Upgrades to New York City’s subway system to reduce delays and make stations more accessible are at risk if a plan to charge motorists driving into the city’s central business district is significantly delayed, the head of the Metropolitan Transportation Authority warned Wednesday.

The MTA, which operates the city’s subways, buses and commuter railroads, will implement the new congestion pricing toll, which will charge vehicles entering south of 60th street into Manhattan’s midtown. Drivers may begin paying the fee as soon as May, but New Jersey is suing to potentially block or delay the program from starting.