I only reason that I know this is because my father used to work on a road-crew for the Cincinnati Waterworks.

Those markings indicate underground-utilities that need to be marked before a road-crew can safely dig there.

Blue means there is an electrical-line
Yellow means that there is a gas-line

If someone doesn’t come out and mark them before a dig, there is a good chance that the road crew could accidentally dig up something important and/or dangerous.

I heard about a guy who accidentally jackhammered into a high-voltage line wile doing a water-main repair. He can breathe okay as long as nobody un-plugs him.