Traffic in central London fell by roughly 25 percent Monday, the first day of a congestion-mitigation plan that was the controversial brainchild of Mayor Ken Livingstone. Under the plan, it costs about $8 per car to enter central London from 7 a.m. to 6:30 p.m., with steep penalties for those who don’t pay. About 80,000 people ponied up the money yesterday, while the rest took alternate routes or public transportation, leaving quiet roads and no gridlock, even during rush hours. Some of the reduced traffic was the result of the beginning of school holidays, but Livingstone and others still hailed the plan as a success. The money generated by the plan will go toward improving the city’s public transportation. Critics of the plan say it unfairly burdens poorer commuters.