It's a jailable offense now to toss a cigarette butt out your car window. Via 9 News:
Firefighters throughout the Metro Denver area want drivers to know they will seek prosecution of those who would throw lit cigarettes out of their car windows. Under Colorado law, anyone convicted of the Class 2 misdemeanor could be sentenced to jail. "It happens daily," said Lt. David Marchbanks of the Denver Fire Department. "All over Denver, we have people running on weed fires that are caused by cigarettes. It's a lot of resources taken from the city."