You can buy packaged emergency safety kits, like this $35 one from AAA, which includes a booster cable, flashlight, first aid kit, and many other items, but the DIY approach is more satisfying and you probably already have many of these items lying around. It's not just about emergencies or safety, either. Below I've separated the checklists by category
There's nothing like breaking down at the side of the road and realizing the spare tire in your trunk has a flat from the last time it happened. (True story.) To keep you up and running, keep these in your trunk:
Spare tire (in good condition), along with a tire jack and tire iron, because without them or someone else to help you, the spare tire is useless. Here's how to change a tire, in case you need a refresher. Also, if your wheels require a special security key, make sure that's always in your car too.
Tire inflater and sealer, like the Fix-a-Flat, which can plug a leak (and help you avoid using the above tools) just enough to get you to the auto shop.
Jumper cables, because dead batteries happen to the best of us. We've got a crash course on how to jump-start a car, but you should familiarize yourself with your engine just in case things are a little different. Alternatively, you can pack an emergency battery booster so you don't have to rely on a Good Samaritan coming along.
Your car's manual, which should be in the glove compartment already.
Tire pressure gauge: As our sister site Jalopnik points out: "checking tire pressure on a regular basis can improve handling, increase fuel economy, promote tire longevity, and even save lives."
Duct tape and WD-40. Seriously, check out these 10 heroic duct tape car repairs.
Car repair information. A business card for your auto repair shop, the number for AAA (if you're a member), and car insurance claim forms should also be stored in your glove compartment.
You might already have an emergency go bag or kit set up. If you spend a lot of time in your car and it's always nearby when you're home, you could just keep that kit in your trunk—or create a second, perhaps lighter version.
In any case, your safety supplies should include:
A few car-specific items:
Seat belt cutter and window breaker. This one's $7 on Amazon. Keep this in your glove compartment, not in your trunk, obviously.
Flares or reflective triangle, so you don't get hit at the side of the road in the dark.
Maps. Yes, the paper kind.
Convenience and Comfort
In addition to the basics above, you might want to keep these things around also:
Paper towels or a hand towel
Tissues or a roll of toilet paper
Pencil and paper
Spare change/emergency money
Recycled shopping bags for those impromptu shopping trips.
Blanket, which comes in handy not just for keeping warm in emergencies, but also at the park, baseball stadium, etc.
Change of clothes: also an emergency item, because if you get drenched in rain or snow, it's no good to sit around like that.
USB mobile device charger