The issue:
Bike theft is rampant in most places in the US. The solution so far has been increasingly huge and heavy locks / chains. Eventually, people will probably have to weld their bikes to nearby objects, or use quick-setting concrete, in order to prevent them from being stolen.
Figure 1: A bike (do not steal)
The idea:
Provide a subtle theft deterrent that would not be obvious to the thief. This way, they would ride off with the bike, thinking they had disabled the only security (the chain / U-lock).
Normally, the deterrent in question here would be a GPS tracker, so the bike could (theoretically) be recovered, with great effort. However, the police are unlikely to raid an apartment building based on the knowledge that a stolen bike is possibly inside.
So we turn to another option:
A extremely loud air horn is attached somewhere on the bike, preferably pointing toward the rider. It must be securely attached and difficult or impossible to easily remove or disable. Perhaps it would be built into the frame, or in a secure metal box.
The air horn is connected via a sensor to the wheel. Once the wheel has rotated a certain number of times (say, 50), if the air horn has not been deactivated beforehand, it activates and doesn’t stop until it has been totally expended.
Figure 2: A “do not steal me” rider-facing bike horn. In reality, it would need to be attached to the handlebars more securely than shown in this diagram.
Presumably, a thief would prefer to abandon the bike instead of be deafened by it or hang around to likely reprisal while the horn exhausts itself.
(In order to deactivate the horn, there would be a button on it that the rightful owner would need to press BEFORE the horn went off. The horn could also make a few warning noises in order to remind an absentminded rider to disable the horn before it was too late.)
PROS: Non-lethal theft deterrent. Presumably would not introduce the owner to liability in the same way as a standard booby trap (e.g. a “bike thief bear trap” would be frowned upon by the courts, even if it was effective in capturing its prey).
CONS: Adds a tiny amount of weight to your bike, so not suitable for the Tour de France or similar competitions. Otherwise, none.
WorstPlans: updated every Monday since July 2014! 
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