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1.
Whenever you are driving on a two lane highway and there are two eighteen wheeler trailer trucks blocking both lanes and driving ten miles below the speed limit which causes a significant amount of traffic.
I was making great time driving on Highway 70 on my way to Kansas City, but then there was a huge cluster truck.
by kevtronic January 03, 2012
 
2.
When a group of two or more semi trucks are blocking traffic.
I would have been here sooner but I got caught up in a couple of cluster trucks on the interstate.
by allthenamesrused October 03, 2013