Just a thought:

Rather than IR, could you use an ultrasonic "transmitter" on each car,
tuned to a different frequency?  A sensor could pick up the approaching
vehicle, identify it based on its frequency, and the doppler shift would
tell you the precise moment it passed the finish line.

Ambient noise is the only obstacle I can think of....

Perhaps using a doppler shift from a light source?  again, frequency to
identify vehicle, and shift to detect time....

Just my $0.02 US, adjusted for inflation.....

--Joe Jansen

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