The UN ECE R134 regulation is a good requirement to follow as it copies the language in the UN GTR #13 regulation. The updated version of this UN document (UN GTR #13 Phase 2) is currently in approval review at the GRSP in Geneva and should be approved by the end of 2023. Nevertheless, since the US Department of Transportation’s National Highway Traffic Safety Administration is a contracting party to the UN GTR #13 development process, they have already begun a review of the Phase 2 document and are currently creating the FMVSS standard language for this new NHTSA requirement. It won’t become a Federal Motor Vehicle Safety Standards standard for at least 1.5 years, so for now it should be considered as a due diligence requirement for automotive FCEV applications. Since ECE R134 represents the latest embodiment of UN GTR #13, this is a long way to say that following R134 puts one in compliance with UN GTR #13 and hence would be following best practices in the USA (and Canada). 

From a vehicle design perspective, it is always best to ensure that locations where leaking hydrogen can accumulate are minimized/avoided, and that hydrogen sensors are placed in strategic locations where hydrogen can potentially collect. Potential ignition sources in areas where hydrogen can collect should also be avoided. If this safety concept is followed, then the 3% warning/4% shutdown strategy will work fine, although some original equipment manufacturers (OEMs) may choose to employ a stricter requirement of say, 2% warning/3% shutdown. In the event of a detected hydrogen leak, there is a requirement for the vehicle CHSS to be isolated but there is technically no requirement for the high voltage system to be isolated. Some OEMs may choose to leave HV on so that the vehicle may continue in limp mode to a safe location, while others might choose to isolate HV as well, leaving the operator with very limited remaining battery power to safely pull over. 

There are no requirements for the use of detectors for hazardous material transportation. Use of detection equipment is dependent upon the manufacturer and operator based on risk assessment.