Traffic Barriers – Meant to Protect?

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?