Answer:
An exponential function
Step-by-step explanation:
So let's suppose that at 9:00 am there were 132 cars. We'll call this 

On the next half an hour, let's say at 9:30, the number of cars trippled, this is:

the next half an hour, at 10:00 am, the number of cars trippled, this is:

and finaly, at 10:30am, the number of cars trippled again, so:

We can now see a pattern there which can be simplified as:

or

where:
initial number of cars
n=number of half hours after 9:00 a.m
number of cars after n number of half hours.
That function we found there is an exponential function, which represents an initial amount being multiplied by the same number several times.