Asimovian:
Based on my own observation, rather than a specific cite, a vehicle moving at a drastically different speed (faster or slower) than the rest of traffic can pose a significant danger. Drivers looking to make lane changes or other maneuvers generally have a very short period of time to look all around them to make sure their path is clear without taking their eyes off traffic directly in front of them for too long. Someone behind them moving relatively very fast or someone in front of them moving relatively very slow is probably not going to be in the area the driver is visually sweeping for safety. So the maneuver they think is a safe one may not be.
And I think this is exactly why some interstate highways in the US have posted minimum speed limits, and driving below that limit can get you a ticket. It is a hazard.