Some road signs are incorrect for their task and others can create problems by not being located exactly where they should be (sometimes because the installer was sticking strictly to a rule book and didn’t use common sense). But in this case the cause of potential danger is different:
So here we have a sign telling drivers to switch off two-way radios and cell phones, but look how close to the construction zone the sign is. The speed limit at this location was 35mph and there was no reduction of the limit in relation to the work being done. So cars travelling at 35mph are doing 51 feet-per-second. Allow the driver the now-standard 1.5 seconds to react and how long — three seconds maybe — to pick-up the phone (illegal in some states) and switch it off and the car will have travelled 230 feet, with a distracted driver and in a location where any construction zone workers who are on foot are extremely vulnerable.
And remember, you only need one driver in this scenario to be distracted at the wrong moment, at any time through the duration of the construction work and there will be a dead or crippled worker.
Then, of course, there is the potential danger from explosives, if that is why the sign is in use. To quote a common Americanism: Are we having fun yet?
This whole set-up is thoroughly unacceptable.