You may have experienced this feeling before. On the horizon you can see the flashing blue lights of oncoming patrol cars. The tracker dogs are barking wildly and there you stand, in the middle of the runway, staring disconsolately at the retreating navigation lights of the Learjet that was supposed to whisk you away to Mexico. What do we do now, someone asks, and your leader shouts back a terse instruction: "We run!" Which is all very well for the escapees of Prison Break, whiling away the months until season two reaches the screen, but what about us poor viewers? We'd served some hard time to get to this point - 20 weeks in Fox River Prison, recovering after every disappointment and knock-back, kept going by the thought of the moment when we could finally shout, "We're free!" - released from our imprisonment in the narrative. Instead, off went Abruzzi's plane, carrying with it our last hope of clean resolution.

We've only got ourselves to blame, of course. We should have known better after the last episode of season one of Lost, which left devotees equally bereft and cliff-hung, staring into the black, bottomless shaft of what seems to be the island's red-herring mine. Again, we'd held on in the desperate hope of some kind of narrative closure. We weren't fools, naturally. We didn't expect a rational explanation for the tropical polar bears or the miasmic swirls of malign energy - but we surely hoped for something that would soothe the itch of the unresolved mystery.

The truth is, though, that popular television drama simply can't afford proper endings any more. This isn't a problem with some kinds of drama. You don't expect ER or The Sopranos or The West Wing to wrap everything up for the season finale - only that most of its internal storylines will be completed and laid to rest. In fact, if you're a fan, you will experience the end of such a series as a kind of death in itself - a sense of bereavement that was particularly strong for West Wing viewers since it coincided with the unexpected death of one of its big stars. But both Lost and Prison Break offered a different kind of contract to the viewer - one that was implicit in their titles. They didn't name places in which action would take place, perhaps indefinitely, but circumstances that demanded some kind of conclusion. If you're lost, you get found. If you break out of prison - in fiction at least - you either make it or you don't.

Unfortunately, the economy of American television rules out such end-stopped creations. No sane producer would ever offer a network a series that couldn't be extended if things went well, and no network would invest in a drama that couldn't be exploited for the easier profits of a second or even third series. The result is a kind of terror of finality - just in case everything is so effectively tied up that you can't plausibly get it untied again. And while this is less the case in British television, you can't help but wonder if the same rules aren't beginning to apply here.

I look forward to this weekend's Viva Blackpool - a single-episode spin-off from Peter Bowker's wonderfully enjoyable karaoke drama - with very mixed feelings. On the one hand, I want more. On the other, I know that one of the things I loved most about Blackpool was the refreshing sense that this was all there was going to be. It wasn't trying to build a franchise, or target a particular audience demographic, or deliver a solid mid-evening anchor point for the schedule architects. It was just itself, heading with beautifully paced determination towards a genuine finishing line. By contrast, every episode of Lost feels a little as if it is vamping until ready - and "ready" means "so stale that we've decided to cancel the show".

The odd thing is that this commercial imperative towards indeterminacy upends the standard prejudice - which is that low art is tritely and tidily concluded, while high art dares to withhold the conventional pleasures of the neat ending. Now it's a mark of artistic integrity for a series to slam the door shut on its own future prospects. Think of the final episode of The Office, when Ricky Gervais offered us the consummation we'd all been waiting for and, in effect, ensured that he couldn't go back on his decision that there wouldn't be a third series.

In America, he wouldn't have had the choice - it would be decided by the studio executives and the Nielsen ratings. And I would suggest that the emotional intensity of that last episode derived at least in part from the knowledge that the programme wouldn't be returning. Open-endedness might have been more lucrative, but the proper ending was far richer.