the cars controls and physics are handled on client. for what roblox physics is, it wouldnt be smart to enable collision because one car could hit the other, while the hit has zero effect on the other car. maybe it would work with a more advanced system that trusts collisions from both clients, and mimics them to the other client. an easier and exploit-proof way would be the server should watch for when cars touch each other, calculate the forces mimicking collision, and tell the involved clients to create that force.
