In this case, being a "traffic engineer" with the ability to sign engineering plans means graduating from an ABET-accredited engineering program, passing both the Fundamentals of Engineering exam and the Principles & Practice of Engineering exam, being licensed as a professional engineer, and passing the Professional Traffic Operations Engineer exam. I think they do a little more than "maximize vehicular traffic flow."
Certifications prove that you studied, and are smart and or diligent enough to pass an exam.
If those certifications try to teach you bad approaches. Then they don't help competence. In fact, they can get people stuck in bad approaches. Because it's what they have been Taught by the rigorous and unquestionable system. Especially when your job security comes from having those certifications, it becomes harder to say that the certifications teach wrong things.
It seems quite likely from the outside that this is what happened to US traffic engineering. Specifically that they focus on making it safe to drive fast and with the extra point that safe only means safe for drivers.
This isn't just based on judging their design outcomes to be bad. It's also in the data comparing the US to other countries. This is visible in vehicle deaths per Capita, but mostly in pedestrian deaths per Capita. Correcting for miles driven makes the vehicle deaths in the US merely high. But correcting for miles walked (not available data) likely pushes pedestrian deaths much higher. Which illustrates that a big part of the safety problem is prioritizing driving instead of encouraging and proyecting other modes of transportat. (And then still doing below average on driving safety)
