Stands for Slow People in Formation. It's a term used for a situation where a large group of slow moving or meandering people effectivly create a road block.
"If we have to drive today, let's skirt around downtown. The street fair has turned the whole thing into a spif."
Student:"Sorry, I'm late, I was stuck in bumper to bumper traffic."
Teacher: "Was there a big crash?"
Student: "No it was just a spif."