I think you grossly overestimate how much space does car infrastructure hold in relation to literally millions of km^2 of free space around it. Assuming your observation is right its impact would be minuscule.
But we're not discussing the free space around roads, I'm talking about the width of the road X length of road X 2.5m where the bugs get splatted. If there are more cars travelling in that space, fewer bugs will be splatted by each car even if the level overall remains the same.
I see what you mean. This still seems far fetched especially when taking into account rural and remote places where even doubling the traffic would mean passing 1 car every 10 minutes.
