Be careful... I managed to temporarily close the A140 a few years ago when I was carrying a load on the 'Paddy Hopkirk' style of bars. The front nearside clip 'popped' and the load (a bedframe wrapped in polythene- and now suddenly a sail !)took off, half of it slamming back into the offside of the car and the other half almost wiping out a following motorcyclist. The nice man from the RAC and the equally nice policemen eventually helped me secure the wreckage onto the car with cargo straps through the windows before carrying on my otherwise event free journey.
No harm done to anyone thankfully but an insurance bill of £4000+ and two traumatised children in the car.
With that experience I now only use the screw fixed type, costs a bit more but worth every penny and easy to fit and remove.