Roof-top bike racks are one of a couple of types of bike racks available for moving a bike from Point A to Point B. These are a great option for people who don't want to take a chance with scratching up the back of their car with the bike's handlebars on a back rack.
Roof-top racks are a great option. They can be used by people with factory-installed luggage racks on the roof. They can also be installed from scratch.
The biggest problem with a roof rack is getting the bike up there. These are not necessarily the ideal option for people who have tall cars. Getting the bike on top of the roof is the first problem and then there is the problem of a low tree branch or anything else over a road.