What are those black things that show up, that cross the entire road?

So for months and months and months the road is clear. Then one day when you are driving you run over these two black things that are mounted to the road, and they are long enough that they cover both lanes.

Are they for getting speed limits of vehicles, and then they store that data, or send it back to somewhere? I've never known what they are, maybe you do, and you can explain it better than I can?