https://www.theguardian.com/environment/2022/may/05/flying-insect-numbers-have-plunged-by-60-since-2004-gb-survey-findsMy guess is that there is something else going on here too. For a bug to be splatted, the car has to be travelling above about 40mph. Otherwise they bounce off (probably fatally) or are pushed up and over. A car only driven within urban areas doesn't usually pick up any at all, because of the speed limits. The peak splatting speed is about 60mph, in my experience.
Motorways and dual carriageways have few bugs because there's nothing green there to interest them.
So, to splat bugs, you need to be on a typical A- or B-road, travelling at over 40mph. That is less common than it used to be, due to congestion and far more long speed-limited stretches than there were 20 years ago. It would be possible to correct for this with GPS logging, but tricky to do.
Also, the reason there aren't as many bumblebees about is that they are all on my lavender in summer
