These lines are often found on the approach to a roundabout or a dangerous junction. They are called a rumble device. They give you extra warning to adjust your speed. Look well ahead and do this in good time. They not only provide a visual warning of an upcoming hazard, but also an audible warning as the vehicles tyres run over them, and a tactile warning through the steering wheel.

They are much less extreme than other traffic calming measures such as speed bumps and speed tables.

As the lines are evenly spaced for a start but then gradually narrow they provide a good reference for your speed because you'll hear the interval between them lengthen as you slow down. If the interval shortens then you are not slowing down enough. 

You can see this clearly on this image of the M45 near Dunchurch.

Sometimes the lines are created in clusters counting down how close you are to the junction. For example in the image below of Somerset Avenue in Weston-super-Mare you'll see five lines, then 50m later there are four lines. 25m later there are three lines, and finally 10m later there are two lines. There is not one line because that could cause confusion with a stop line at the junction itself.

In Axbridge, these speed awareness lines mark the exit road off the A371 onto Cross Lane.

If the lines are raised they are often not used if there are houses nearby because they create additional road noise.