If you have a roadside emergency kit in your vehicle, you're a step ahead of most drivers. But you should have 3 very critical things within that kit. If you don't have these things, you could very well be a sitting duck when an accident occurs. With that in mind, consider adding these few things when putting your emergency kit.
Lights (Flashlights, Flares, Etc.)
Do not dismiss this. Many accidents occur in utter darkness. Roads aren't always brightly lit, and neither are freeways. If you're going to be driving long distance, or working late, it's important that you have lights.
Food and Water
You never know when you're going to be stranded. You could be miles and miles away from help, and that could mean dehydration, and hunger. These things can lead to loss of lucidity, and could even cause some to pass out. Have these just in case you're stuck waiting for a long period of time.
Extra Batteries
Your flash light needs batteries, your smartphone will need recharging, and you may have other devices that need power. Always have extra, so that you're able to stay safe amidst any situation.