Organizations:

International Municipal Signal Assoc. (IMSA) - www.imsasafety.org
This is THE organization for the people who do the hands-on installation and maintenace of traffic signals. They have GREAT training.

Institute for Traffic Engineering (ITE) - www.ite.org
This is where most traffic specs originate. Lots of engineers and consultants here.

Intelligent Transportation Society of America (ITS) - www.itsa.org
provide the tools for skilled transportation professionals to collect, analyze, and archive data about the performance of the system during the hours of peak use. Having this data enhances traffic operators' ability to respond to incidents, adverse weather or other capacity constricting events.

American Traffic Safety Services Association (ATSSA) - www.atssa.com
has been representing companies and individuals in the traffic control and roadway industry since 1969.

Vendors:

For your reference, here are some of the new manufacturers' web sites. This is not an exhaustive list - it is what fell to hand at the time. Inclusion does not constitute an endorsement, nor does exclusion constitute disapproval.