The Eisenhower Memorial Tunnel is the highest vehicular tunnel in the world, and it feels every inch of it, at least when you're approaching it from the west, as I did during a roadtrip in 1996.

Signs along Interstate 70 warned drivers that vehicles may be subject to vapor lock as the altitude increased. I remember, at one point, looking off to my right and seeing clouds below me, an experience which had previously been reserved for plane rides.