I drove a Scion xB for 13 years which had all the sleek aerodynamics of a toaster. When I got it in 2005 after driving a sedan for years, I IMMEDIATELY noticed how much more buggy the windshield got during the summer. It was one of the few things I disliked about that ugly, lovely little Lego brick.
By 2015, the bugs has drastically diminished enough for me to remark upon it before coming across any studies. The confounding factor here though is that the county I was in had a lot of development in it over that time. My routes stayed mostly the same (meaning both ‘where I drove’ and ‘how many buildings were along them’) but I can’t discount the expansion of the surrounding sprawl as a big factor.
Even after I moved to a neighboring more-rural county though, with a longer commute to the same workplace, I did not really see an increase.
This does not seem to have any influence. Even with 'oldtimers' you get less squashed bugs nowadays.
Even accounting for this, there is sufficient evidence for a significant drop in insect populations. While it's hard to have exact numbers, the estimations in decline of population range from 70% to 90% since the 1980.
I remember that a couple of years ago, there was a study that was based on squashed bugs on licence plates. Basically they used the standardized surface of the plates to measure the bugs in a certain area. No idea what came from it.
There's also memory. I remember when driving at night 35 years ago I saw a lot more insects in my headlights than I see these days. The difference is very noticeable.
There definitely more to it than aerodynamics.
Actually average CdA (drag area) would certainly have gone up in most parts of the world due to SUVs becoming popular. Looking at values on Wikipedia [1] definitely does not show an obvious reduction over the the last 20 years.
