This is a very good question and one to which there is no clear answer, though car manufacturers and safety organisations are working together to find an industry standard.

Most electronic handbrakes will not work if activated once while the car is in motion, but if you consistently hold the brake most have an over-ride function that will bring the car to a halt. In some, such as the Nissan LEAF, activating the hazard warning lights allows some immediate manual control over braking systems from the passenger seat. In some of the latest models, there are built-in systems that recognise driver inattention and will gradually bring the car to a halt in such circumstances, especially if driver alert systems are ignored. We agree it's a real challenge in many electric cars and embrace any developments that mandate safety systems to improve road safety as a result.