'When compared to other forms of advertising' is a cop out in my view.
Those other forms are generally avoidable with either zero cost, or a (debatable) net gain.
For example, one can simply choose to not watch broadcast TV, or read newspapers, preferring instead ad-free sources (which are generally better in my experience anyway).
I can't opt out of outdoor advertising without leaving employment opportunities behind or moving away from family and friends.
So that makes the baseline zero against something.
We have Piccadilly Circus/Times Square style moving LED billboards popping up in more and more places now with inane products on them. I don't want to look at pointless consumerist nonsense all day, I want to see life, not capitalism. If you think that's easy to just 'block' I suspect you're quite different to me.
Taking the LU as an example: there are ads above and in front of your head at all times, at stations (every minute) there are ads on the walls (which sometimes move), the escalators have moving LCD screens with advertising on them. Then you get outside and there's probably an advert in front of your face (because the station is a heavily trafficked area).
TV is no comparison! That's 10 minutes completely ad-free with a break that you can stand up and walk away from.