Here's info on signal light bulbs, or any incandescent lamp for that matter.

From Lumitron Corporation:

"Where lamp replacement is difficult, light output can be sacrificed for longer life. A reduction in the applied voltage will increase lamp life significantly. Lamp life varies inversely to the 12th power of the ratio of the applied voltage versus the design voltage.

Candlepower is directly proportional to the 3.5 power of the ratio of the applied voltage to the design voltage, and can be increased at the expense of lamp life." [The modern unit is the candela ≃ 1.02 candlepower.]

So you can see that life increases much faster than luminous intensity decreases as one lowers the voltage. Of course all of this is academic now that we have LEDs. Daiso is selling 10W (60W equivalent) 25,000 hour LED bulbs for $2. The incandescent's day is nearing its end.