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A lot fewer than if they started in Belgium!
Maybe 80% of their riders come from between Manchester and Vienna and Geraadsbergen is accessible without flying for most of them. I know they have to keep it interesting by changing things but there is a conflict between that and all the stuff they say about carbon footprint.
Getting back to the UK from further east than Croatia or Poland by land is always going to take 2-3 days though.
How many people will travel not by plane to both the start and from the end do you think? Train from Constanta to UK doesn't sound much fun on top of 4500km.