When driving on a road with two or more lanes going in one direction, and vehicles occupy all lanes. Said vehicles drive at the same speed and evenly, barring other vehicles from passing. This event native to Denver, where many "undocumented workers" usually from Mexico attempt to drive without a valid driver's license or automotive insurance. These illegal immigrants therefore drive significantly under the speed limit, effectively infuriating all other drivers on the road.
Sorry I'm late for work today Mr. Jenkins. There was a Mexican road block on Interstate 25.