"The EZ-Pass transponder works by listening for a signal broadcast by the reader stationed at the toll booth. This 915 MHz signal is sent at 500kbps using the IAG protocol in 256-bit packets. Transponders use active Type II read/write technology. This system is proprietary to Mark IV Industries.[17]
The next generation of automated toll collection systems will be more open and likely to based on the 802.11p specification currently in development. [18]"
This link shows a transponder with driver feedback - a light and sound. If you connect it to a fusion brain, it will record each time it activates. A simple comparison of GPS position and a lookup table would allow it to find the toll. You could even have it pop up a keypad for entry if the toll location was not found.