ISA has been mandatory on new UK and EU vehicles since July 2024, but Thatcham's real-world testing exposes a measurement con-trick: approval bodies score accuracy across distance driven, not at the moment a speed limit actually changes, which flatters the numbers considerably. Even the best vehicle tested managed 98.4% on the distance measure but only 90.3% event-by-event; the worst dropped from 91.3% to 74.3%, meaning roughly one in four speed-limit readings was wrong, with some systems inventing limits of 5mph or 100mph that don't exist on UK roads. Any duty-of-care or driver-coaching policy leaning on ISA accuracy should treat the headline compliance figures as optimistic, not operational (Thatcham Research).
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