First release of EPB was implemented by BMW on 7 Series (E65) back in 2001.
Electronic Parking Brake as a system has been designed to be integrated component of vehicle network system.
It exchanges a lot of info/data, via Power-train CAN-Bus, with Engine, DSG, ACC, IC, BCM, etc.
Additionally EPB is a certified replacement of Manual Parking Brake
and thus compliant with the following standards
FMVSS 105,
FMVSS 135,
ECE 13H.
And this means that EPB can also be used as a secondary/emergency brake
.
At glance EPB appears to be just 2 servomotor calipers controlled by the switch
but this is only one part of this very sophisticated system
.
Generally servomotors are being used at speed of 8km/h and lower
which means that they are designed for stationary use as a purely parking brake.
Applying EPB at higher speeds is the second part of this system.
In such a scenario EPB controller sends CAN messages to the ABS and requests hydraulic braking to take place.
This braking is called "Emergency Braking" and it is capable of safely stopping vehicle in very controlled manner from any speed.
It uses anti-lock functionality and by default it brakes using all 4 wheels with predefined intensity.
EPB adaptations allow for a lot of adjustments and tweaks,
among which "Emergency Braking" intensity and braking axle selection are the most important.
There is a selection of 7 intensity levels and "Emergency Braking" can be set to use rear axle only or both.
So taking into account all of the above and what is most important safety of all road users EPB can not be used as a stand alone system.
The most obvious concern would be applying EPB via servomotors and locking rear axle wheels at high speed
.