Unfortunately it is not that easy to simulate traffic, especially not on city scale or larger.

The most important input to a traffic simulator such as this is the so-called "traffic demand", i.e. the routes that vehicles follow. Typically this is provided in the form of origin-destination matrices, but this data is not freely available.

Next up is the way in which traffic lights work. Reality is very hard to model here, again because the data is not freely available.

And then, due to numerous modeling errors in vehicle density, in the way that roads differ from e.g. OpenStreetMap, and how traffic behaves, the simulations are highly unrealistic, unless one spends some time to calibrate it.

It costs quite a lot of money to set up a realistic simulation, and most governments use commercial tooling that is easier to use, such as Vissim or Aimsun.