"Stress-free" and "car trip" might seem like a very unlikely combination, but a little bit of basic planning will make a big difference: you can't do anything about traffic, weather and other drivers, but you can make sure you are ready to take them on in a (relatively) relaxed manner. All of the below apply even more if you are travelling in a foreign country or unknown area, vehicle or climate.
(1) Give yourself enough time.
This is probably the most important of the ways towards a stress-free car trip. There is nothing more stressful than having to hurry when driving, and it's also likely to seriously impact your safety. When calculating the time for the trip, make a realistic estimate of the driving time (route-planning websites like Google maps, or your own SatNav, will give you a good idea of driving time in normal conditions), and then add corrections for conditions (winter, wet weather, first and last days of school holidays and similar) as well as enough time for scheduled and unscheduled stops. On the other hand, driving overnight on a clear, dry and warm night or driving through largely empty country will mean that your average speed will likely be better than the machine estimates.
The further you go, the more likely you are to need unscheduled stops. The more people you have in the car, the more likely you are to need unscheduled stops. If those people are very young, or old, you better factor in those stops in as scheduled ones, it will be less stressful
If driving on a motorway where service stations are elaborate little villages requiring detours, each of your scheduled stops is likely to take longer then if you can just stop in the roadside lay-bay for a toilet break, stretch of legs and (if there are smokers in the party) a cigarette.
(2) Factor in breaks.
This one follows on from the previous one, but is about more than just allowing time for the necessary stops. Tiredness is a major factor in road accidents, and tired drivers are more stressed drivers. It is recommended that breaks are taken at least every 2-2.5 hours of driving, and the best break involves stopping for more than just a drivers' swap. Get out, walk about, breathe some air that is not car fumes. Stretching and a little bit of exercise is not bad either.
(3) Stay hydrated and fed.
Make sure that you have water or other fluid (fizzy, sugary drinks are not the best) as well as some food. Sweets are good for an instant boost, but can lead to energy dips and crankiness. Slow release carbs – nuts, sandwiches and fruit – may be better. If driving a longdistance, try to plan for a meal break. If you can afford it, stop at a restaurant or a nice country pub rather than a service station feeding station. Don't eat too much, though, to avoid sleepiness.
(4) Know where you are going.
If you are traveling to a destination you have not visited before, make sure you actually know the directions and have them handily accessible to you and/or your navigator. Take a map even if you have a SatNav, and include contact details for your destination with the directions. If there are a few possible routes, agree on one with your partner or co-driver beforehand.
(5) Carry means of communication.
Make sure your mobile phone is charged up, and have a in-car charger just in case. Have phone numbers for your destination, your breakdown service provider and your insurance providers easily accessible. If bypassing large cities, listen to traffic information on local radio for diversions and traffic problems.
(6) Check the car and make sure it's roadworthy.
Nothing can make for a more stressful journey than a constant worry about the state of the vehicle. Before a particularly long journey, especially abroad, or with a heavy load, have the car serviced. Before a shorter journey check the levels of tire pressure, oil, break fluid, water and windscreen wash. Fill up at the beginning of the journey and don't run out of fuel: if crossing a particularly unpopulated area or driving late at night in the countryside, make sure you always have enough fuel to get you to the second next fuel station. Carrying jump-start leads, a tow rope, basic tool kit and a manual (Haynes manual, for example) for your car model might add extra reassurance.
(7) Mind the kids.
Long car journeys are not the best way to travel with children, but if you have to undertake one, plan for even more breaks and take plenty of entertainment (but not the car-sickness inducing types like books or console games). Audio books, music, games to play and bribe-treat packs are all excellent. If traveling with small children, consider seriously driving at night, as having them asleep will make a massive difference to your comfort and stress levels (don't drive at night in countries or areas where it's not safe, either because of crime or wildlife hazards).
Planning for a less stressful car trip will not only make the journey less stressful, but also safer, for you, your passengers and others.