This ad is a good example of the Gestalt principle of similarity. Similarity is when "items are perceived as being related because they have a similar shape, color, size, etc". This uses inverse-similarity to grab and keep the viewer's attention.
unbelievable - At night, the back-lit poster was reflected in the adjacent glass pane. This revealed the complete McDonald’s ‘M’ logo and the message “open all night” could be viewed the right way round. Bus shelter Campaign by DDB Sidney