Just drove from Chicago to Boston using the toll roads, thruway and pike. I have an Illinois I-Pass transponder in the RV, an FR3 30DS, towing a Chevy Sonic 4 down. The I-Pass was accepted in the automatic lanes of all the states. I checked the billing and the charges reflected the 4 axles.
Illinois: $9.30 (from I355 at I55)
Indiana: $27.61
Ohio: $11 (includes extra $1.50 for getting lost in Cleveland)
New York: $25.75
Massachusetts: $12.10
I did not have to use the trucker's lanes or stop to take any tickets, it's smooth sailing, at least until the credit card bill hits.
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