Blonde Driver

A blonde is driving down the highway.. very slowly mind you.. and a cop pulls her over. he says,

"Mam, do you know how fast you were going?" the blonde says,

"Duh! I know how to drive! I was going 17 mph! haha silly!"

The cop looks at her strangly and says,

"Do you know the speed limit?" she again laughs at him

"Of course! Dont you?!"

he says,

"Mam.. dont be smart, humor me and just tell me what the speed limit is."

She looks at him patiently and smiles,

 "Its 17 mph now, and in a couple minutes it will be 18 mph, Geesh! How hard can it be? I read the green posts on the side of the road! The only thing i dont understand is why dont you pull everyone else over for speeding?!"

"Because Mam," the officer began... "Those green posts you were reading are called mile markers!"