Los Angeles police will abandon their weekly drunk-driving checkpoints in the San Fernando Valley after Friday because the four-month-old experiment is not working and alcohol-related traffic accidents in the Valley actually have increased. Operating the roadblocks has required at least 30 officers each week. Statistics for the Valley showed that drunk-driving arrests decreased from 2,215 motorists last year to 1,625 for the same time this year. There were three alcohol-related traffic deaths last year compared to 10 this year, and 53 major injuries caused by drunk driving compared to 66 this year. Although the weekly roadblocks will end this coming weekend, traffic officers will set up occasional sobriety checkpoints in the Valley, just as they do in other areas of the city. In West Los Angeles, for example, traffic officers set up sobriety checkpoints about once each month, Sgt. Tony Morgan said.