Way back in the day when I lived on the west coast and commuting was done uphill both ways in a driving rain/wind storm every day, I did occasionally use a car bike rack. The rack I used was roof mounted. I remember driving from Portland to San Francisco with my bike on the top of my car and when I arrived my bike was covered in dead bugs. I think I left them on for a while and told people I just ride really, really fast =)
Using a car rack for long trips does pose a security issue. You certainly don't want someone taking your bike or bikes when you are away from your car. Reader Thomas, who sent along the free cassettee spacer hack we posted a while back, also sent the following text and picture. If you have bike transport advice feel free to contact us.
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Here's a method to make it more difficult for would-be bike thieves to snag a bike from your vehicle. I was on a cross-country drive with 5 bikes on a rear rack, and I did not want to go through all of the hassle of undoing each and every one in the pouring rain, hauling them up an elevator, then re-securing each one again in the morning.
So in addition to using 4 locks on the bikes over night, I backed the van up against a light pole. That pole prevents someone from being able to take the bikes off the rack without moving the car or the pole... or using some serious cutting tools. One could also use bushes, a wall, or ever another vehicle to back up against. I highly recommend a spotter while you're backing up to make sure you don't crush any of your babies! You want to be really close to the object.