When we were planning a road trip, we asked our friends if we could borrow their storage rack for the top of the car. We wanted to try it out before buying one of our own. They have the X-Cargo X-Treme Car Top Carrier, which they purchased at Sears for I think around $250.
This particular model holds 100 pounds of gear. You can put camping equipment, luggage, strollers, etc. in this very handy necessity. We have a Toyota 4Runner SUV and found that this carrier fit quite nicely on top. It comes completely assembled, so no need to worry about having to put this together. It also locks to prevent someone from stealing your items. The material that the carrier is made of is a very strong plastic type material. The only downfall to this is there is kind of an increase in the sound of wind, but we got used to it after awhile.
When we arrived back home, we went to Sears and purchased a carrier because we can not imagine having to take a road trip without one of these.