Useful signs - volvoman
On the way to Stansted this morning at around JCN 2 of the M25 I caught sight of a multitude of brake lights in the distance and immediately expected mayhem to ensue. What happened next was that various matrix signs illuminated and a much larger version flashed a warning "stranded vehicle ahead". It appeared the incident had only just happened so I was pleasantly surprised at just how quick the response was. I believe the extra information provided as to the nature of the incident was very helpful in:

a) convincing those ahead to slow down (let's face it we've all seen numeric matrix signs flashing for no apparent reason and many people now seem to ignore them) and

b) alerting everyone to the type of incident we were approaching.

I've now idea how much these things cost but I'd like to see more of them in use.
Useful signs - Sooty Tailpipes
"Jack Straw is late for a meeting"
"The Beckhams feel threatened"
and other legitimate messages will hopefully follow.
Useful signs - GrumpyOldGit
On the M4 recently while in a huge and stationary tailback the signs said 'Think don't phone while driving' or some such useful message. What a total waste of money.

They should constantly display 'Return left after overtaking', 'Don't tailgate' etc, then be updated quickly in the event of an incident.
Useful signs - volvoman
Agree - a lot of the stress involved in being stuck in traffic comes from not being informed of what's going on ahead. It's a bit like waiting for a train that's late and not being told why. There's nothing worse for getting people worked up whereas a simple explanation reduces the tension. If drivers knew there'd been a fatal accident up ahead, for example, I'm sure most would calm down and act accordingly.
Useful signs - Hawesy1982
I too agree with all the responses here, i've recently driven on the M25 in heavy terrential rain resulting in standing water, and heavy fog reducing visibility dramatically. On both occasions the signs, if illuminated at all simply read "Think Don't Phone While Driving", whereas "Reduce speed - standing water" or "Remember to use your lights in fog or rain" would have been much more appropriate and useful.
Useful signs - BazzaBear {P}
Now that is a fantastic idea. Why can't the base state of thee signs, if nothing else is going on, be to remind people to keep left?
It might actually get the idea through their thick heads.
Useful signs - GrumpyOldGit
Can't see it happening as it's logical, and the Government don't do logical!
Useful signs - pdc {P}
We've had "Think Dont Phone When Driving" and "Think Don't Drink and Drive" and now this week's message on the M60 is "Think. Don't Drive When Tired"

Will next weeks be "Think. For Yourself" ?
Useful signs - No Do$h
Let's hope that Paul McKenna never gets hold of the controls for these signs.