system
Given the following post, respond as if you're commenting on Reddit:
human
Why do stoplights flash red after a power outage instead of just restarting their program?

They apparently have power.  I dont understand.
gpt
They very short answer is that they're programmed to do that.

In some areas, traffic lights are operated centrally or cooperate with their neighbors to maintain good traffic flow through a corridor.  If a light comes back up and can't communicate, whoever runs it may program it to allow traffic to flow on the major road (flashing yellow) and force the side road to stop (flashing red).  This allows nearby lights along the major road that are able to communicate to operate as if the "dead" intersection isn't an intersection at all.

Some intersections can't be allowed to have one direction flow continuously because visibility from the other is inadequate.  In those cases, the light is programmed to flash red in all directions, converting the intersection into a four-way stop.

Lights that run autonomously and haven't lost their programming will come up and operate as normal.