After spending 30+ years responding to traffic accidents, more than 20 of them as an EMT, I figured out what I needed more than anything, so, even tho I had a pretty good kit, compete with oxygen, hard collars, etc, what I stuck right on top, for easy access, a bunch of ziploc bags, each one packed with a stack of 4x4's, a roll of Kling (or whatever is available now, a roll of stick to itself gauze), a roll of surgical tape with one corner turned under (peeling up the end of tape with gloves on is a bummer), and a pair of those gloves. Add a pocket mask, with one way valve, if you know CPR, and a pair of EMT shears. We had "body blankets" in the vehicles, but add anything to help keep body heat in and the weather out. Down a tad deeper a couple of sanitary pads, for lacerations larger than a 4x4.

If you have the trng, some vaseline gauze for a sucking chest wound, and some sterile saline if you are lucky enough to encounter an abdominal evisceration. But with enough of those 4x4's you can do a lot of good...