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as bp says - yes, difficult to enforce but having the rule there would lend a lot more weight to organisers enforcing bans in the first place. OK triathlons are in the main much smaller events (London Tri has over 10000 taking part but that's over a number of races over 2 days so each race is relatively small in numbers) so it's easier to enforce as you can spot people putting ipods on in transition and stop them as they leave. it would be more difficult with mass starts of multiple '000s but DQing at the finish should be enough to help to prevent their use.
and within the BTF rules is a clause that Race Directors can use to introduce variations to race rules for events provided competitors are informed equally and the BTF are notified. If UKA had similar, then this could be invoked for some races where the use of headphones is unlikely to cause too many issues - e.g. multi lap track endurance races, some ultras, some off-road events etc.
UKA are copping out
or perhaps UKA don't really give a toss about whether people wear earphones, and so are quite happy to allow race organisers to apply such rules on a case by case basis. Sounds sensible enough to me.
Now I hardly speak from vast experience, being a complete newcomer to road-running.
However, whereas I initially couldn't do without music or a spoken word podcast while out on training runs, I can completely take it or leave it now.
I went out last week with headphones on, and it was about 3 miles in that I realised I hadn't started anything playing. It's actually becoming my preference to hit the roads when there's not a soul out there, and just enjoy the quiet.