they have to ensure gritters are fully operational before the snow starts to fall, they would look pillocks if they said we know its snowing but all the gritters are broken and cos we never tested em
simples 
The best bit of disorganised gritting was in York a couple of years ago, 2 am gritters were out gritting, 5 am road sweepers out road sweeping lol, prob being driven by the same guys.
I emailed the council when i got home, asking if this was part of there recycling policy, got a snotty reply back off them, idiots.