LSTM Time series prediction for multiple multivariate series
I think one question you need to answer is how the traffic is correlated across cities. 

If they are not correlated (which is likely) then the city can be a categorical input variable. The network can learn what is common across all cities and what is specific to different cities.

Here is a reference that may be helpful. In this example, the inputs are clearly related, so that may be a difference in your case.

You also may consider developing a model for one city, and then improving it by adding other cities.