As for the cars...
More cars and more car miles this year than in yesteryear means fewer bugsplats per vehicle mile. I would assume that, once a vehicle has swept past a volume, the bug population in that volume is restored by a "random walk/fly" process that depends on the average number of insects per unit volume in the area surrounding the highways.
Also vehicles pass in clumps depending on traffic patterns and time of day, so a bug that might have splatted on your vehicle is instead swept up by a driver in front of you.
Over time, insect populations for flying insects near roadways may decline, given the death rate is higher there. OTOH that may vary depending on whether the species is attracted/repelled by light, heat, roadkill, fumes, etc.
As for the article...
The article said: "the Krefeld Entomological Society... in 2013 .. When they returned to one of their earliest trapping sites from 1989, the total mass of their catch had fallen by nearly 80%. "
Note the word "one" as in one of "100 nature reserves in western Europe". The article continues:
"Through more direct comparisons, the group—which had preserved thousands of samples over 3 decades—found dramatic declines across more than a dozen other sites."
So, perhaps 12 of 100 sites show a "dramatic decline". That's about 8% of sites, so what about the other 88 sites? Did numbers there go up, decline or what?
Like the article says, time for some good science. But NOT time for alarmism. The sky is not falling.
All-in-all, pretty anecdotal stuff that could also attract some fun modeling. But first maybe, let's get some good solid science.