One-pedal-drive and the brake lights

When driving any car, as you apply the brakes using the brake pedal, a switch connected to the pedal is “closed” and the brake lights are turned on. The point of this is to signal the driver directly behind you that you are slowing down. This signals the following driver to also slow, or change course, to avoid a collision. (I once had to repair the switch in my brake pedal and learned a little about this.) Similarly, when stopped, keeping your foot on the brake makes it clear to other drivers that you are stopped on the road so they can prepare to stop.

After driving my new BMW i3 for a while and adapting to using the one-pedal driving technique, it occurred to me that I didn’t know what was happening with the brake lights when I would slow down or stop (using the regenerative brakes). At first, I tried watching other i3’s around me to see that their brake lights came on (and they did), but I couldn’t tell whether these drivers were using the brake pedal or just the regenerative braking from the motor. I tried applying the brake pedal while coming to a stop (to ensure the brake lights lit), but that made my stops jerky and uncomfortable.

Finally, I figured out a way to learn more about how the brake lights were activated. The Ford F-150 sometimes comes with a large, “chrome” grill. When one would pull up behind me as I approached a red light, I was able to check my reflection in the grill to determine when my brake lights would be on. I learned that once the regenerative brakes are engaged roughly 40% or more, the brake lights come on. They stay on until the car comes to a stop (because the “brakes” are no longer engaged). So, if I slow gradually, the brake lights stay off. If I brake abruptly, like approaching a traffic signal, the brake lights stay lit until the car stops. For many safety reasons, I have learned you should keep your foot on the brake pedal while stopped at a traffic signal. This prevents the car from rolling and signals the cars behind you that you are not moving.