Question

why the highway road are tilted

Answer 1

It's because of physics. When a car drives along curve, the momentum of the car wants it to go straight off the road because its easier than turning. This is why if you turn a car suddenly, it will skid in the direction you were going before you turned. By making the road curved, the force of the road pushing against the car (known as normal force) helps push the car round the turn. You can think of like it being difficult for the car to skid uphill. Furthermore, think if the road was tilted 90°, so it is basically a curved wall. The car would have to turn as it drives alongside the wall because it can't go straight through the wall. A tilted road is a less exaggerated version of this. I can give a more physicsy description if you'd like.