Rik, mostly something else. Car design extensively uses computers to reduce airflow resistance and improve performance. The bugs are being pushed out of the way. Just drive with your head out the side window with your mouth open. A beautiful example of an uncontrolled trial being used as 'proof' is here:
https://www.nhm.ac.uk/discover/news/...-20-years.html.
There are fewer leafy avenues as roads have straightened and widened. A far better study does present a general decline with variation for a range of factors. Only one of over 300 references mentions driving and that is for the US.
Acorn