Believe it or not, many new vehicles come without a spare tire. Manufacturers have a few different reasons for that, including weight savings, space efficiency,
Instead, these vehicles come equipped with an inflation kit and/or a can of sealant.
Sealant is a gooey substance in an aerosol can that's designed to coat the inside of the tire due to centrifugal force once you get rolling again, hopefully sealing the puncture. These products, such as Fix-A-Flat, have been on the market for decades and tend to work pretty well on a minor puncture. They're not a permanent fix, however. Your speed should be limited after using Fix-A-Flat type products, and you should see about getting the tire repaired or replaced as soon as possible. In addition, most of these products freeze at temperatures below 32 degrees and may not be usable in cold weather.
The other alternative on new vehicles is an onboard compressor which usually plugs into the cigarette lighter. These little compressors actually work quite well and can refill a tire in a few minutes' time, getting you back on your way again.
This is all well and good, but...
Many times, a tire which fails at highway speed is going to be shredded by the time you can get off the road, or at least permanently damaged and ruined. No inflation kit or can of sealant can help you in that case.
No tire can be repaired if it has a hole in the sidewall or the shoulder. In that case, you've got no other choice but to spring for a new tire.
You can always invest in a spare tire and a jack if you're really concerned about it, but in many new vehicles, there's not even space for a spare.
The upshot? You might want to just make sure your AAA membership is paid up!