[vasco]

Those signs usually say the current kilometer you are on the road, which is extremely useful to report accidents or damage to the road or surrounding structures with large accuracy. This was specially relevant before everyone had a GPS receiver in their pocket, but is still useful - it's still easier for police or firefighters to go to X km on some road then to a GPS waypoint as its easier to communicate verbally and more likely that people will immediately know how to get there. Not sure about every 20feet though, different countries have different distances (every 100 meters in some).