Sometimes, you are all the help you are gonna get. It is vitally important to be prepared for traumatic injury to yourself or whoever you come across at the scene. Emergency medical responders might be minutes or more away; but time can't wait. Whether you are police, fire, EMS off duty, or a prepared citizen; always keep a kit nearby and know how to use it.
At a minimum, it is a good idea to keep a tourniquet on-hand. Better yet, keep a couple tourniquets readily available. It is recommended that you keep any kind of tourniquet with a sturdy windless so you can really be effective with it. Keep tourniquets in the car and keep them in your pack. Some people even keep them on their belt because seconds matter. If there is space, add to that some trauma dressings like the OLAES Modular Dressing and gauze for wound packing. At least. Do this for yourself and do it for others who need your help.
Of course, the most important trauma supply you can have on-hand is know-how. Get training. Learn to stop heavy bleeding and learn to treat the most catastrophic wounds. Do this so you can remain effective until help arrives, or be the help someone else is praying for.