Driving down a highway, you'll usually find there are two lanes--one traveling in each direction. On larger freeways, there may be multiple lanes and a shoulder for emergency stops. The New York State DMV Driver's Manual reports that a broken-up or solid white or yellow line divides the travel lane from the shoulder of the road. This means that the travel lane is always defined as a lane for moving traffic.
The right travel lane on a multiple-lane highway is always where slower traffic should drive. This allows people the ability to pass them if necessary to keep traffic flowing.
The far left lane is always used for passing slower-moving traffic. Most highways have a posted speed limit of how fast and slow you can drive on the road, and the passing lane is used for those who want to drive the highest speed limit allowed, or faster.
The Massachusetts DMV reports you should use lanes only as marked. This means you should never stop in the travel lane if there is a shoulder of the road available. If you have an emergency while on the road, pull as far off the road as possible.
The shoulder of a highway or freeway should not be used as a travel lane according to the DMVs across the United States. This lane, even in a heavy traffic jam, should only be used for emergency stopping or for emergency vehicles to drive past the traffic.
When driving on a highway or freeway, it is courteous to follow general driving rules of slow-moving traffic in the right travel lane. Failure to do so can lead to traffic jams and frustrated drivers who wish to drive the posted speed limit.