On most US highways (i.e. multi-lane limited access roads), it's customary to leave a path in the left 'passing lane' for any traffic that wants/needs to go faster than you. If cars match speeds across lanes, it impedes faster traffic.
The speed limit itself is a separate convention and regulation. In some places you can be cited for obstructing traffic by going the speed limit in the passing lane if you are matching the speed of cars to your right, effectively blocking the road.
The speed limit itself is a separate convention and regulation. In some places you can be cited for obstructing traffic by going the speed limit in the passing lane if you are matching the speed of cars to your right, effectively blocking the road.