What constraints are you trying to meet?
I would like to be able to identify when a rider enters and leaves the course so that I know how long they are on course for. Each rider needs their own unique identity. Start and finish are significantly far apart to need some form of wireless communication. Will need to handle multiple riders on course at one time.
What sort of distance will there be between the start/end points?
I was considering an Xbee pro for the transmitter that has a theoretical outdoor line-of-sight range of a mile. These races take place on hillsides and often in quite dense woodland but the start is always at a much higher vertical elevation than the finish.
How tolerant will you be of false/missing results?
This is not for any regulatory body but I would rather avoid any if possible.
Does it need to be tamper-proof?
Not really, this is just for use between friends and clubs so I wouldn't expect any attempts to cheat or manipulate the system
Can you get away with simply banning overtaking?
No, but we could theoretically only allow one rider on course at any time. However this is not desirable as on a longer course (3 min for a fast rider or 5+ min for a slow rider) with many people each requiring 2 runs, you can quickly run out of time.
Is there anything stopping you having a human element involved in the normal operation of this?
It needs to be as automated as possible, I don't want someone writing down riders numbers as they finish for example.