It is not true that you can ALWAYS dial 911 on an unsubscribed cell phone. Some carriers permit it, but some do not. It does not work in Portland, Oregon, for example. Some states require carriers to enable it, but not all do.
Personally, I rarely drive out of the greater Portland/Vancouver area and I see no need to pay for extra gas to haul around a trunk full of camping supplies and food stuffs when I know that I'm not going to be more than 1000 feet from a good restaurant. For the urban communter, the car emergency kit is much different:
Subscribed cell phone (you can get one for as little as $9.95 a month in Portland).
$100 in cash. You can get towed most anywhere and get a cab home for $100.
A fire extinguisher. A fire under the hood can really do a lot of damage fast and even if there's a fire station just off the next exit, in rush-hour traffic, they won't get there fast enough. I've passed enough burned out wrecks just as the fire trucks pull up to know.
A good jack. Check the one that comes with your car. They're often flimsy and will need to be replaced.
A good tire iron. The one that came with my car was made of bent sheet metal, a fact that I did not discover until it was to late and I had to go to the cell phone and $100 cash just for a flat tire. Now I carry a 1/2" socket wrench with a long handle and the appropriate socket.
The key for your wheel locks if you have them. Without that key, the AAA man is going to have to get violent and that will be expensive.
Some cheap rain boots. Cars always seem to break down when it's pouring rain and you're wearing dress shoes.
An inexpensive rain jacket for the same reason.
If your car has a unibelt, as mine does, a spare. It's about a $20 item, doesn't take much space, and can be installed in about ten minutes with only the aforementioned socket wrench required.
An assortment of hose clamps and a few lengths of hose appropriate for your car.
A gallon of water in a well-sealed container.
Some cheap work gloves.
Any other hand tools that you feel qualified to use for minor repairs.
A good city map.
A working radio and know which station has the best traffic reports (AM 620 in Portland).
A flash light.
Chuck