In general, single broken lines mean passing or lane changing is allowed, single solid white lines mean lane changing is discouraged or prohibited, and double solid white lines mean it is prohibited, as it often is in tunnels. On two-lane roads, a single broken center line means that passing is allowed in either direction, a double solid center line means passing is prohibited in both directions, and the combination of a solid line with a broken line means that passing is allowed only from the side with the broken line and prohibited from the side with the solid line. Lanes with double broken yellow lines on each side are reversible, and lane control signals are used to indicate which direction traffic in such lanes is supposed to travel. The solid white line on the right side is called the 'fog line' used to help cars stay in their lane during foggy conditions and help pedestrians stay off the road.