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Using pictures of staring eyes on signs warning people to clear up is another possibility, says Workman. 

Studies have shown such techniques can be effective. At Newcastle University bike thefts were reduced 62% by putting pictures of staring eyes above cycle racks. A 2010 study found similar techniques made it more likely that trays would be cleared away.  

"There's something about human eyes and the feeling you're being watched," says Workman. - Eight radical solutions to the problem of dog mess By Duncan Walker, BBC News Magazine