Solid yellow lines on a roadway are designed to separate traffic that is moving in different directions. Double yellow lines are only seen on roadways where there is two-way traffic and indicate whether passing from either direction is allowed.
The presence of a broken yellow line on some roadways indicates passing may be allowed. If there is one broken line and one solid line, the vehicle on the side of the broken line may pass. A solid line indicates that it is not safe to do so because you cannot see far enough into the distance to determine if opposite traffic is coming. A single broken yellow line means that it is safe to pass from either direction.
Solid yellow lines can also be used in other situations. For instance, two sets of double lines means that vehicles are to stay out of that area. This yellow line configuration is used in some areas to prohibit entry into a carpool lane.