Where do we find a grant of jurisdiction in Traffic court?
We don't.
This page is http://www.fu.gq.nu/6756.html

1.  Jurisdiction is one subject in the Cadillac motion.  It is a challenge to the jurisdiction of the court. Various versions of the Cadillac motion are linked to my main list of winning litigants. http://www.circuitlawyer.8m.com/traffic.html

You are near the top of the list on that page, Theo.  You used a version of the Cadillac motion.  Here is a link to your winning challenge to jurisdiction: http://www.fu.gq.nu/theo.html  You can search for the word "jurisdiction" in this winning motion.

2.  Jurisdiction issue must be resolved before the court can do anything in the case.  This is the ruling of Rhode Island v Massachusetts: http://www.lawyerdude.netfirms.com/rhode.html

3.  You posted the jurisdiction clause from the constitution.  This clause prescribes the jurisdiction of the federal courts. But we are not talking about federal courts.  Do you go to federal court with your traffic ticket? no.

4.  We would look to the various state constitutions for jurisdictional grants regarding traffic court, but the various state constitutions were written before automobiles.  Some constitutions have been amended recentely.

5.  Also, we might look to state statutes for a grant of jurisdiction. However, the statute would have to rely on the constitution - perhaps a general clause such as "health and welfare".

6.  It is likely that local judges invented traffic court and vested it with power in violation of their oath of office.  This is a question to ask immediately when you get to court. Ask it in writing. This would be called  a "Rhode Island" motion - if that pleases you.