The other 144 is utterly essential to keep lights on when unexpected demand arises from heat waves, outages or maintenance downtime.

FORBES: Brownout

The batteries are not essential during flight as they are mainly used on the ground during maintenance activity, Sinnett said, such as providing navigation lights and braking while the airplane is towed.

CNN: Boeing announces changes to Dreamliner battery system

The lights will be more visible to drivers in sunny conditions and require less maintenance so they are cheaper to run.

BBC: Cumbria traffic light improvements begin