I was curious about this so I went and looked it up. It's here:
View attachment 1966
Clear as day. If the vehicle engages regen braking when the driver lifts off the accelerator, the stop lamps must light up if the retardation is greater than 1.3 metres/second squared (about 3mph per second).
I can't say
for certain that lifting off in the MG5 (sorry, I know this is the ZSEV forum but I'm interloping for the sake of this discussion) causes a braking effect of more than 3mph per second, but it certainly seems plausible.
Likewise I can't say
for certain that the brake lights don't come on - only that I have been actively looking for them in my mirrors and haven't seen them, and that I have been flashed from behind (and worse - more on that in a minute) multiple times in my first couple of weeks of owning the car.
In my opinion it's not such an issue if you're coming to an EV from a traditional car, because unless you're
really intent on downshifting to slow the car via engine braking, you'll be in the habit of using the brake pedal all the time.
I've just taken delivery of my MG EV after three years in a BMW i3, which has heavy regen braking that you can't turn off and which absolutely does trigger the brake lights.
You quickly get used to not using the footbrake very much.
The regen isn't as strong on the MG, but on KERS 3 it's not far off - so the 'one-pedal driving' habit, of slowing by lifting off rather than by applying the footbrake, remains. But inattentive drivers behind seem to be easily caught out if you decelerate without illuminating your brake lights, and they seem to get very angry about it.
Twice now I've been slowing, with indicators on, to turn off at a junction. The following car has swerved out round me and carried on at some speed while leaning on the horn.
I've just fitted a rear-looking dashcam in the back windscreen because I'm fairly certain it's inevitable that I'm going to get rear-ended.
I'm also working on retraining myself to use the brake pedal even when regen would slow me down adequately without it.