Oiling the RoadJuly 8, 2013

Last week I visited Pittsburg and stayed at a friend’s house who recently moved there. I noticed the road sign below at several different locations around his home. I was rather flummoxed by this sign. Are they really going to place oil on the road? Wouldn’t that be very slippery and dangerous? Perhaps this ‘oil’ is a colloquialism for some other process. My friend was just as confused. Does anyone know what this means?