In addition to the above...
A towel...for standing on the road, waiting for the wrecker or for traffic to clear when its 100 degrees out,to wipe off the sweat.
A couple of bottle of water...for scenarios like the above.
Alcohol based hand sanitizer...to wipe the blood off your hands and arms so that you don't get it all over your car.
Traffic Vest. Chartreuse colored. Ours have our Dept name on them. Since I wear black and grey I am invisible at nighttime. That vest has kept me alive more than once. Also to respond to emergency's when off duty. We are required to wear it then so we can be easily identified. It works well.
I've been using a Hatch Patrol bag for around 10 years. It probably weighs 50 pounds with everything in it.
A towel, to wipe puke off of you when you apprehend a drunk.
An extra knife.
A towel, for the little unexpected things.
Some disposable wipes are good for when you step in dog poo. Or some heathen spits on you right before he gets beaten to within an inch of his life.
Did I mention a towel?