[adhesive_wombat]

I'm also not sure what would happen if a car with an automatic speed limiter at 40 in dense traffic, it suddenly spots a 20 sign on an adjacent road and drops a gear to slow down, and gets rear-ended.

Technically the car behind needs to keep the distance safe, but also it's been functionally brake-checked by the speed-limited car, and without any brake lights coming on.

[Schroedingersat]

Bit of a non-sequitur, but the easiest way to implement such a speed limiter is to make it such that the car won't accelerate when over the limited speed (and either warn the driver or reduce power gradually so it slows down gently) without manual intervention, rather than jamming on the brakes

[adhesive_wombat]

That's how my old car does it, but the new one will downshift if it wants to slow down a lot (either if you reduce the cruise speed a lot, or it imagines it saw a lower speed limit sign). Seems dangerous.

[Epa095]

That's how my cruise controll works. It don't really work downhill;-)