All federal and state highways currently have traffic signs that read "slow traffic keep right". This is not a rule enforced by any agency nor explained in driver's education. Consequently, traffic on American highways runs inefficiently with slow cars clogging the left lane and increasing aggressive driving on the road. Often, semi drivers purposefully slow down traffic in the left lanes.
The Autobahn system in Germany has a strictly enforced Keep Right rule that allows traffic to move quicker and safer than US traffic. Traffic fatalities in Germany are exponentially less than the US.
If the Slow Traffic Keep Right rule were enforced and the driving public educated about it, our roads would be much safer, more efficient, and far more pleasing to use.