It's practically required to carry a transponder in your car on the east coast as there are many toll roads and they are in the process of phasing out cash.

There are companies that want to turn your cell phone into that transponder so that you can just use your phone as a universal transponder.

This seems likely to happen. Once it does, will having a smart phone with this feature enabled be required to drive on a road? If so, it is seems likely that this requirement will be challenged in court, but what's the difference between being required to have a plate on your car or an app on your phone?