Didn’t read it but very usefull information. At least to start the search:

Visual Function: An Introduction to Information Design
By Paul Mijksenaar

See it at
http://books.google.com/books?id=-j7JcB2al7sC&pg=PP1&ots=7zkuczWIME&dq=Visual+Function:+An+Introduction+to+Information+Design&sig=naIYHnWqIrb_2dcW2tf8pwM630A#PPA26,M1

António

Massive topic...can you be more specific?

As far as font legibilty etc is concerned...
http://typophile.com/node/14300

Bit more history here.

There’s always Wikipedia.

Sorry these are very British. James M has got to be your man in the States.

Have you looked at the MUTCD? (Manual of Uniform Traffic Control Devices) its a mega tomb and not very modern but it does describe what is currently in place on the Federal Roadscape. Don Meeker, my partner in the Clearview project has developed a proportional grid design system he is currently trying to get the Feds to adopt. I know the system is very modular and basically sends a good portion of the MUTCD to the waste bin.

There may be some useful information on our Clearview web site:

http://www.clearviewhwy.com

http://en.wikipedia.org/wiki/Traffic_sign

Ireland has a Traffic Signs Manual, but I can’t find the complete manual online in a quick search. A new Chapter 8 is here.

The UK:
http://www.dft.gov.uk/pgr/roads/tss/tsmanual/publicationoftrafficsignsma....
http://www.dft.gov.uk/pgr/roads/tss/trafficsigns.pdf
http://www.standardsforhighways.co.uk/dmrb/vol8

The Vienna Convention on Road Signs and Signals; a PDF version is here, but may be out of date; a Wikipedia article.

These are specific to the United States:

Manual on Uniform Traffic Control Devices (MUTCD)

Standard Highway Signs

MUTCD publications

MUTCD home page