Based on the premise that signs telling you not to do something are a sign of people trying to do it, the fact that this one is telling you not to travel in a lift (that's "elevator" for people abroad) with cryogenic fluid is a clear sign that somewhere in the city there are people trying do exactly that.
As the sign says, "Do not travel in this lift with cryogenic fluid. It could kill you". It then documents the process for transporting cryogenic fluids in an unattended lift by having people at both ends. Presumably if you are on any intermediate floor, the site of a large container and some hi-viz tape will put you off. That is something to experiment with, possibly in the Trenchard Street car park.
For our special winter festival quiz on Bristol's Transport Issues:
- What is Cryogenic Fluid?
- What exactly is it for?
- Why would you want to transport it in a lift?
- Why could it kill you?
- Where in the city is this sign? [ no cheating by looking up phone numbers. Ill-informed guesswork for consistency with the rest of the site.]