The 8ft mobile speed sign was set up on the pavement in Queen Street, close to Queens Retail Park, this morning (Thursday, December 16).
Rush hour drivers were urged to "Please drive carefully" as the sign flashed its 50mph speed limit warning at them.
But Queen Street has a 30mph speed limit - 20mph slower than the limit suggested by the electronic road sign.
Lancashire County Council has been approached for comment.
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