Without a doubt, this prestigious distinction must be awarded to a certain blow-up doll I happened to encounter while commuting with a friend to a class we'd been taking at a local university. In truth it was not a truck to which this inflatable companion was affixed; rather, it was haphazardly tied to the rear bumper of a large white van with no windows in the back. Hopelessly vexed, my friend and I began to speculate about the origin and purpose of this sight, and began to run through the following possible scenarios:
Storage: I believe that a simple lack of storage space for the doll, quickly remedied by tying it to the bumper, is the most convincing scenario. However, this leaves several questions. First, why affix it to the bumper? Surely it would make much more sense to tie it to the roof of the van, traditionally the first line of defense in loading a vehicle with far more objects than it was ever intended to hold. Furthermore, the bumper seems a precarious place to load such cargo, as it would assuredly be destroyed in even the most minor rear-end collision. Second, why the doll and not some other object? I suspect that the driver of this van is a private person, evidenced by the aforementioned lack of windows in the cab, and such a private person would not want such an item drawing attention to himself. There must have been a better choice of cargo with which the doll could trade places. Finally, and perhaps most pressingly, why not just deflate it?
Safety: The doll affixed to the bumper could plausibly have been thought of as a safety device, but again this is a scenario plagued by inconsistencies. It is true that the doll would absorb some of the impact by another vehicle in the event of a rear-end collision. However, I believe that this would have a fairly minimal effect on the actual damage to the van, especially at high speeds. In this case, the doll could again easily be replaced by another object that would absorb some impact in a collision, such as a kiddie pool, which I would argue to be far superior in its effect. And anyway, if blow-up dolls had any significant safety benefits, wouldn't they have become a standard feature in all automobiles?
Thrift: I have occasionally heard of people using blow-up dolls in order to use the carpool lanes on a highway, in order to get discounted tolls. This scenario is also plausible, however, I imagine that in order to receive a carpool discount, the passenger or passengers must be seated in the car rather than tied to the bumper. Furthermore, I believe that transporting passengers tied to the bumper of your vehicle is generally illegal (except in New Hampshire). Finally, if this is what the driver was aiming at, he should have probably dressed up the doll, rather than leaving it stark naked.
Negligence/Act of God: The final scenario, and the one most open to interpretation, is that the doll somehow became tied to the bumper by sheer accident. This is admittedly also the most improbable, but if it is true it will greatly restore my opinion of the reasoning capabilities of the unknown driver. Personally, I like to think that he unknowingly backed into the window of a sex shop housing the doll, and drove off, unaware that it had become stuck to his bumper.
I know that this write-up presents far more questions than it does answers, but I thought that I would share my thought process in the hope that someday I will find the answer to this mystery.
edit:
shaogo has been kind enough to offer another suggestion: that the driver was the victim of a prankster or pranksters, who tied the doll to the bumper. I consider this a very likely explanation, and if true it shows a measure of ingenuity on the part of the prankster(s) in managing to attach so noticeable an item to a vehicle unbeknownst to the driver. However, I don't know how much a blow-up doll would cost (no really, I don't. Stop looking at me like that.) but I imagine it is more than the ordinary amount of money one would part with for a joke. Unless it was stolen, but that's just icky.