Police officers wreck police car driving over rise-and-fall traffic bollards They said accidents were caused by cars tailgating the buses and branded guilty motorists as "selfish and silly." It is not known whether the two officers were following a bus when the accident occurred. The bollards are designed to allow buses and emergency vehicles through. The vehicles activate a sensor which lowers the posts, but they rise again immediately after the vehicle has passed. As well as a series of signs, motorists are alerted by flashing lights and audio warning messages. One on looker said: "The police officers were absolutely mortified at what happened and were clearly very embarassed not least because shoppers and other onlookers were laughing at them. One old guy was giving them a lecture about driving properly in Manchester whilst another was shouting 'karma' because he got fined the previous weeks for doing a U-turn. That wait for the tow truck mist have been the most embarassing moments of their lives. The pair of 3ft-high telescopic posts were installed in 2006 after it emerged that the road was one of the worst accident hotspots in the city. They use sensors to sink into the street and allow buses through, before rising to bar other traffic. But they caused controversy when four cars were trashed in five days as they tried to drive over them. In a recent interview Manchester City Councillor Pat Karney said: "I just cannot believe it that we have this precious piece of road which ensures maximum safety for pedestrians... but selfish and silly motorists think they can take on these bollards. We've got four warning signs here. What bit do they not get on the four warning signs? It really is, when it comes down to it, selfish behaviour.