tends to stick around for a very long time,” Taft said. 

She said the toxic chemicals from those discarded cigarettes can then leach down into the water supply, causing future problems.

While the drop in the percentage of drivers littering  is impressive, Taft said a certain group millennials are the most likely to toss their trash.

“Littering can happen in any situation, with any person. But, we do find that it’s those teens and young adult males that continue to be the primary offenders,” she said.

Taft said food is the most likely thing to be tossed from a driver’s window. She said while food might be biodegradable, it could still take years to disappear.

EDITOR'S NOTE: This story has been updated to reflect the correct percentage drop of drivers who admitted to throwing litter out their window.

Updated 11/10/2015 at 9:10 a.m.