A55 a few years ago, car passed me with bikes hanging off the back, straps obviously loose, tried flashjing lights at the driver, he wasnt looking, a mile up road on Holywell hill the traffic stopped dead, llgv accross both lanes small gap to squeeze through, bikes had fallen off car lgv had hit them and crushed them then got a puncture ad a pedal had gone through tyre.
Result a lot of unhappy motorists late, a damaged truck, 2 damaged bbikes and a large fine for the driver, and the police randomly stopping car carrying bikes for a few months after.
Now heres a tip, get a steel cable lock, fix through frame and then under roof bars and lock, if bikes come loose they just hit vehicle and not endanger any other road user.