Measuring Distance by Headlamp Luminosity

["Greg Koehler (Volt Computer)" <a-gregk@microsoft.com>]

Al said:

>Not THREE seconds later, an old late 50's Chevy went past in the 
>other lane!!  The headlights were so dim that I suspect he may have 
>been running off his battery to get home.  
>
>The point: his lights fooled me.  Until then, I didn't realize that 
>slightly dim lights could look MUCH farther away than they were.  And 
>it damn near got him and me killed in a 60+ MPH head-on, because he 
>looked at a leat a mile farther away then he actually was!"

I'm not an expert, but I know you can't measure distance on the basis of
brightness alone.  This is a well established fact in optics.  If you
had
hit the Chevy it would have been entirely your fault for using this
false
assumption.

You can measure distance by watching the rate at which the light source
changes position, but never by the relative luminosity alone.  In fact,
if you
look up in the nighttime sky, many of the brightest stars are the ones
farthest away (they're brighter because they are so much bigger than
other
stars).

Anyway, if you see headlights when passing, make sure you wait until
you see how fast the headlights are moving so you can determine the
speed of the other vehicle, or try to see the silhouette of the other
car
before passing.  It's best when you see the other lights come around a
corner in the distance before they hit a straight road section. This is
pretty much impossible if you are driving at night and the other car is
coming straight at you, in which case you should just stay put. 
That said, I agree that drivers should try to make their cars clearly 
visible to others by using good daytime and nighttime lights.

One more thing.  It's way dusty in Eastern Washington.  Don't assume his
headlights were substandard.  They could have just gotten dirty by
driving
on a gravel road or something.  Could happen to anyone. 

Greg Koehler
Kirkland, WA
'90 80Q