<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by the-ca-goat:
You don't have to wonder...you can figure it out!
If you and the small plane are converging head-on 1/4 mile is nothing--i.e., if you're traveling 400 mph on the Horizon plane and the small plane is going 100 mph--the two planes are traveling towards each other at 500 mph or over 8 miles per minute. The aircraft will collide in just over 7 seconds. Yikes, ehe?
Finally! I've figured out how to use all of that stuff I learned in 8th grade math..."if train A leaves Denver at 4:40 and train B leaves Sacramento at 4:45..." 
Though I never did too well, then so don't look TOO closely at the #s now. 
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It's actually less than that: 500 mph is 7.2 seconds per mile. A quarter mile therefore goes by in 1.8 seconds.
The good news is that you won't be scared s***less for long, one way or the other.