Karl,
  I believe the DOT has a required minimum clearance of 14 feet from road
surface for heavy trucks. (tractor-trailer) If it's less than that there
should be a yellow diamond warning sign that gives the true height. So
rack-up your bikes, check your tires, (van empty w/ little gas in tank) and
measure the height.  My '80 vanagon wasn't much over 6 feet and most bikes
are less than 4 feet. You should have no problems....
Dan

>
> One concern I would have with a rooftop bike rack is the clearance.
> I don't want to be flying down the road and smack my bikes into an
> overpass and peel back the roof like a sardine can.  Anyone have
> experience with this w.r.t. height and typical clearances?
>
> -Karl
>
> Karl and Kristina Bloss, Trexlertown, PA
> '87 GL Weekender  "Beverley"
> VW shop list: http://www.enter.net/~bloss/vw/vwshops