Anecdotal evidence is way too localised to have any accuracy.

I spent nearly 20 years commuting every couple of weeks between Stirling (central Scotland) and Leeds, Livingston and Reading. Certainly I needed to soak off thousands of tiny corpses when I drove around Scotland, and almost none when I was hacking over motorways or pottering round Berkshire.

But I would attribute that to preponderance of sheep and cattle farming (not so much cereal), much more lush vegetation, woods, hedegerows, and rough pasture, less cutting of verges, 24-foot single carriageways, and an abundance of rainfall. Large areas of England are barren inhospitable deserts by comparison.

However, I have no idea which of my hypotheses above might carry greatest weight. Not the streamlining maybe - it's been a Volvo 940 or 760 estate almost all the time. I also considered whether modern acrylic paints are less adhesive for splat and chitin, but it's still grey and it's still Volvo.