Im not sure this fits, they saw a much larger drop (18%) in heavy duty trucks entering the city, and a smaller drop (9%) in passenger cars. I am not sure the public transit options are close alternatives for heavy duty trucks.
I suspect that this is due to the elimination of toll shopping/avoidance. Per [0] and [1], the only way to avoid a toll entirely is to drive from the West Side Highway or FDR Drive to the Brooklyn Bridge, but commercial vehicles are prohibited on FDR Drive and the Brooklyn Bridge has weight restrictions [2], so heavy trucks don't have a legal way to dodge the tolls anymore.
If you need to reach Long Island, the incentive to avoid the (tolled) Throgs Neck, Whitestone, Verrazzano, and RFK bridges are gone; now you're paying for the privilege of sitting in Manhattan traffic.
Do we know that those heavy duty trucks were formerly used to do things you need heavy duty trucks for? It seems more likely that 18% (or more!) of the usage was by people who think heavy duty trucks look cool and wanted to show off theirs.
That's the difficulty with the Light/Medium/Heavy Duty categories. It doesn't tell you a huge amount about what the vehicle is being used for but most of them heavy duty mean commercial or utility. There are a handful of popular models that tip into the Heavy duty class and those are usually 3/4 ton pickups. Not sure how popular those are in NYC though.
