Why exactly did the chicken cross the road? Well in the case of Napa, California back in 1975 it was simply because they were on patrol and had to cross the roads in order to complete their duty.
Rather than installing speed bumps or other speed restriction methods, it was decided to try an experiment whereby they would use chickens to slow down traffic on one of the streets that ran nearby their main park. A total of 85 were released and able to roam the area freely and in the 9 months that the experiment lasted a total of 12 were killed by drivers.
Apparently it worked as people were anxious about the possibility of mowing down a chicken so they were driving slower than normal even though there were still some drivers that felt that they were just chickens so there was nothing to worry about.