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The problem with stuff like this is it kills people!  Not the mattress itself, but it's the side effect of the drivers following and how they will handle the unexpected situation.

I ran over a mattress in Atlanta once.   It almost took me down, but I stiff armed the handbars and me and the bike weathered the storm.

My worst tho, was I was following a van in Oakland, CA that ran over a full 2x4 stud which threw it in the air.   It sailed right over the top of me with a foot or so of clearance.   The whole incident was a mere flash lasting perhaps a second or two, but later I was shaking thinking about what coulda?

Since the night when an old truck lost one front wheel right in front of my bike and the thing came rolling my way, most of my attention on highways goes to identifying possible sources of unpredictable danger, like that one, or cargoes poorly tied, rescue vehicles with an open platform from which tools may fall, light trucks driven by amateurs - an endless list. Thanks, Serow.