Was there another car on the other side of the road? Speed cameras can be on either side - and some flash you as you come toward them and some as you drive away from them. If you were doing 40 the whole time you were between the lines it seems unlikely it was you who tripped it, but if you saw a flash and you know you were speeding... sounds as though you'll be getting a photo in the post soon.
DH always makes Driving Miss Daisy comments to me, because I'm SO paranoid about getting caught speeding!!! I just can't afford it!
x-posted with Genau! - we have two that are on the other side of the road, but I guess now that I think about it, it is pretty unusual.... So maybe it wasn't you!