This question has been bothering me ever since I went to a Kohen shuir.
If a Kohen is driving in a car and about to enter a tunnel. Just before he enters the tunnel, he sees a funeral procession entering the tunnel. He is 100% sure that the nifter is a Jew. Can the Kohen enter the tunnel?
No. (Of course, if pulling up suddenly to avoid the tunnel will involve a risk to life, he may not do so.)