I've got a different list. Essential?! On a road trip I think that you worry about your survival and/or how to get a stopped car going again. For a gasoline car, why are they on the side of the road? #1 is clearly running out of gas. Now, this may be more of a "trunk" item, but "Extra Fuel" is what you need in your wife's car so that when she calls you out of gas, you can talk her through filling up a bit and save yourself driving out to wherever she ran out: Amazon.com: 1/2 Gallon Extra Fuel - Emergency Fuel Solution NON Flammable 5 Worldwide Patents: Automotive Starter fluid would be good to have on-board if you have room (the mini-bottle at O'Reilley's is about the size of your lunch milk in school). #2 is probably a dead battery. There are small products for that: Clore JNC300XL 'Jump-N-Carry' 900 Peak Amp Ultraportable 12-Volt Jump Starter with Light : Amazon.com : Automotive Also helpful is a spare alternator belt or women's panty-hose in case a thrown or broken belt is what caused you to have a dead battery in the first place. #3 I'd think would be a flat tire. Easy: Amazon.com: Johnsen's 3950 Non-Flammable Quick Flat Fix with Cone - 13 oz.: Automotive #4 might be a coolant hose giving way. Easy patch: Victor Hose Bandage (2 in. X 10 ft.) - VICV302 If you want to go nuts, how about an adjustable wrench or a multi-tool: Crescent FR28SWMP 8-Inch Adjustable Ratcheting Socket Wrench - Amazon.com Couple of spare fuses, maybe?! OK, that's great for the car...how about for you on the side of the road?! #1: A reliable flashflight (handcranked!): Amazon.com: 2pc Wind-Up Flashlight Set: Home & Kitchen #2: Nice Mylar emergency blanket: Emergency Mylar Thermal Blankets (Pack of 10): Amazon.com: Industrial & Scientific #3: First Aid Kit: Medique 40061 First Aid Kit, 61-Piece - Amazon.com #4: Bottled water #5: USB car charger for your phone: Amazon.com: GTMax Mini USB Car Charger Vehicle Power Adapter - Black for Apple iPhone 4 4G 16GB / 32GB 4th Generation: Automotive