I think that the AASHTO spec will tell you if its the same.
when i was Dealing with AASHTO the certs more often than not would list all similar grades. i no longer have access to the spec.

You might want to check the AWS D1.5-2008 Bridge Welding Code.

I found this too.  Looks like it might be similar to ASTM A 709. 

http://www.chapelsteel.com/A709-grade50-grade-345.html

My reply from yesterday got nuked; apologies if it reappears and this is a duplicate.

Where can you find this?  In the spec itself.  If you actually read it, you will see in one of the first couple of paragraphs where it says Gr. 50 meets or exceeds the requirements of A 572.  These specs aren't just number and letter designations; they're real documents with real requirements and real descriptions, and if you have AISC certification you're supposed to have copies of the relevant specs.

AASHTO M 270 is the same as ASTM A 709, maybe a year behind because AASHTO takes its updates from ASTM for this spec.  There used to be a footnote in M 270 stating this explicitly, but it seems to have fallen off a few years ago.  But up at the top of the AASHTO spec it also lists the equivalent ASTM spec.  So M 270 lists right under the title which edition of A 709 it's equivalent to.

ASTM A 709 Gr. 50 is basically A 572 Gr. 50 plus Charpy requirements.  A 709 and M 270 explicitly state that particular grades meet or exceed the requirements of A 36, A 572, A 588, A 992, etc.

D1.5 discusses the AASHTO & ASTM specs.  D1.1 doesn't mention AASHTO specs at all; I guess they figured only the bridge people would ever be messing around with AASHTO specs.  What kind of job are you working on that calls for D1.1 but AASHTO steel?

Hg

One difference between A 572 - Gr. 50 and A 709 Gr 50S is that there is a 85% Tensile to Yield ratio requirement in the ASTM A 709.