While roof racks are fine and very convenient, keep in mind that bicycle windage is murder on your fuel consumption. So if you're planning a long trip, you might do better with the bikes inside, or on a rear rack of some kind.
Also, learn to note and read overhead clearance signs. Can't count the number of people I know that wrecked their bikes on the roof, usually when using indoor public parking, but also driving into their own garages.
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