Hundreds of motorists were flashed by a speed camera after practical jokers erected a rogue 40mph speed limit sign in a 30mph zone.
Locals in Wyke, Bradford, said the camera had been flashing like a "disco strobe light" as motorists sped by at the wrong speed.
Highway chiefs condemned the prank as "crassly irresponsible", reports the Bradford Telegraph and Argus.
Bradford Council has now removed the sign, which it believes was installed several days ago.
Local resident Dave Clarke said: "People were coming at 30mph but this sign brought them back up to 40mph so people were picking their speed up and setting the flash off.
"Some evenings it's been like a strobe disco light. There have obviously been a lot of people who have been caught by it."
Council engineer Darren Badrock said: "The 40mph sign was not put up by the council and appears to have been placed there by a member of the public. We have now removed it.
"Whoever put it up may think it was just a practical joke but it is actually crassly irresponsible.
"We lowered the limit to 30mph on Woodside Road because there had been 157 casualties on the A641 over three years. Encouraging people to drive faster than the speed limit may put lives at risk."
He said anyone who received a fixed penalty notice as a result of the fake sign should contact the authorities.