There are no international standards (ISO or ECE) for rear cycle carriers.
However, there are UK regulations to keep lights visible and ensure a safe load.
ECE 48 defines requirements for lights and their visibility. Reverse, fog, stop, indicator, brake and number plate lamps are mandatory on all vehicles.
The Road Vehicles Lighting Regulations 1989, reg 21 states,
“For A vehicle which carries a load or equipment which obscures any obligatory lamp, reflector or rear marking, additional lights shall be fitted and all the installation, performance and maintenance requirements shall be complied with.”
There is a proviso that between sunrise and sunset if the visibility is not seriously reduced, you only require, brake and indicator lights.
The angle of visibility vary depending on the lamp, but if you achieve sideways 45° and up/down 15° you will in compliance. Remember the fog lamp should be on the drivers side.
Road Vehicles Regulations 1986 says,if the rearward extension is less than a metre, and the lateral projections less than 305mm then no special measures are required.
Finally Road traffic act 1991 states,
“A Person is guilty of an offence if he uses, or causes or permits another to use, a motor vehicle or trailer on a road when: the weight, position or distribution of its load, or the manner in which it is secured, is such that the use of the motor vehicle or trailer involves a danger of injury to any person.”
A cable lock through the bikes and the carrier frame back to the vehicle adds security and piece of mind.