yeh bare it mind curly cos to many people and it'll be hard to organise/fit into a day etc..
set someone off every 30seconds, for 50 people that will take 25 mins to do the first runs then do that again it will take 1hour to do 2 timed runs (if everything is on time)
(the walkies would only be used for the first start or in case someone doesn't start)
then every 30secs or 1min, the guy at the top lets one guy leave and the one at the bottom just get the time or arrival (or deduce it from the time of departure since he knows it)
Better have 2 people at the finish line, easier ^^
on most sports watches and some phones you have a lap timer where you push a button when the person crosses the line (obviously the timer has been started at the start) and it records there lap time not sure how you could work it unless you have two times down the bottom with walkies and a guy up top with a walkie saying when they go