On my 2012 Prius C4 the HAC would operate regardless of sloped conditions. It was great while in a drive-thru line, just deep press the brake and the HAC would engage with the Slip Indicator Icon lighting up. On my 2015 C4 I thought the HAC wasn't operating because nothing would happen on level surfaces when I deep pressed the brake pedal. The car would move forward from idle and the Slip Indicator Icon would not light up. I took it to the Dealership and they put the car on the wheel alignment rack and elevated the front end. Sure enough the HAC activated but there was no Slip Indicator Icon lighted up. The dealership said the Slip Indicator Icon no longer lights up and the HAC only works on sloped roadways now. However, the Quick Reference Guide still mentions the Slip Indicator Icon should light up when HAC is activated. Is the Dealership trying to get out of changing a light bulb or is that the way HAC operates now? No more lighted icon and on sloped surfaces only?
According to my 2012 owner's manual the HAC only engages for 2 seconds. So it would seem to me to be a bit dangerous to rely on it in a lineup if you're distracted by, say, fishing change out of a pocket or something.
