I also got an answer via email that I think is worth sharing.
I'm not answering officially as I’m probably wrong but…
The main point mentioned is that because flies have such a short
lifespan, so they should evolve faster than elephants or other slow
growing species. While this is true, and there are many examples of r
(ruderal) species evolving rapidly* evolution is not a predictable or
sensible process, and there are many other factors that contribute to
evolution other than a short generation time.
One main question to consider is… how many flies are there? And how
much of that population is affected by the road fatalities? Whilst
there are over 23,000 insect species in the UK, your question
specifically concerned flies so… An average fly lives for around 35
days and can lay roughly 750 eggs in her lifetime, so fly numbers are
bloody huge! Although a small proportion of flies may be killed by a
car, the proportion will be so small as to make no impact to the
overall population; therefore the driver for evolution is not strong
enough to have an impact on the species as a whole.
Another reason why motorway deaths may not have a significant impact
on the fly population is due to when the flies breed. Even if
motorways killed 90% of all flies, the fact that many insects breed
hours after hatching from their egg means that the population is still
able to reproduce. Killing something after it has bred is again not a
driver for evolution, it would have to be a very strong force that
kills a high number of individuals before they have chance to breed.
In summary, motorway deaths affect such a small amount of individuals
that it does not have a significant impact upon their populations, I
imagine that many many more of them get killed in spider webs each
day, however the idiots are still flying into those too!
Hope this helps :)
*The peppered moth changed colour from mottled to black due to the
population from the industrial revolution and then to white once the
air pollution had cleared as the ones that stood out had such a high
*in the last 50 years some fish species have evolved a much smaller
body size due to the advantage of escaping through a net and being
able to carry on breeding