Spray asphalt markings are done to mark places where the construction crew needs to proceed with caution due to plumbing, wiring, etc, being under the asphalt at that point.
The spray is often made with fluorescent colors, because of the importance of the crew noticing and preparing for the markings when digging and ripping the asphalt off.
You'll see these markings especially on places where telecom companies are marking the cables to be replaced when they upgrade their systems from, say, copper wires to fiber optic ones.
The markings may be there for weeks before you are going to see any digging done, especially for municipal works, but also for the work that requires a lot of planning, such as telecom related work.
Sometimes asphalt constructor crew bosses use these non-permanent sprays to mark different instructions to the asphalt crew.
There are several picture on Flickr on typical construction markings on the asphalts around the world. For example, this one from Boston:
From Spray Asphalt page to Asphalt Guide index