The 'safety' angle really gets me. Don't get me wrong - I'm all for safety. If there's a road where there's constantly fatal accidents (due to speeding) then anything to slow people down has got to be a good idea.
As an example of 'unnecessary post mounted cash boxes', a small section of road near me in Potters Bar running from the M25 exit (J24) into the town is about 1/4 mile long. I can't recall in 14 years there ever being a crash there, let alone a fatal one. However, the powers that be have decided that it's such a dangerous stretch of road, that they have installed not one, but three cameras! Ffs! You just get through one, have about 200yds of road and there's another.
On the positive (but somewhat ironic) side, the road is usually jammed solid anywhere near rush hour, so there's not much chance of getting done. ;-)
Steve.