the big metal 'things'

So, in certain parts of Dallas... usually the older parts. There are these huge (maybe 8 in diameter and 4 in in height) metal circles that separate the left turn lane from the rest of the lanes. Some newer installations of these 'things' are plastic, but still just as large.

What I would like to know is... What are these? What is their purpose besides separating the lanes? Do the benefits of these 'things' out-weigh the risks/damage that could be had by driving into/over one?

Ok. I can see why they were put there. They will control traffic going into the turn lanes too late. But, as we discussed earlier, street signs in Dallas are HORRIBLE, so unless you know where you are going (or have GPS) it is impossible to know what the street is until you are up close to it... and then if you are turning left... too bad sucka.

Additionally, I think these pose more of a hazard than a benefit. Say you were going along and there happens to be a big branch in your way. You could pass it in the left turn lane... but NO! If you run into/over these metal things your tires/rims are messed up.

Why oh why waste tax money on these dreadful things? Whose idea was this?