Legally, in a situation involving your horse, YOU are the ultimate authority.
On one hand, you are probably much more knowledgeable about horses, horse behavior and horse safety than the "emergency responders" (ERs Fire, Law Enforcement, Animal Control, Ambulance).
On the other hand, if you are hysterical and interfering with a rescue in a way that makes the rescue more difficult or less safe for the ERs, you will be set aside and your "words of wisdom" will be disregarded.
Most ERs do NOT know how to handle horses or other large animals. If you see a responder doing something with an animal that you KNOW is harmful, you need to step in before someone animal or human is further injured. It helps if you can speak their language and you know basic scene protocol.
There are three steps you can take toward ensuring your horse's safety in the event of any incident where you would expect your local ERs to help out.
The first would be to connect with your local ERs, especially your fire department. By talking with them you will learn just how much they know about rescuing large animals. That is your "baseline". Explain to them your concerns about THEIR safety and the safety of your horse. Showing your concern for THEIR safety first and your horse second will win you "Brownie Points"! If they don't have much experience, offer to help. You can introduce them to your horses, teaching them about horse behavior and how to keep themselves safe around horses. (You can use the "Horse Awareness and Safety class" offered at Red Jeans Ink)
Second: By learning about the Incident Command System (ICS) you will further your cause by understanding the protocol for emergency response and your possible role in a response. (You can take a free class on ICS on the FEMA website)
Third: Learn about Large Animal Rescue. Once you understand how to communicate with ERs you will need to know what to say. A good place to start is this Large Animal Rescue website.
Here are a few tips about trailer accidents for both you and the ERs to start you on the road to LAR safety.
DO NOT open ANY door to see what’s going on inside. Find the smallest opening possible and peek in. An open door is an invitation to the horse to escape, even though he is injured or trapped; even if the opening is impossibly small.
DO NOT approach the scene of an accident with squealing tires and horn blaring. If you are at the scene before the ERs arrive, send someone down the road to stop them, or when you call 911 have dispatch tell them to approach silently. Have them turn off sirens and flashing lights.
QUIET! Remove as much of the chaos as possible. If the incident happens in a crowd, establish an area around the horse that's empty of everyone who does not need to be there and is quiet. If it's a trailer accident, SAFELY stop traffic and keep bystanders away.
DESIGNATE a "safe area" before you remove the animal from the trailer. You want to avoid a "secondary incident" safely remove a horse only to have him jump up and run into traffic. Surround the scene with vehicles in a circle. Use fencing if you have it available. Use fire hose to create a barrier, reinforced with humans to "encourage" the horse to stay in the area.
DO NOT use the horse's legs, tailbone or head as handles. Horses are not built like humans and are easily broken.
Hopefully these few tidbits of information will pique your curiosity about LAR and you'll want to learn more. Your horse's life could depend upon it.