Mmm, thinking about this, it occurs to me that, after the exceptionally wet winter and the state the road was in after it, a 'good load' of stone was added, to improve the surface, which could, perhaps, be a good bit higher that it has been - or was, whenever the clearance was last assessed.
I'll see if I can remember to take the means to measure it, whenever we next go there.
The reason for being so concerned about not provoking an emergency goes back to a time on the Big Island, when I found a chap in need of an ambulance, collapsed under a bridge on a disused railway, through the middle of an urban cemetery.
As is common there now, I ended up talking to a controller who had no real local knowledge and just wanted a Postcode for the location, which didn't have one, of course, even if I would have known it. I knew where I was, but he just didn't understand. I gave him the number of the A-road that ran over the bridge and described the location again, making it clear that we were under the bridge on a disused railway, in the main cemetery, within the town itself.
He typed things into his system and came to the alarming conclusion that we were under a different railway bridge, about two and a half miles away. Despite my protests, he convinced himself that I was actually under a bridge on a four-track section of the electrified West Coast Main Line, with the casualty, and we were both about to be electrocuted and run over. It was as much as I could do to stop him turning the power off.
I remember saying to him "The last train was in 1976 and there aren't even any rails here now!"
I nearly needed treatment when the yellow van turned up...