This advertisement does not seem like value for money

Escalator steps stacked up in front of an advertisement for Douglas College, Granville SkyTrain station, Vancouver, BC

The long walls of the underground SkyTrain stations can provide big canvases for some rather striking advertising. But stuff happens. The advertisements can get defaced or damaged, or in this case obscured.

How do advertisers ensure that they are getting the full value for their expenditures? Will the advertising seller offer extra time to make up for the days, or perhaps weeks, that this advertisement is obscured by the piles of escalator treads?

If Douglas College is still paying for this advertisement, it does not seem like value for money.