# Re: Measuring Distance by Headlamp Luminosity

```[from Greg Koehler. Quoting Al]

Since most cars are about 6feet wide (plus or minus a beer can or two), I
use the spacing between the lights to give a rough guage of distance. The
Saturn is one of the few that come to mind with narrower than normal
lights.

(Got this idea from the Spitfires. Dem WWII fighters that have a ring in
their firing sights to judge when best to hit the red button. The ring was
calibrated for single wing fighters of the time. When enemy plane fills the
ring in the sight, the 6 guns will be at the perfect alignment to lump the
most lead together at one point. Convergence)

But I digress.

Don't ask me how far a Harley Davidson is away from me.
>>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
>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

Ernest Wong
at esw5@cornell.edu

```