First the the Dodge minivan ABS fiasco and now the runaway Toyota fiasco.
At first glance it seems like a backup brake system might be difficult to design. The driver has panicked and won't let up on the brake pedal. The accelerometer/brake pedal force shows something isn't functioning properly but how do you phase in the backup system?
Even if the driver did let up on the brakes for a second it's not clear if the driver could brake without locking up the wheels after being spoiled by ABS.
Conventional dual braking systems like the primary and secondary air brake systems in heavy trucks operate at less than 100% when one system is down.
Dual parallel ABS systems however, could both operate at partial load each, say, 50 - 50 or 30 - 70 when both are working. When one is down the other would pick up the slack and work at 100%.
Brake by "wire" might not be such a bad idea after all.
Bret Cahill