A New York state precedent known as the "rule of two" stipulates a driver must commit two traffic misdemeanors when a pedestrian or cyclist is struck for prosecutors to bring a charge of criminal negligence, according to CityLab.
In essence, drivers must be violating at least two traffic laws when they fatally strike a pedestrian to be charged with a negligence. The rule of two isn't a law, rather it's a legal precedent which is arbitrarily applied. Sometimes, drivers who were committing two traffic violations and killed a pedestrian or cyclists still aren't charged with a crime. The act of hitting a pedestrian doesn't count toward a motorist's two strikes. Drivers need to be violating two traffic laws in addition to killing someone to be charged with a crime.
New York officials are trying to enact "Cooper's Law," named after the 9-year-old boy killed by the cabbie, which would allow the city to revoke the license of cab drivers who hit pedestrians who had the right of way. There is already recent legislation which makes drivers who fail to yield to pedestrians and cyclists with the right of way "guilty of a traffic infraction," but the law can't supersede the precedence until the 'rule of two' is challenged in court.
The push for Cooper's Law comes at a time when New York mayor Bill De Blasio is pursuing his 'Vision Zero Action Plan' with the goal of eradicating traffic deaths. Last year 290 pedestrians died in the city, according to the New York Times.