There is a difference between accidental deaths and intentional killing. Veganism is about stopping animal exploitation as far as possible and practicable.
Accidentally killing an animal and eating their flesh is something completely different to deliberately killing an animal and eating them.
If you happen to find a dead animal and eat it, you are not exploiting them for their life, just their dead remains.
Roadkill is a side effect of our advancement as a civilization. Unfortunately there is no way to avoid using cars or transportation if you want to keep living in our society.
Roadkill is akin to crop deaths, a side effect of our advancements. No other way to keep our society, but animal farming can be completely avoided and it's easy and cheap once you get to it.