Depending on what your interest in transportation engineering is, you may find yourself pretty disappointed. I can probably opine a little on this based on my background. I did primarily unix systems and security for the better part of a decade before getting a masters in urban planning, spending a few years working for a metropolitan planning organization including in their travel modeling group (these are usually the major public sector transportation modeling agencies due to federal requirements), and am now back in school working on a PhD in planning. I'm going to give you the traditionalist's view here, which may look a little drab / discouraging.

If you want to "fix traffic" I highly recommend running away now. You will not fix traffic as transportation modeler (I assume transportation modeling is what you mean by traffic simulation). The state of the art in travel modeling is not good enough that obvious solutions present themselves very often. Even when obvious solutions to some narrowly-constrained problem do present themselves, model results are usually one data point for consideration in a delicate dance between bureaucrats, politicians, and a general public with diverse and competing interests. You will also find yourself very poorly compensated compared to your peers who graduated in CS.

Another thing to keep in mind walking into anything transportation related (private or public sector), is that you are going to have a big ramp-up on domain knowledge / will start at the bottom of a low-paying totem pole. In my group in a public agency, we had a handful of people with PhDs, lots with masters, and I'm struggling to think of one person with a bachelors. And to be clear, a masters degree in civil engineering, planning, or public affairs qualified you for an entry level (assistant) position or maybe an associate level if you had some decent experience on top. I don't bring this up because I'm under the delusion that degrees make the person, but do beware that you're likely to run up against some biases if not hard degree requirements. In terms of domain knowledge, the transportation research board (TRB) is one of the national academies of science. I'm not sure the best scholarship always happens in this domain, but I bring this up to say that there's lots to learn... Also, transportation modeling is a very stats-heavy domain. Getting into the guts of transportation models is likely to require more than a passing familiarity with graduate level statistical methods.

In the public sector, the MPO I worked for was actually developing its own next generation travel modeling package, but opportunities in the public sector for that are as rare. The public sector has been running so-called "trip-based models" for decades. The software for these has been an off-the-shelf affair for some time, with actual model runs set up through either through monotonous clickety-click or ghastly macro language by intern-level positions guided by more senior modeling staff. Activity-based models are the next generation, and are being tested out by more ambitious agencies now. Once the benefits of these new models are proven out, smaller MPOs and city planning offices will likely adopt them. However, I don't see the the second wave hiring in-house coders--they'll wait until they can get an off-the-shelf package (or if there is justice in the universe, some usable open source version). I mention this because, despite the breakneck pace at which academia produces new and novel statistical and modeling techniques, consumers of these models are crazy slow at adopting new model types. This obviously has implications for what kinds of coding jobs exist.

You might find an in as a coder for a company that writes one of the major existing travel modeling software packages, but my understanding talking to them, is that they tend to be fairly small operations and have coders who also tend to have some domain expertise in travel modeling. I could imagine them hiring a straight-up coder, but breaking into the subject matter might be hard, as you could very easily end up doing pretty generic coding tasks with little relation to traffic engineering.

Reading through this, I'm realizing I sound pretty discouraging. If you constrain yourself to transportation modeling, which is what I assume you meant by traffic simulation, you may find yourself disappointed by the opportunities. This is the boring traditional world of transportation modeling. If you're willing to look outside transportation modeling, you may find some more interesting transportation-related coding jobs that are friendlier to people with more of a coding / entrepreneurial bent. Particularly in the startup space. I think there's still plenty of greenfield left for people wanting to sensor-up the city, ferry those data around, and then make sense of it all.