Officer James Holmes said officers were called out early Saturday after witnesses found a child strapped in a safety seat in the middle of an intersection.
The boy wasn't hurt. He's now in the custody of Arizona Child Protective Services.
Authorities say the child's mother, 19-year-old Catalina Clouser, her boyfriend and their friends had been smoking marijuana earlier in the evening at a nearby park.
Upset that her boyfriend was arrested for suspicion of driving under the influence, police say Clouser went to the home of friends and smoked more marijuana.
Clouser left around midnight. Police say she apparently put the sleeping baby on the roof and drove off, forgetting he was there.