Ambiguity of "Dogs must be carried on this escalator"
The sign is not ambiguous because — as you said — everyone knows exactly what it means. Ambiguity can creep into the most precise and lawyerish of writing. The sign shouldn't be re-worded because it's a good sign. Its job is to tell people what to do, in as few words as possible, and it accomplishes that.
"Dogs must be carried on this escalator" can be interpreted in a number of ways.
- All dogs should have a chance to go on this wonderful escalator ride
- This escalator is for dog-holders only
- You can't carry your pet on the other escalators
- When riding with a pet, carry it
The most salient message is that the escalator is dangerous for dogs, but you can't exactly write:
This escalator is also a dog skinner: hoist your pet when riding
Lift your dog or I will eat it, om nom nom — the escalator
Friends don't let best friends ride escalators — designate a dog carrier
Standing on moving stairs may cause brain damage in dogs
I'd keep the sign as-is, but if I had to change it, I'd say:
Don't put your dog on the escalator
This is a short unambiguous directive, and implies that people carry their dogs on escalators. I was thinking that perhaps "don't put your dog down on this escalator" would be clearer, but that just leads to a joke about not calling your dog fat while it's on the moving stairs.
You could tweak the original sentence to say "Any dogs must be carried...", meaning if you have a dog you must carry it. But if the real goal is to say "Dogs may not stand on the escalator", it would be clearer to say that:
Dogs are not permitted to stand on this escalator; carry them.