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It must have been a construction zone (or recent zone that wasnt updated) where it was tagged as 55 (and because of that only +5mph). I take it you didnt notice a speed limit icon on the AP display? If you see that again, check the speed it sees on the dash display (and should also get an alert that you are limited to +5 in that area). I've been up/down I5 on AP as fast as 85 (I think the hard limit is around 90 on open freeway)
No limit on actual freeways or limited access highways... secondary roads or special situations are limited to 5mph over the posted limit. If you must, you can push on the accelerator to override. But, this is a safety issue.