The percentage of illegal parking in spaces reserved for the physically disabled was monitored under three sign conditions: ground markings, ground markings plus vertical signs, and vertical signs containing a message that concerned citizens were watching the spaces. Illegal parking dropped from 69.3% of 102 vehicles during the initial ground-sign condition to 57.3% of 36 vehicles in the first vertical-sign condition. Following removal of the vertical signs, illegal parking increased to 68.7% of 43 vehicles. During the second vertical-sign condition, illegal parking dropped to 53.7% of 32 vehicles, followed by an increase to 69.5% of 68 vehicles after the vertical signs were removed. The lowest rate of illegal parking (27.1% of 78 vehicles) occurred in the vertical sign-plus-message condition. Illegal parking subsequently increased to 34.6% (of 94 vehicles) when the message sign was removed, followed by an increase to 65.2% (of 105 vehicles) when the vertical signs were removed.
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