Greetings! I’m making a car racing game and I’m a beginner …so the problem is that as i click on play my car falls through the ground…I have imported the car using Unity Standard Assets & hence the colliders are already applied to all the four wheels…I also have attached a screenshot for reference.I created a long bridge using Autodesk Maya & I’ve imported the bridge in Unity …My car is moving perfectly on a plain surface or terrain without falling through the ground however when i placed my car on that bridge the car doesnt seem to move and it falls through the bridge…I tried adding “box collider” & “rigid body” (I checked the option- use gravity) as well to the bridge but still the car is falling through the bridge …Please help me since 2 weeks I’m trying to figure it out but I haven’t achieved any success …Any help would be appreciated ! 
I think it’s something very, very simple. You do not need rigidbody and gravity on the bridge. I have a feeling that your box collider is too large(or too small) that when you place the car, it is actually inside the collider. That will cause it to fall through since it’s already inside the collider. You can adjust the size and the position of the collider, independent of the bridge.
Can you take a screenshot while the bridge is selected, so we can see the inspector on it?