what could be the visualization equivalent of the resistors in series?
The cars analogy doesn't model what's going on.

A better analogy is water in a pipe.  Let's say you have a pump (like voltage source) that keeps a constant pressure from one end of a pipe to the other.  With just a section of open pipe, the flow rate (like electric current) will be quite high.

Now imagine installing a constriction (like electric resistor) somewhere in the pipe.  The flow rate will be less.  Now install a second restriction further down the same pipe.  The flow rate will be even less.