It takes a signal (1 km)/(200,000 km/sec) = 5 μsec to propagate from one end to the other of a 1 km (fiber) channel. Hosts at either end of the channel have a 5 μsec window in which they can operate as if they had uncontested access to the channel. With 1 Gb/sec and 1,500 bytes/frame, 5 μsec is almost half a frame.
It takes 1.2 μsec to transmit a 1,500 byte frame at 10 Gb/sec, well within the 5 μsec window. Collisions can occur after a frame has been sent, and in those cases the sender generally can’t use the collision to determine if its frame has been damaged. In these cases a higher-layer reliability mechanism, such as a network-layer time-out, is required to recover lost frames.