Collider for road track in Unity3d car tutorial

Hi , i am new to unity3d. i am doing a sample car project . i have loaded my city and added first person controller . when moving through the city , the first person doesn't collide with objects in the city. if the enable Generate Colliders in the mesh , the first person object collision happen. is this only way to make collision? i have explored the unity3d car tutorial , where how the collision work with Road fench guard and car ? how can i make thing simply