WidthIllusions

An observation that confronted me recently: when driving on a bridge that passes above the Capital Beltway, that eight lane thoroughfare seems far narrower than when one is on the Beltway itself (or when one is driving beneath the highway through an underpass.) Why?

When it happened to me in an unfamiliar neighborhood I actually couldn't believe, at first, that the road I was crossing above was a major interstate highway. It seemed somehow 50% too small, like a scale model. I don't think the problem is explained by simple masking, the edge effects caused by the bridge's abutments.

Could this be related in some way to the "rising moon" illusion? Or to the false size difference that appears when two arcs are nested one above the other? I'm mystified ...

TopicScience - Datetag20040330

Just guessing, based on far too little data: Maybe you're comparing the width of the bridge to the width of the highway. I remember the local overpass being about five or six lanes wide (2 traffic lanes, two copious shoulders, and 2 sidewalks) despite only being a two-lane road, and that makes it almost two-thirds the width of the entire eight-lane highway below. I couldn't verify this pre-hypothesis without further information, though. - RadRob

(correlates: MandatoryInversion, UnfortunateBillboard, StupidityAndConspiracy, ...)