As Spamcan says, excepting pre planned stuff like recent closures of A43, the data on the gantries is reactive. Similarly, once the problem is solved it takes a certain amount of time for message to get back to 'control' and for signs to be switched off.
If the gantry message says queue, incident or debris it almost certainly means such a thing existed. It might have dispersed or been resolved/removed by time you get there but it doesn't mean the alert was wholly spurious.
Gantry messages, along with radio alerts and observation of traffic as far ahead as you can see, are clues that stuff might be amiss. They're not gospels.
I do appreciate them being used to give long range alerts for closures and diversions. A message in Lancs about trouble on M6 in Staffs/Brum gives me time to select a preferred diversion; A 50, M6 toll or urban roads through Black Country. Much better than wig wags half a mile before it all grinds to a halt.
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