Braking by wire?

yes, the system is electronic, with a failsafe hydraulic backup.

the smoother transition is toyotas refining of the system over 5 generations

 

no, i'm just assuming it is a refinement. i haven't seen anyone who have owned previous gen's mention anything drastically different in feel or performance.
i know it started at least in gen 2 if not 1. it has been completely documented here, and of course, toyotas 16 billion dollar 'runaway acceleration' debacle5738a3c4-af69-11e3-9627-c65021d6d572_story.html
in which stepping on the brakes didn't slow the car.
and also discussions of how the brake pads work in conjunction with regen, and what happens when 'brake by wire' fails.

 

As far back as Gen 1, there has been a pedal that pushes brake fluid into a spring-loaded "stroke simulator" that feels like applying brakes. The pressure developed in that fluid is measured as an input to the brake ECU to indicate the driver's desired amount of braking.

The brake ECU negotiates that number with the hybrid vehicle control ECU (called the power management control ECU in some generations) to find out how much regenerative capacity is available, and whatever amount remains is achieved by controlling the valves in the brake actuator to apply pressurized fluid to the brakes.

If the ECU loses power, or loses confidence in itself, its electrical outputs to certain valves in the actuator drop, and those valves return to a fail-safe position where the fluid pressure produced by the pedal is routed to (some of) the brakes.

There have been changes over the generations: Gen 1, Gen 3, and Gen 4 have a 'booster' at the rear of the master cylinder where the system's pressurized fluid is used to assist the driver's pedal pressure (the way a vacuum booster would in a more familiar car). In those generations, the fail-safe braking is still assisted. Gen 2, by contrast, does not have such a booster (though the pump in Gen 2 is still called the "booster pump"); if a Gen 2 fully loses electrical power, its fail-safe braking is unassisted. To compensate for that, Gen 2 had a backup electrical power supply for the brakes.

You'll notice I haven't said anything directly about Gen 5 here. What usually happens around here after a new generation comes out is that someone will eventually be interested enough to download the diagrams and details from the new generation's New Car Features manual, and then we can pore over them and see what was changed from the generations before. For example, this thread shows the way that went when Gen 4 came out:

https://priuschat.com/threads/prius-4th-gen-brake-diagram.208330/

(Regrettably, the image attachments in that thread didn't all survive the recent PriusChat upgrade, which kind of hurts in a thread that's all about comparing details in diagrams.)

As a good thread like that generally starts with someone being interested enough to go get the new diagrams and details from the New Car Features manual, if there isn't such a thread here already, it may just mean that no one has been that interested yet.

 

There are 21 sub-sections of the braking section of the NCF for the gen5. If you can narrow things down a bit, I'd be more than happy to post some.

If I'm reading it correctly(no guarantee), the brake pedal is still connected physically to the master cylinder.

 

It kind of looks a lot like Gen 4, but if my eyes don't deceive me, they've gone completely without an accumulator for the first time in Prius history. Which must mean they're counting on the electric pump to instantly serve up your boost pressure just-in-time every time.

I suppose they were tired of accumulator leaks. Strikes me as a rather gutsy move though....

 

Accumulators, pumps, and combinations of each have been around longer than hybrids for diesels and forced air intake engines. May not be as gutsy as it appears.

 

I gotta admit, I kinda like having my accumulator, knowing it holds around 20 or 30 strokes worth of brake assist pressure, with the pump running occasionally to charge it up. If the pump flakes out, I'll get lights and a beep well in advance before it's hard to stop.

It feels like I'd have to be more gutsy to want to count on the pump for the actual braking in every instance.