Hacker News

The spark plug only works on tempered glass, on which these hammers also would also work great. The issue the article is pointing out is that many modern cars use laminated glass, not tempered, which is much harder brake, and on which the spark plug trick would porbably work even worse than one of these hammers.

Also, carrying around a tiny piece of a broken spark plug in your car, in manner where you can quickly and reliably access it in an emergency is probably difficult.