They mean that no vehicle should cross them to pass or turn. Depends where you are but in many places double lines in the middle of the road mean overtaking another car is not allowed.
It seperarates the direction of traffic flow
In the US, both solid white lines and double white lines are used to mark areas of prohibited lane changes in multi-lane traffic. Broken (dashed) yellow lines, solid yellow lines, and double yellow lines are used to separate traffic moving in opposite directions.
The two white lines on the roads in Perth, WA mean that you are on a 2-way road. You may not cross double lines, whether they are white or yellow, or you will find yourself in the path of another auto going the opposite way.
When there are broken double white lines in the road, it means you can cross the lines to pass as long as it is safe. You must be able to safely be back in your lane before you reach a solid white line.
White lines on the road can mean several things. Double white lines down the centre of the road means do not cross or overtake in a vehicle. Dashed line on your side means you can overtake if it is safe to do so.White lines are used to mark lanes so drivers can stick to a lane and not veer across the road. Look on any road for examples.
The yellow lines refer to parking controls on UK roads. When double yellow lines are in force, you can't. (note that double yellow lines are not always operational at all times. The restriction prevents parking on or adjacent to double yellow lines. There is no mention in the regulations about which side of the yellow lines and therefore they are effective for vehicles parked on pavements or verges. Look at the Highway code text next to the pictures of double yellow lines in the Road Markings section.
The Car Coach - 2011 What Do the Lines on the Road Mean was released on: USA: 8 September 2011
Double yellow lines.
In america, the double R road sign means RAILROAD.
When two parallel solid yellow lines separate lanes of traffic, the only legal maneuver you can perform is turning left into a driveway on the opposite side of the road.
In the UK double solid white lines down the centre of the carriage way indicate that you should not cross them except under certain strict conditions.
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