I got downvoted to oblivion a few months back for suggesting that maybe cars were just getting more aerodynamic. I still wonder if anybody has really considered that. I assume so, it seems like a plausible first guess.
I have a car that's not very aerodynamic (Jeep Wrangler) and when I'm driving through rural areas the insect mess at dusk or dawn is bad enough that I have to run the windshield wipers. Conversely, I never have to worry about it around my house, which is in the middle of large urban area (SF Bay Area).
Now, consider that there has been massive human migration into urban areas over the last century, plus cars getting far more aerodynamic in the last 40 years, and I think you can make a case for observation bias. However, I think you can also say very firmly that urban areas = less insect populations, so as our urban footprint grows, it is likely costing us. I think suburban sprawl may be even worse, because of the habitat destruction your typical housing development causes without the benefits of density that a truly urban environment offers.
So honestly I think you could be right and wrong at the same time. It is definitely worth correcting for.
Interestingly, this article [0] states that the opposite is true: "The research included vintage cars up to 70 years old to see if their less aerodynamic shape meant they killed more bugs, but it found that modern cars actually hit slightly more insects."
That seems plausible. Perhaps an car with poor aerodynamics pushes a wall of air in front of it that deflects bugs up over the car, where a slippery car may allow the bugs to smash right into the glass.
Now that I've made my comment, I'll go read that article. Thanks for sharing ;-)