It kindsorta depends on the agency, the location, and the vehicle. When I was a highway cop working the L.A. freeways, in addition to our personal stuff, hats and helmets, we always had at least three boxes of 30 minute flares, tops cut off, and all of the end caps already swapped around, so that we could flare off three lanes of a frwy as fast as possible. Didn't need much in the way of first aid supplies, the fire dept and ambulance usually beat us there anyway. Spare tire, 4-way lug wrench, jumper cables, and the main part of the company radio stuck back there. Fire extinguisher.

When I transferred out to the boonies, we had not quite as many flares (I-5 only has two lanes in each direction there, but you might have to keep a flare pattern going much longer), I became an EMT and had a pretty well stocked (and bulky) kit, a shovel, wire and staples (my idea) for patching holes in a fence to keep the critters in, and half a dozen traffic cones. One gallon cowboy canteen. Radios got bigger, more parts in the trunk. Newer bigger fire extinguisher.

Then we got Ford Mustangs, which had a much smaller trunk. No cones, fewer flares, GI E-tool (mine) instead of a full sized shovel, EMT kit now in the back seat. Then we got Chevy Cameros. Look and sound really cool, accelerate like being launched off the nose of a carrier, NO trunk at all. Blow-up spare, one case of flares if you were lucky, jumpers, 4-way, EMT kit, your gear, fire extinguisher, and of course those radios, all crammed behind the back seat, on the back seat, on the back floorboards (what little space was there).

In addiiton to all this, I had a sample case kind of thing with clipboards, extra forms, spare loaded mags in GI mag pouches, spare shotgun ammo, when we got rifles spare 30 round 5.56 mags in GI pouches, half a dozen pair of spare cuff, flexcuffs, 100 ft tape, the list goes on and on...

My best friend was the sniper on the sheriff's SWAT team, he carried flares (we gave 'em to him), shovel, jumpers, fire extinguisher, a parachute bag with all his SWAT gear in it, two hard cases with his sniper rifle in one, and a Colt H-BAR in the other, plus little goodies to make his life as a sniper more comfortable (Thermarest pad, food, water, blackwidow spider spray, I don't know what all).

As you can probably guess, it takes quite a bit of time just to carry your personal gear out and get it stored, plus check to see that all of the issue stuff is actually there...