Re: The great <insert nation> public
"I think these are now treated as a 'cry-wolf' these days by most drivers, so tend to get ignored."
Yes, mostly because of impatience and arrogance on the part of drivers. I lost count of the number of times that I got calls telling me that the signs were set and "There's nothing, here, no incident, nothing." Not all of them were as funny as the prat driving into the back of a van, but most of them were plain wrong. The calls related to incidents such as a truck carrying cows to market rolling over with cows all over the motorway and vets and stockmen running around trying to catch them, police officers trying to stop someone jumping from a bridge onto the road, police chases (which a driver will rarely see) where there's a need to slow down the traffic to stop it catching up with the suspect/police cars and on two separate memorable occasions, an angry driver who said he couldn't see a vehicle on fire despite the fact we could see on CCTV that it was just a few hundred yards from where they said they were. In all these cases the callers were adamant that there was no reason for the signs.
For some reasons when they get behind the wheel of a car may people think that they are omnipotent and don't realise that someone in a motorway control room can see all of the motorway and the driver can see a very small part of it. Also every driver seems to think that they should be getting a personalised service and don't realise that with only two or three lines of text to display a message they are not going to get a nice set of Tweets about the current state of the motorway.
"As people get used to them telling you pointless things (Tiredness kills etc). If the messages on the boards are mostly not real impending warnings, then people will stop looking at them in the first place!"
The designers know this. It's a basic of good interface design that you don't overload the user and that displays should not show irrelevant garbage. This is how the signs were planned to be used - if there was text on a sign it would be relevant and important.
Then politicians got involved and were very angry that a sign costing £lots was blank. One of them, I forget which, wibbled that when he was on holiday signs in other countries show time, temperature, details of local events and handy advertising for credit cards. He wanted UK signs to be the same. There was an argument and the politicians lost on some points - be thankful that there's no advertising - and won on the display of gash information just to make it look like the signs are being used.