road slug 

n. The driver in front of you who is driving ten miles under the speed limit

Sorry I'm late for work, I kept running into road slugs on the way here.

road slug by Liz Silva July 22, 2011

Road slug 

Extremely slow driver on the road.

Honey, I'm gonna be late, there's a road slug in front of me.

Road slug by Amalright November 19, 2013