This diagram shows the relative volumes of passenger cars on routes along the New York State Thruway for 2012. (click here to view 5-axle trucks)
Thanks to the cash tolls (which use tickets) and EZ pass systems, the Thruway Authority is able to track the entry and exit points for almost every vehicle. There are even trips which start and finish at the same toll plaza, which might be attributable to Thruway employees or those working at rest areas. This entry/exit data is then aggregated and made publicly available through data.ny.gov.
The source data includes multiple vehicle classes and years; I used class 2L which should reasonably represent passenger cars, and selected only data from 2012.
Google Charts Sankey Diagram renders the data, but there is a D3 sankey plugin if you prefer. (hat tip to Chris Whong)
To do: get the data to load from the data source dynamically; for now the data is included statically with this page.