Only the Highways Department can tell you for sure, but, based on what
I have had to do in the past, I would expect that to mean that
measurements are taken from the point at which the centre line of the
drive meets the edge of the footway furthest from the road. The splay
would then extend 1.5m either side of that and 1.5m back into the
drive.

Colin Bignell
pedantic
Nicely put, IIMSS, though for the avoidance of doubt I think you mean
the splay extends 1.5m either side of the edges of the drive. See for
example figure 3 in
http://www.scotland.gov.uk/Publicati...093/Q/Zoom/100
/pedantic

--
Robin