Everybody should know who and where
Not just the trained people.
In an emergency the useful knowledge is distributed to the wrong people. The trained responders know what to do; everybody else needs to know how to get them, and that is the part usually left to a poster nobody has read.
The information that has to be instantly available to any staff member is short: how to summon a first aider, where the nearest kit and defibrillator are, and the site address to give to the ambulance service. Three facts.
Testing whether they are known is a two-minute exercise. Ask three people at random. The results are usually poor, and they are poor in organisations with excellent training records, which is the point.
Signage carries most of this if it is placed where somebody would look under stress rather than where it is tidy. Above the equipment, at the exits, and at reception rather than in a corridor of notices.
The address matters more than people expect. Large sites, business parks and multi-tenanted buildings routinely cannot state their own emergency access point, and that costs minutes at the worst moment.
The other thing worth posting is what not to do. Untrained bystanders frequently move an injured person, offer food or drink, or crowd the space, all with good intentions. A short list of what to avoid is as useful as the list of what to do and is almost never displayed.