fritz:

Don't worry, Eunice, they are sprung and sink down into the road surface when you drive over them, so they would not send your car sliding off the road. Smiley
They also make a very satisfying "thump" noise to tell you that you are crossing a lane line.
They really are amazing in poor visibility conditions when even the white lines on the road don't show up any more and I have often wished we had them on the Autobahn here. Smiley

That sounds sophisticated, thanks for explaining.