Firstly yes, there are plenty of bored chavs in Portugal, but bless them they tend to chav up things like Renault 4's
I cannot by any means claim to be expert on this, and therefore I am only putting across my perception and opinion now.
I have never seen a rear end smash at one of these lights, the lights themselves are quite a way from the speed sensor so the process is quite sedate, doesn't promote panic braking.
Some of these lights are placed with no corresponding junction, however to jump one would be a really bad idea. The penalty is extremely harsh, and if caught you will be fined on the spot (involves the policeman driving you to a cashpoint to pay there and then!)
Again, only what I have seen, I have never seen anyone jump one of these lights. Logic dictates that it must happen though.
As far as I am aware accidents do reduce after the installation of these devices, I will have to search for figures on this, I am not sure if the DGV make them public, but will endeavour to find out.