So we spend a ton of money putting in transponders and readers in cars which still have various failure modes, or we just put cameras on the highways and intersections and say "car with tag 123123 went from gate 1 to gate 2 in x minutes, those are y miles apart, average speed had to be > speed limit, issue ticket to 123123".

The toll roads could trivially automatically enforce speed limits. They already precisely know when each car goes through each gantry, they know the distance between each gantry, so they know everyone's average speed.