The idea does seem odd, even trying to get into the mindset of a believer.
Is the blessing supposed to make the grit/salt work better, bending the laws of physics, and altering coefficients of friction or the melting point of water?
I'd have thought that praying for [deity] to subtly influence drivers not to drive like idiots in winter weather would be a more plausible path.
Though why would a deity need praying to in order to do something like that?
Wouldn't it already know that people thought car accidents were a bad thing?
Regarding an experiment, I'd guess there would be any number of confounding factors, either varying over time, or possibly between areas.
As well as variation in weather and general driver behaviour, there are changes in car design (ABS/traction control, accident survivability, etc) changes in grit/salt type, changes in gritter-driver training regarding where to concentrate gritting, changes in weather forecasting accuracy, changes in planning of where to grit and not to grit.
Weather-related changes could be complex.
Do fatal accidents vary closely along with hardness of winter?
Does a hard winter make people warier about driving?
How many accidents are caused by an patch of ice that's unexpected (and possibly un-gritted) due to the weather and roads not being generally bad?