h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

If you're going to be somewhere crowded, commuter trains, standing area at Wimbledon, Tina Turner concerts etc. an umbrella, preferably one of the old non telescopic ones, is a good way of preserving some personal space .

That depends on which side of the umbrella you are on.

I don't think there is any crowd more jostling and space-stealing than the ones trying to get on and off the subways in New York City. Generally I stand back and let the first wave of foolish fiends push and shove their way around, then I casually slide in. Even this strategy has garnered me a few pokes and stabs by umbrellas thrust forward like swords to preserve someone else's personal space. When there is no space to be had, this practice is quite dangerous.

I know. Ouch.

You're right, it does depend which end of the brolly you're facing

Being responsible this can only be recommended, as a last resort, when the people you're warding off have some choice. Such as standing round the outside courts at Wimbledon. Which is the only time I've deliberately used the umbrella this way. I came to the conclusion the people behind were trying to make things so uncomfortable that I'd go away. I didn't.
On trains its only fair to use something blunter! Or some other technique; sneezing fits, nose blowing, extended coughing sessions...start arguing loudly with yourself - that'll clear the carriage in no time, with no harm done to anyone.