Why would streetlights be going out right as I drive past them?

Dear Car Talk

Dear Car Talk | Aug 01, 1998

Dear Tom and Ray:

There's something that's been bothering me over the last several years, but you
must not laugh. At night when I'm driving around in my '93 T-Bird, street lights
sometimes go out as I pass. This seems to happen every five minutes or so. When
I told my wife, she thought I was nuts. She came along for ride so she could
ridicule me. Well hi-dee-ho; she was all freaked out because it happened three
times over several miles. We then went to pick up the kids and went to Dairy
Queen. And as we pulled up to order, the whole building went dark (I can get
sworn statements that this happened!). Can you explain these weird events? --

RAY: Of course we can explain these events, Gary. Most street lights these days
are high-pressure sodium fixtures. And the flickering is a sign that the bulb
needs to be changed.

TOM: Most household light bulbs fail completely when they wear out. But when the
tungsten in the street light is failing and can't keep the lamp hot enough, or
the sodium gas is slowly escaping and there isn't enough pressure, the street
light shuts off, but then continually tries to restart itself. And when
conditions are "borderline," you'll see a lamp on sometimes and off sometimes.
The closer it gets to complete failure, the more frequently it cycles on and

RAY: So the cycling bulbs are a sign to the local electrical department that
it's time to change the bulb.

TOM: The old mercury vapor lamps that they used to use didn't do this. They just
got dimmer and dimmer. And the problem with that was that it happened so
gradually that nobody noticed until they couldn't see anymore. "Fred, didn't we
used to be able see on this street at night?"

RAY: As for the Dairy Queen, my guess is that they were closing. Did you notice
that all the workers went home and said goodnight after the lights went out?
That would be your hint, Gary.

* * *

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