Oh wow...that's terrible. I just got home from Red River Gorge and Natural Bridge State Park in KY and one part of it is the "Nada Tunnel" - which is a one way tunnel with very limited turn around spots since you are in the Gorge itself. About 2 miles out I see a sign that finally tells me the height inside the tunnel is 12ft, with no tape measurer and the bikes on top I had to measure from the group up with a piece of 10 1/2 inch paper and make sure I was fine. Luckily I was bout 8-10 inches shy, turning around would have been at least an hour to our starting position and put us 3-4 hours out from where we were headed.

Red River Gorge : http://en.wikipedia.org/wiki/Red_Riv...ge#Nada_Tunnel
Nada Tunnel is on that page

Natural Bridge State Park:
http://en.wikipedia.org/wiki/Natural...Park_(Kentucky)

Overall I like them on top because getting in and out the rear door is already troublesome enough for someone like I, that does not have a rear popper. I personally would not want to have to swing an arm or two to get into the back. But that is just personal preference. However I said it earlier, it is NOT EASY getting them up and mounted in those tracks. I am 6'2'' and the height is a little bit of an issue. I could see it being much easier if my style of rack was one where you took off the front tire, since the spinning of the front tire is the biggest hassle when the bike is over your head.