[chillingeffect]

that's an example of non-linearity. probably the car only applies throttle to regulate speed and not brakes.

home heating systems are similar in that the usually only add heat.

in both cases, the PID would be generating the proper output, but it doesn't have a means to drive the system consistent with its output. e.g. it can't turn on the brakes, or air conditioner.

[jagged-chisel]

Both vehicles do have some mechanism to slow the vehicle. It feels like a variation on downshifting; it doesn't feel exactly the same as when I'd downshift (whether a manual transmission, or an automatic transmission in 'let the human decide the gear' mode.) When they're "too far gone" (in mine, it's 2mph over, in mom's it's 5mph over), it starts actively reducing speed in this odd way.

[csharptwdec19]

If it's a newer vehicle, it's possible there's actually a CVT rather than a traditional automatic. Some Manufacturers will put in 'fake gears' in a CVT because drivers feel like the car is going faster when it shifts (even though it actually doesn't, the CVT itself can keep the vehicle in an optimum power band all the time.)

Actually, CVT Management is an interesting control system question in and of itself, especially when fake gears are involved.