A roadside emergency kit is a kit that contains the most likely tools you will need in the event of a breakdown. These kits are widely available and generally include: jumper cables, roadside flares, antifreeze, ice scraper, basic tools (screwdriver, pliers, vise grip, wrench), and a flashlight.
Road and Travel Magazine recommends keeping a working cell phone in your car at all times. The phone need only have a minimal plan and thus can be very inexpensive. Keep the phone in an easily accessible location, such as the glove compartment, rather than the trunk. This way, if you get into an accident that leaves you trapped, the phone is within reach. Depending on the situation, you also may not feel safe getting out of your car and walking to the trunk.
Depending on where and when you are traveling, keeping an extra set of warm clothes or blankets in your car can literally save your life. This is particularly important when traveling with children. According to Jan Null, adjunct professor of meteorology at San Fransisco State University, 31 children died from hypothermia in the United States from being left in cars in 2009. Bundle the clothes in a trash bag and store them in the trunk or under the seat of your car.