I never felt very confident with the boot mounted rack that secured with straps, although we did do a trip from Penzance to the New Forest and back with 2 road bikes on it, and I certainly didn't stick to 60mph! We stopped about every 2 hours on the way there, and the straps needed tightening every time. Did the same on the way back, but I'd positioned the "doughnuts" better, so the straps hadn't moved.
We replaced it with a towbar mounted rack, which was much more confidence inspiring. It was the type the bikes sit on, and are secured to the wheel plates and to an upright bar. We drove from Penzance to the Channel Tunnel, and then all the way down through France and Spain with 2 bikes on it, and nothing moved. I was doing 90mph on the French motorways quite happily, but the bike rack and roof box made for quite a few fuel stops! (This is all past tense because we're now car-free, having sold the car, roof box and bike rack.)