Car tires are more important than many people give them credit for being. They are literally your link with the road. The most dangerous place for you to be is out of or in poor contact with the road. In this predicament you are not able to have any input into your course of direction and that is a bad thing when you are going 60 plus miles per hour with no way to avoid crashing into something. Friction is what we are going for and there are some things that you have control of and some things that you dont in this area. You are responsible for your own and anyone elses life in the car you are driving and so you had better be sure that you have taken all the measures that you can to prevent a problem.
Now the contact of the road has several factors as already mentioned. The majority of the friction comes from the contact of rubber of the tire with the concrete or asphalt of the road. The action of gravity on the mass of your car is the only thing that is causing this contact and so naturally more weight equals more friction between the road and your car tires. A good thing but can turn against you in the form of inertia after the friction has been overcome say after...