> From the colours, he must have been confusing "Obligatory right/left turn from this lane" with the "Sharp curve ahead" signs.

No mistake. They were "sharp curve ahead" signs, specifically on the turns on off-ramps of Rt. 55 (I-91 in the U.S.). They were well off the main road, after exits that didn't have exit-only lanes. There were five or six in a row, and the EXACT size, shape, and design, except for the color, as the familiar yellow and black sharp turn signs.