In general, you should keep to the right at all times unless passing, and you definitely should remain in the right lane when traveling below the speed limit.
Because everyone is trying to pass the slower moving traffic and most highways are posted with the sign, "Slower traffic keep right" or the left lane (passing) is marked "Through Traffic".
http://definitions.uslegal.com/i/impeding-traffic/
When entering or merging with highway traffic, it is safest to match the speed of the traffic already on the highway. This helps to maintain a consistent flow and reduces the chances of causing a disruption or collision.
An easy traffic jam could be due to a car driving slower than the rest of the cars.
If you are a normal car you should use the middle lane unless you are turning of.
You should maintain a safe following distance at all times, which may require you to slow down below the speed limit, particularly at times when it's not feasible or safe to pass traffic which is traveling below the speed limit.
Slow traffic keep right sign
The slower traffic keep right sign means that drivers who choose to drive less than the speed limit should stay in the right-hand lane. For example, 18-wheelers going up a hill often experience decreased speed because of the weight they haul. Unless passing an even slower vehicle, the slower truckers need to stay in the right-hand lane. Vehicles driving significantly below posted speed limits should also use their flashers until they are able to match the posted speed limit.
The slower traffic keep right sign means that drivers who choose to drive less than the speed limit should stay in the right-hand lane. For example, 18-wheelers going up a hill often experience decreased speed because of the weight they haul. Unless passing an even slower vehicle, the slower truckers need to stay in the right-hand lane. Vehicles driving significantly below posted speed limits should also use their flashers until they are able to match the posted speed limit.
The slower traffic keep right sign means that drivers who choose to drive less than the speed limit should stay in the right-hand lane. For example, 18-wheelers going up a hill often experience decreased speed because of the weight they haul. Unless passing an even slower vehicle, the slower truckers need to stay in the right-hand lane. Vehicles driving significantly below posted speed limits should also use their flashers until they are able to match the posted speed limit.
When you are in a country that drives on the right, then generally you should drive in the right lane; the left lane is for passing. When you are in a country that drives on the left, then generally you should drive in the left lane; the right hand lane is for passing only.
Avoid distractions; avoid speeding; avoid going slower than the average traffic speed; avoid going through red lights