New cars these days don't all have this option: electronic starters have keyless ignition, activated when the key is nearby.

http://www.worldwidemotors.com/blog/how-to-use-push-button-s...

Personally, I'm concerned there is no manual brake override.

If I drove a Mercedes I'd test using manual ignition, maybe pulling the key out would stop the car.

There is no such thing as "manual break override". You will always be able to use breaks in any car, regardless of what systems they're running. You might be without a booster, but still.

You can also just shut the engine down with the ignition switch.

That isn't true. There are many models with no mechanical linkages for the breaks.

You're confusing the parking brake for the hydraulic braking system.

The brake pedal is alway mechanically connected to a split/redundant hydraulic braking system.

Park break, yes. Regular breaks, no.

At least in Audi’s, holding the start stop button will turn off the car at any speed.

Source: Don’t rest your hand in the center console on the freeway.

One would assume some sort of molly-guard, precisely against that.

>activated when the key is nearby.

throw the key out the window?

I tested this when my dad got one of the push-button start cars. Doesn't work, at least not in his Acura.

Give it a few minutes and it will start to complain, and then you will lose engine, but not electrical power.

Doesn't help on any keyless car I've ever used - the car will beep at you when the key leaves the vehicle, but the engine won't shut off until you hit the stop/start button. And in this case, that button apparently did not work.

I've never been in a car with a push-button ignition that would let you turn the engine off while the car was moving. Too easy to bump the button.

You typically have to hold it for multiple seconds