Nielsen Media Research just did a study that found shorter (10 and 15 seconds) ads are more 'effective' than longer (30 seconds) at gas pumps in the US.
Okay, their measure of effectiveness is simply recall - which may or may not be a good one - but it seems intuitively correct. We probably shouldn't be thinking in terms of 15 vs. 30 second at all when it comes to out of home advertising. But TV is where all the parameters have been created for measurement of effectiveness.
I am sure that processing ads at gas pumps is different to processing ads at home in the context of entertainment. A less linear narrative and more visual branding elements are probably more appropriate - somewhere between billboards and TV commercials. We have a long way to go to better understand the effectiveness of different executions in different out of home contexts.