Definitions by Mr. Underhill

Mormon Block 

Two cars driving side-by-side at the same slow speed on a highway, thus blocking others from passing. This slang imagines a family with many children forced to take two cars and then drive in a manner that is oblivious to other traffic and others around them.

Those two cars are doing the Mormon Block and slowing the entire highway down to 40.

Mormon Block by Mr. Underhill May 27, 2016