After a long journey, it seems to me - and I've heard other people say it - that cars are less besmattered with flies than they used to be maybe 30 years ago.

Is this the case? Has anyone measured it?

If it is true, is it because:

1. There are fewer insects?
2. There are more cars, so they get fewer flies per car?
3. Unnatural selection has favoured high-fliers?