Traffic Safety

Traffic barriers can be made of concrete or plastic, and the plastic ones are usually hollow on the inside so that they can be filled with sand and/or water.  Keep in mind these can also be called ‘barrier barricades’.  Now, my question is, how can a concrete barrier really be safe for a car to hit?

There are a lot of traffic barriers out there that are approved by all sorts of traffic safety standard, but how much do they ‘protect’ a car when you hit it?  I would assume a concrete barrier is not as forgiving as a sand filled plastic one, but I could be wrong.  Many of these barriers are supposed to be designed so that the cars ‘bounce back’…how safe is that?