Not disagreeing with the sentiment of your post but curious how you’d stop people from standing on the pavement of a public highway? As essentially that’s what fans waiting outside the training ground are doing.
I'm regularly in Lancing and passing the entrance to the training ground. Up until last season I think I'd seen kids waiting on Mash Barn Lane maybe 4 or 5 times in total over the years. Since last season it's common to see that many at once lining the road and it's a regular occurrence now.