Question

control signals could prevent such tragedies.
what is another way that
morgans traffic signal made city
traffic safer?
morgan invented an electric traffic signal with three
positions. besides stop and go, there was a signal for all
vehicles to stop at once in order to let pedestrians cross in
safety. his signals could operate 24 hours a day, with a
spotlight for nighttime use.
main idea
morgans electric traffic signal made
city traffic safer.
key detail
it helped
control
carriages,
bicycles, and
pedestrians.
key detail
it worked 24
hours a day.
it caused
traffic jams.
it helped him
buy his car.

Explanation:

The text mentions that Morgan's traffic signal could operate 24 hours a day. This is another key detail that shows how it made city traffic safer. The other options like "It caused traffic jams" and "It helped him buy his car" are not relevant to making traffic safer.

Answer:

It worked 24 hours a day.