I remember reading somewherer that the best way to escape a snake that
is chasing you is to run in a circle. Nobody believes me. Can you find
some some supporting evidence on the web. The evidence should come
from at least two reputable sources.

Running in a circle is certainly not the best way.

Me? I would throw a few grenades in its direction and then blast it
with my Kalashnikov.

All the snake would do is stay stationary and strike as you pass. 
Unless its a tree climbing snake to up a tree.

Seems like there are many theories of what to do when chased by a
snake (Probonopublico's theory notwithstanding):

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"...If ever chased by a snake, which I find very hard to believe, run
in a zig-zag pattern..."

http://www.geocities.com/RainForest/9003/dangers.htm

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"...There?s a little myth that snakes can outrun humans. If you are
unlucky enough to be chased by a snake, the myth claims than you
should run straight up the nearest hill, because snakes can?t go
straight up, but have to weave back and forth across the track..."

http://www.abc.net.au/science/k2/moments/s641636.htm

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"...My mother would warn me that if I was ever chased by a snake I
should run in a straight line, since snakes slithered from side to
side -- unless, of course, I happened to be running downhill. Then I
should weave from side to side because a determined serpent would turn
itself into a hoop by taking its tail in its mouth and roll down after
me..."

http://www.endicott-studio.com/rdrm/forbiser.html

Thank you Bowler-ga for your last entry, where the hoop snake's tail
is as firmly in its mouth as your tongue is in your cheek.

Folks, just jump back and run a short distance.  The snake will not
chase you.  If it does, jump back and run a bit farther. farther.

Every person I know, three, who has been bitten by a venomous snake
admitted that it was not the snake's fault -- it had no choice,
considering the circumstances from the snake's point of view.

A snake will not chase you.  Despite the fact that some of them seem
to move very fast when they slither along, you can outrun them easily,
and they know it, so they aren't going to even try to chase you.  If
we are talking about snakes other than boa constrictors, their
interest in humans is purely self-defense: you run; they're happy.
Their only speed is in striking from a coiled position, at best
lunging half their length.

It's crocodiles and alligators that you should zig-zag from, because
they're fast sprinters but rubbish at changing directions. So I was
always told when I was growing up in the sewers, anyhow.

i dont think running around in circles will help!!!

What would worry me ...

How can I tell if the snake is a poisonous or a tree-climbing variety? 

Or not?

Bryan, you obviously were not an US Boy Scout.  
Most N. American poisonous snakes are pit vipers that have a somewhat
triangular head  - and aren't noted as tree climbers:  rattle snakes
and moccasins, which from my experience are rather lethargic.
The other poisonous snake is the coral snake, which has red, yellow
and black bands around and is quite a bit smaller than a fully grown,
six foot rattler, and looks quite similar to a harmless snake with the
same colouring.
But some clever soul composed a ditty to differentiate between them
based on the adjacent colours of their bands:
"Red and black, friend of Jack;
Red and yellow, kill a fellow."

I hope this assuages any fears you have, should you chose to visit all
your Googly girlfriends in America.

Cheers, Myo (Life Scout)